Oxford Aviation Jeppesen-Meteorology

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JOINT AVIATION AUTHORITIES

AIRLINE TRANSPORT PILOT'S LICENCE

Theoretical Knowledge Manual,\,-

050 METEOROLOGY

APPROVEDThis learning material has been approved as

JAA compliant by the united KingdomCivil Aviation Authorttv.





AMENDMENT SERVICE

An amendment service to this series is provided free of charge on the Oxford Aviation Trainingwebsite at http://www.oxfordaviation.net/products/studyaids/amend.htm

First Edition: May 2001

Second Impression: October 2001 - incorporating Amendment List 1o Edition 1

http://www.oxfordaviation.net/products/studyaids/amend.htm

http://www.oxfordaviation.net/products/studyaids/amend.htm



CHAPTER ONE - THE ATMOSPHERE

Contents

Page

1.1 A DEFINITION OF METEOROLOGY. 1 - 1

1.2 REASONS FOR STUDYING METEOROLOGY 1 - 1

1.3 A DEFINITION OF THE ATMOSPHERE. . 1-2

1.4 THE CONSTITUENTS OF THE ATMOSPHERE (BY VOLUME). .... 1 -2

1.5 PROPERTIES OF THE EARTH'S ATMOSPHERE. . 1 - 2

1.6 THE STRUCTURE OF THE ATMOSPHERE 1 - 3

1.7 THE SIGNIFICANCE OF TROPOPAUSE HEIGHT. 1-4

1.8 TEMPERATURES. . I - 4

1.9 ATMOSPHERIC HAZARDS 1 - 5

1.10 THE INTERNATIONAL STANDARD ATMOSPHERE (ISA) 1 - 5

1.11 ISA DEVIATION. . 1 - 6

1.12 THE ICAO INTERNATIONAL STANDARD ATMOSPHERE.

ATMOSPHERE QUESTIONS

1 - 8

....... 1 - 9



METEOROLOGY THE ATMOSPHERE

1.1 A DEFINITION OF METEOROLOGY

"The branch of science dealing with the earth's atmosphere and the physical processes

occurring in it."

1.2 REASONS FOR STUDYING METEOROLOGY

a) To gain a better understanding of meteorologists' deductions.

b) To gain a better understanding of meteorologists' documentation.

c) To gain a better understanding of in-flight hazards.

d) To gain a better understanding of data and its collection.

e) To gain a better understanding of self-forecasting.

Weather is the one factor inmodem aviation over which man has no control, a knowledge of

meteorology will at least enable the aviator to anticipate some of the difficulties which

weather may cause.

Weather - influenced accidents to UK transport aircraft

Table! Transport aircraft acoidents, 1975-94

(ajAllaccidents

Aeroplanes Rotorcraft

20

20

• rncludes ramp and other minor g rrund acc idents, hence low percentage figures.

(b J Acddenl ii excluding.elec tedramp and other occurrences

Aeroplanes

*WI;Weather_influenced

Table 2 Weather -influence accidents to transport aircraft by element of weather, 1975 -94

1-1 © Oxfo rd A via tio n S erv ic es L im ite d




Element PercentageoftotaJ Percentage of total

Visibility

Icing/snow

Wind and turbulence

Rainlwetrunway

Lighming

All cases

Table 1.2.

For this course a knowledge of advanced physics is not required, but a knowledge of the

elementary laws of mati on, heating, cooling, condensation and evaporation will be useful.

1.3 A DEFINITION OF THE ATMOSPHERE

"The spheroidal gaseous envelope surrounding a heavenly body."

1.4 THE CONSTITUENTS OF THE ATMOSPHERE (BY VOLUME)

Nitrogen 78.09% Argon 0.93%

Oxygen 20.95% Carbon Dioxide 0.03%

Plus traces of:

Neon Nitrous Oxide Helium Nitrogen Dioxide

Krypton Carbon Monoxide Xenon Sulphur Dioxide

Hydrogen Ammonia Methane Iodine and Ozone

Plus water vapour and solid particles.

The proportions of the constituents remain constant up to a height of at least 60 kms (except for

Ozone), but by 70 kms the force of gravity, being less, causes the proportions to change.

Although the trace of ozone in the atmosphere is important as a shield against ultra violet

radiation, if the whole of the layer of ozone were brought down to sea level it would only be 3

mmthick.

1.5 PROPERTIES OF THE EARTH'S ATMOSPHERE

The earth's atmosphere varies vertically and horizontally in:

a) Pressure.

b) Temperature.

1 -2 © Oxford Aviation Services Limited




c) Density.

d) Humidity.

The earth's aunosphere is a poor conductor.

The earth's atmosphere is fluid.

The earth's atmosphere supports life only at lower levels.

1.6 THE STRUCTURE OF THE ATMOSPHERE

a) The Troposphcre..

rhar layer of the earth's atmosphere where temperature decreases with an

increase in height.

ii) consists of % of the total atmosphere in weight.

iii) contains almost all th e weather.

b) The Stratosphere Illay be defined as that layer above the troposphere where the

temperature remains constant with an increase in height. (In fac t temperature shows 1 1

gradual increase with hC!p'ht,especially at the top, where th e temperature is zero at 50

kms. This is due to the abs~rpth)nd}lhe SUIl 's ultra violet radiation by the concentration

of ozone at higher levels).

c) The Tropopausc..

marks the boundary between the troposphere and the stratosphere and is where

temperature ceases to fall with an increase in height, (Practically taken as the

height where tile temperature fall is Jess tban z=C per 1,000 n.)

ii) is not a continuous line - there is usually a gap at 40 degrees of latitude between

the so-called polar and tropical rropopauses.

~a....\.:...;, \iii) is nat uniform i n h eig ht it varies with..

I) Latitude.

2) Season of the year.

3) Temperature prevailing on the day.

4) Time of day.

1 - 3 © Oxford Aviation Services Limited




90' JANUARY 90'JULY

70' 70'60'

Figure 1.2. The Mean Height of the Tropopause at the Greenwich

Meridian

o o 1. ~,.,.,~

1.7 THE SIGNIF1CANCE OF TROPOPAUSE HEIGHT

The significance of the tropopause height is that it usually marks;-

< I) the maximum height ofthe cloud.

b) the presence of Jetsrreams.

c) the presence of Clear Air Turbulence (CAT).

d) the maximum wind speed

1.8 TEMI'ERATURES

Temperature in the troposphere increases from the poles to the equator.

Temperature in the lower stratosphere increases from the equator to the poles in summer but

reaches max temperature in mid latitudes in winter

The lapse rate (the rate of change of temperature with height) in the troposphere is produced by

ILo. ; ! i f - : " " rising air, whilst that in the stratosphere is produced by solar radiation, and is in fact reversed.





1.9 ATMOSPHERIC HAZARDS

As aircraft operating altitudes increase, so concentrations of OZONE and COSMIC

RADIATlON become of greater importance to the aviator.

Above 50,000fl, normal concentrations of ozone exceed tolerable limits and air needs to be

filtered before entering the cabin. The heat ofthe compressor system will assist in the breaking

down of the ozone to an acceptable level.

Cosmic radiation is not normally hazardous, but at times of solar flare activity a lower flight

level may be necessary .

' T . 2, v.~ {l\'''. ''>!.\h''' ......

Advances in meteorological forecasting and communications should result in pilots receiving

prompt and accurate information regarding high altitude hazards. but it is important that they

should be aware of these hazards and prepared to take the necessary re-planning action.

1.10 TI:lE iNTERNATIONAL STANDARDATMOSI)UERE (ISA)

~,CI ~ ~ '-'_

For a variety of reasons it is necessary to establish a standard average atmosphere, describing

variations in temperature, pressure and density throughout altitude.

There have been several different Standard Atmospheres, but the one i _ 1 1 genernl usc now is the

'ICAO {SA', dated 1964 which covers an atmosphere from -16.400ft(-5km) to 262,464ft.

The ISA is needed for.-

a) the calibration of aircraft instruments

b) the design and testing of aircraft.

The lCAO ISA is defined as follows>

a) a MSL temperature of + 15° Celsius,

b) a MSL pressure of 1013.25 millibars,

c) a MSL density of 1225 grarnrnes I cubic metre,

d) from-Skill, a lapse rare of 1.98° Celsius/lOCO ft (6.5 degs/km) up to 36,090 n (11 kms),

e) a constant temperature of -56.5° Celsius up to 65,617 n (20 kms),

1 ) an increase of'temperarure 01'0.3° Celsius /1 000 ft (I deglkm), up to 104,987 ft 32 krus)

1 -5 © Oxford Aviation services Limited




Tem erature tC

pressure rnb

Relative Density

-'13£..

122.,

112..

102.. I

90_1

80_

70_

60_

50_

, Upper Limit of Wright Air Development:l Centre Atmosphere J jO 000 ft

I I I I I i £ .I I/,1 I I ~

I I I I I

: /~ ~t"top~us 1Q4,9,87ft "

~ 5 6 J ' e i i 3 2 i k l ' ~~ I I I I I ....

i T '"~'~'"I'~'~q':l'7ftII-56.e- c I I :, I

f--~ i~ ~r()Ol':"~131:6k'mOi_Qft

~ . ; l ' ( . :: < t : : I

; ~ . < 1 ' I ? ~ ~ : :' ~ ~ " O S ' , , / ' ~ : I I

: " " ~ " : : :I I 'I I

I I 1'-

I I II I I

I I I

, "

40

30

20

10

I.H [SA DEViATION

Figure 1.2. The International Standard Atmosphere (ISA)

Although meteorological observations are made in absolute figures, it is usual, when making

calculations involving aircraft performance or corrections to instruments, to consider them

relative to the ISA. These are known as "ISA deviations".

lffor instance, the observed temperature were SoC warmer than that expected in the ISA, then

the deviation would be+5°C

For the temperatures below, calculate the ISA deviations;-





Heighttft)Temperature ISA [SA

(0C) Temperature Deviation

[500 +28

17,500 -18

24,000 -35

37,000 -45

9,500 -5

5,000 +15

31,000 -50

57,000 -67

If the limiting deviation for your aircraft at an airfield 5,000 ft AMSL is ISA +10, what is

the maximum temp at which you can operate?

If the deviation at 3,500 ft is +12, what is the ambient temperature?

1-7 © Ox fo rd Av ia tio n Se rv ic e s L im ite d




•1.12 THE rcxo INTERNATIONAL STANDARD ATMOSPHERE

Height (kms) I Height(n) I Temp (,C) Pressure Height Change Density(%)

(mb) (permb)

32.00 104,987 -44.7 8.9 1.1

30.48 100,000 -46.2 11.1 1.4

27.43 90,000 -49.2 17.3 2.2

24.38 80,000 -52.2 28.0 3.6

21.34 70,000 -55.2 44.9 5.8

20.00 < " 1 ; , 0 -56.5 56.7 7.2

15.24 50,000 -56.5 116.6 15.3

13.71 45,000 -56.5 148.2 19.5

11.78 38,662 -56.5 h"'2oo 103 ft 26.3

11.00 36·,090 -56.5 228.2 91 ft 29.7

9.16 30,065 -44.4 .3.9Q 73 ft 36.8

5.51 18,289 -21.2 • • 5 0 ! ! 48 ft 56.4

3.05 10,000 -4.8 696.8 37 ft 73.8

3.01 9,882 -4.6 700 36 ft 74.1

1.46 4,781 +5.5 850 31 ft 87.3

0 0 +15 1013.25 27 ft 100

Note:

The above height change figures show how the pressure against height change equation is ......._,;

m odified as a ltitude changes but the figures offered only rela te to ISA conditions of T em perature

and Pressure. We can assess changes outside these conditions by using the following formula:

96TH=-p

where H =height change per Mb I Hpa in feet

T = Actual Absolute Temperature at that level

P =Actual Pressure in Mb I Hpa

K =96 (the equation constant)

The 4% Rule:

The 4% rule is an extension of the above which states that when the ELR temp' is ro-c

away from ISA a 4% height change error is generated at or through any given altitude

change. e.g at Fi360 (H) =96 x 226.5 divided by 228.2 = 95ft per Mb height change atthat level which equates to 4% difference from the ISA change of 91ft.

1-8 ©Oxford Aviation Services Limited




Atmosphere Questions

1. The international standard atmosphere assumes a lapse rate of:

a) 2°C/IOOO ft

b) 1.5°CIlOOO ft

c) 3 °C /IOO O ft

, d) 1.98°C /IOOO ft

2. The tropopause is:

-1 a) The line where the temperature no longer decreases with increase of height.

b) The layer between the tropopause and the stratosphere.

c) The layer beyond which only CI cloud occurs.d) The line indicating clear air turbulence.

3. One of the most important characteristics of the atmosphere is:

a) Density is constant above 10000 ft.

1 b) The air is a poor conductor of heat.

c) Temperature lapse rate is very frequently above 3°e per

1000 ft.

d) The air is a good conductor of heat.

4. Most of the vapour in the atmosphere is contained in the:

a) tropopause

b) stratosphere

J c) troposphere

d) stratopause

5. The captain of an aircraft needs to know the height of the Tropopause because:

,j' a) it normally represents the limit of weatherb) density starts to increase

c) there are no longer jet streams and CAT

d) it indicates the height of the thermal wind





6. The main Ozone layer is to be found in the:

a) thermosphere

b) troposphere

c) mesophere

,,'d) stratosphere

7. The level in the atmosphere where the air temperature ceases to fall with increase in height is

known as:

a) The troposphere.

b) The Stratopause.

c) The Stratosphere.

" d) The tropopause.

8. Which statement is correct when considering the lower layers of the atmosphere:

a) the majority of the weather is contained in the stratosphere and its upper boundary is the

tropopause

.J b) the majority of the weather is contained in the troposphere and its upper boundary is the

tropopause

c) the majorityofthe weather is contained inthe tropopause and its upper boundary is the

troposphere

d) the majority of the weather is contained in the troposphere and its upper boundary is thestratosphere

9. The atmosphere is a mixture of gasses of the following proportions:

v a)

b)

c)

d)

oxygen 21%

oxygen 21%

nitrogen 78%

nitrogen 78%

nitrogen 78%

hydrogen 78%

argon 21%

oxygen 21%

other gasses 1%

other gasses 1%

oxygen 1%

hydrogen 1%

10. In the ISA the temperature is isothermal:

a) Up to 36 090 ftlll kms

,. b) From 36 090 ft/II kms to 65 617 ftl20 kms.

e) From 36 090 ft/l l kms to 104987 ft!32 kms.

d) From 36 090 ft!11 kms to 45 090 ftl13.75 kms.





11. The International (leAD) Standard Atmosphere assumes that the sea level atmospheric pressure

is :

v' a) 1013.25 mbs and decreases with an increase inheight

b) 1013.25 mbs and increases with an increase in height

c) 1013.25 mbs and falls to about half this value at 30000

d) 1013.25 mbs and decreases with an increase in height up to the tropopause. Above the

tropopause it remains constant

12. At se a level the ISA density is stated to be:

" a) 1225 grammes per cubic metre

b) 1252 grammes per cubic metre

c) 1013.2 mb (hpA)

d) 29.6 inchesof mercury

13. Which of the following statements is most correct when describing ISA:

a) the MSL pressure is 1013.25 mbs and the temperature is + 15°C

b) the MSL pressure is 1013.25 mbs and the temperature is +15 C with a lapse rate of

1.98°C/1000 ft

c) the MSL pressure is 1013.25 mbs and the temperature is +15 C with a lapse rate of

1.98°CIlOOOft up to 36090 ft above which there is frequently an 'inversion

\- d) the MSL pressure is 1013.25 mbs and the temperature is +15 C with a lapse rate of1.98°CII 000 ft up to 36090 ft

14. The following is true for the International Standard Atmosphere:

a)

J b)

c)

d)

at mean sea level the foilowingconditions prevail: temperature + 15 C, pressure 1013.25

hpa, density 1125 gmlm

within the troposphere the temperature decreases by 6.5 C per Ian

the tropopause is at a height of 36090 AGL

the temperature at the tropopause is 226.5 OK

IS. Pressure will \.eV\~DJ-._~ with increase of height and in the ISA pressure will be _'_CO_at

10000fiand '\.,,--0 at30000ft

a)

'b)

c)

d)

Increase

Decrease

Increase

Decrease

800 mb 400 mb

700 mb 300mb

200 mb 800mb

500 mb 200

1-11 ©Oxford Aviation Services Limited



METEOROLOGY

ANSWERS

Ques Answer Ques Answer

1 D 9 A

2 A 10 B

3 B 11 A

4 C 12 A

5 A 13 D

6 D 14 B

7 D 15 B

8 B

1-12

THE ATMOSPHERE

© Oxford Aviation Services Limited



CHAPTER TWO - PRESSURE

Contents

2.1 INTRODUCTION .

2.2 ATMOSPHERIC PRESSURE ..

2.3 THE BAROGRAPH

2.4 ISALLOBAR.

2.5 TYPES OF PRESSURE

2.6 VARIATIONS OF PRESSURE

2.7 PRESSURE DEFINITIONS

2.8 SYNOPTIC CHARTS .

PRESSURE QUESTIONS .

Page

2 - I

........... 2-1

.......................... 2-2

.......................... 2-3

............................ 2-4

.... 2-7

...... 2 - 8

... 2 - 8

. 2-9

© Oxford Aviation Services Lim ited



METEOROLOGY PRESSURE

2.1 INTRODUCTION

Variations in pressure have long been associated with changes in the weather - the 'falling glass'

usually indicating the approach of bad weather. The Handbook of Aviation Meteorology makes

the statement:f"'11\~ I . , . , , , , p" ,, ~~<>_ \<> \ c . .. .. . ~ -i:l",o ~~,<

"The study of atmospheric pressure may be said to form (he foundations of the science of

meteorology. "

2.2 ATMOSPHERIC PRESSURE

Atmospheric pressure is the

force per unit area exerted by the

atmosphere on any surface in

contact with it. If pressure is

considered as the weight of a

column of air of unit cross

sectional area above a surface,

then it can be seen from the

diagram that the pressure (weight

of the column above) at the

upper surface will be less than

that at the lower surface.

A COLUMN OFUNITCROSS_

SECTION

TOTAL WEIGHT OF-ATMOSPHERE

ABOVE

TOTAL WEIGHT OF/ATMOSPHERE

L_ABOVE

Figure 2.1. The Weight of the Atmosphere on the

Thus atmospheric pressure will Surface of the Earth.

decrease with an increase in

height.

a) Units of Measurement. The standard unit afforce is the NEWTON (N) and an

average for atmospheric pressure at sea level is 100,000 Newtons per square metre.

(Pascals) This pressure is sometimes known as a BAR To measure small variations

in pressure, it is convenient to divide the bar into 1000 parts and so the standard

meteorological unit of pressure is the MILLIBAR (Mb). In some countries this is

known as the hectopascal. Other units which are still in use are related to the height

ofa column of mercury in a barometer (see below) and thus:

1000 mb = 750.1 nun =29.53 in = 100,000 N/M2

Note: It is possible to convert Mbs to Inches by using the formula __x_x_x_ and

1013.25 29.92

therefore if we are given (for example) IOOOMbswe may insert this into the formula and

find ___!_QQQ_x_x_ which gives us an answer of29.53In5 of mercury.1013.25 29.92





b) Mercury Barometer. The basic instrument

used for the measurement of atmospheric

pressure is the mercury barometer. The

atmospheric pressure is measured by the

height of a column of mercury and thisheight can be read in terms of any of the units

shown above. The USA still uses inches of

mercury as their measurement of atmospheric

pressure.

Figure 2.2. A Mercury Barometer

c) Aneroid Barometer. A more compact means of

measuring atmospheric pressure is the Aneroid

Barometer. It consists of a partially evacuated

capsule which responds to changes in pressure by

expanding and contracting, and by a system of

levers, these changes of pressure being

indicated by a pointer moving over a scale.

Figure 2.4. An Aneroid Barometer.

To enable a continuous record of pressure changes

to be made, a paper covered rotating drum issubstituted for the scale and the instrument then

becomes a barograph. This instrument is used

by the meteorologist to measure what is known

as pressure tendency, the rise and fall of pressure

over a period of time. Pressure tendency is an

important forecasting tool.

2.3 THE BAROGRAPH

Figure 2.5. Met Office AneroidBarometer

2-2 © Oxford Aviation Services Limited



METEOROLOGY

Figure 2.6. A Barograph

2.4 [SALLOBAR

An isallobar is a line joining places of the same pressure tendency.

Full and dashed lines represent ./ - - ......isobars and isallobars respectively. / '\

Unit of isaliobars:millibalS per hour. / .- -v , \

\ I I Isallobaric low

- = 4 ; II II./

Figure 2.7. An Isallobar Chart

2-3

PRESSURE





2.5 TYPES OF PRESSURE

a) QFE. The atmospheric pressure read from a barometer on an airfield will give

the aerodrome pressure, otherwise known as QFE.

Figure 2.8. QFE.

b) QFF. This is the barometric pressure at the surface (QFE) reduced to MSL using the

observed temperature at the surface (this therefore assumes an isothermallayer from

MSL to the surface). QFF accounts for the effect that temperature has on a pressure

reading. From Figure 2.8 it can be seen that although the pressure at the airfield was

980 mb/hPa, ifthe airfield was taken to Mean Sea Level, the pressure would be greater,

but an account rnust also be made of the effect that temperature has had on the pressure.

This allows us to accurately draw surface pressure charts. The correction to be made

to the surface pressure will depend on the height of the surface (or airfield) AMSL and

the temperature prevailing at the time.

The range of QFF so far recorded, low pressure to high pressure, is from 856 to

1083 mb, but meteorologically the range is taken from 950 to 1050 mb.

c) QNH. This is the barometric pressure at the airfield (QFE), reduced to MSL using the

ISA temperature at the airfield. This will provide a pressure which does not account for

any temperature deviation away from ISA. The correction 1'0 be made to the surface

pressure will depend solely upon the height of the airfield AMSL

In order to get QNH and QFF from a barometric reading at a surface we must use a

formulae which will be shown on the next page. It is not necessary to know the

formulae as such, but it is vital to know the difference that the temperature deviation

will have when being asked to analyse QNH and QFF.

2-4 © Oxford Aviation Services limited




The correction M (in hPa/mb), to be added or subtracted to the barometric pressure is

given by:

M~p(10m_1)

Where'" =

18429.1 + 67.531 + O.003h

and p = barometer level pressure in hPa/mb

t = the observed temperature at station level in °C (for QFF correction use

observed temperature, for QNH correction use [SA temperature)

h = the height of the station, in metres, above the level at which the corrected

pressure is required i.e. above or below mean sea level for QFF and QNH,official aerodrome elevation for QFE and touchdown zone elevation for runway

QFE. Note that h will be negative if below sea level.

Example I:

I) What is the difference between QFF and QNH given:

Station pressure = 1020 hPa

Station height = S O m BELOW mslTemperature = 30° C

Station BELOW sea level, temperature WARMER than ISA.

a) Calculate QFF using the formulae on the previous page

M = -5.6 hPa

The correction to be applied is:

Station pressure 1020 - 5.6 =QFF 1014.4 hPa

b) Calculate QNH using the formulae on the previous page

M = -5.9 hPa

The correction to be applied is

Station pressure 1020 - 5.9 =QNH 1014.1 hPa





Example 2:

I) What is the difference between QFF and QNH given:

Station pressure = 920 hPa

Station height = 300m ABOVE msl

Temperature = _200 C

Station ABOVE sea level, temperature COLDER than ISA.

a) Calculate QFF using the formulae on the previous page

M =41.2 hPa

The correction 10be applied is:

Station pressure 920 + 41.2 = QfF 961.2 hPa

b) Calculate QNH using the formulae on the previous page

M = 36.9 hPa

The correction to be applied is:

Station pressure 920 + 36.9 =QNH 956.9 hPa

SUMMARY

Stations ABOVE MSL a) HOTTER than [SA QFF<QNH

b) COLDER than ISA QFF>QNH

Stations BELOW MSL a) HOTTER than ISA QFF>QNH

b) COLDER than (SA QFF< QNH





2.6 VARIA TIONS OF PRESSURE

a) Height. Although pressure will decrease with an increase in height, density will also

decrease and therefore the reduction in weight of air above a surface will not varylinearly. In the ISA, a reduction in pressure of I mb would give a height difference of:

27 feet at MSL

3 6 feet at t O , O O O ft

73 teet at 30,000 ft

See Figure 1.3.

b) Diurnal Variation. There is a change in pressure during the day which although small

(about I mb) in temperate latitudes, can be as much as 3 mb in the tropics and would

need to be taken into account when considering pressure tendency as an indication ofchanging weather. The variation is shown in Figure 2.10.

THE DIURNALVARIATION INTEMPERATELATITUDES ISLESS THAN

1mb ~

MEAN PRESSURE

RECORDED PRESSURE

Figure 2.10. Diurnal Variation.

The variation is difficult to explain, but is probably due to a natural oscillation of the

atmosphere having a period of about 12 hours, this oscillation being maintained by the 24

hour variation of temperature.




METEOROLOGY

2.7 PRESSURE DEFINITIONS

QFE

QFF

QNH

FORECAST QNH

(RPS)

QNE

ISOBAR

ISALLOBAR

2.8 SYNOPTIC CHARTS

Isobars on normal

synoptic charts are

Mean Sea Level

Isobars (QFF) and are

normally drawn for

every even whole

millibar, (i.e. 1000,

1002, etc.). Figure

2.11. illustrates the

isobars on a synoptic

chart.

On larger area maps

the spacing may be

expanded to 4 or more

millibars but this willbe stated on the chart.

PRESSURE

The value of pressure, for a particular aerodrome and time, corrected

to the official elevation.

The value of pressure reduced to MSL in accordance with isothermal

conditions.

The value of pressure, for a particular aerodrome and time, corrected

to the MSL in accordance with the ICAO standard.

A forecast, valid for one hour, of the lowest QNH expected in any part

of the Altimeter Setting Region (ASR).

The height indicated on landing at an airfield when the altimeter sub-

scale is set to tOI3 mb or 29.92 ins.

A line joining places of the same atmospheric pressure (usually MSL

pressure QFF).

A line joining places of the same pressure tendency.

Figure 2.11. Isobars on a Synoptic Chart.





Pressure Questions

I. The barometric Pressure at the airfield datum point is known as;

0) QNE

b) QNH

v c) QFE

d) Standard Pressure

2. The instrument that gives a continuous printed reading and record of the atmospheric pressure

is:

a) barometer

b) hygrometer

c) anemograph

" d) barograph

3. The pressure of the atmosphere:

a) decreases at an increasing rate as height increases

b) decreases at a constant rate as height increases

oJ c) decreases at a decreasing rate as height increases

d) decreases at a constant rate up to the tropopause and then remains constant

4. When considering the actual tropopause which statement is correct:

a) it is low over the poles and high over the equator

b) it is high over the poles and low over the equator

c) it is the same height of36090 ft all over the world

d) It is at a constant altitude of 26000'

5. Atmospheric pressure may be defined as:

a) the weight of the atmosphere exerted on any surface with which it is in contact

b) the weight of the atmosphere at standard sea level

~ c) the force per unit area exerted by the atmosphere on any surface with which it is in

contact

d) a pressure exerted by the atmosphere of 1013.2 mbs

2-9




6. The QFF is the atmospheric pressure:

a) at the place where the reading is taken

b) corrected for temperature difference from standard and adjusted to MSL assuming

standard atmospheric conditions exist

- J c) at a place where the reading is taken corrected to MSL taking into account the prevailing

temperature conditions

d) as measured by a barometer at the aerodrome reference point.

7. With 1013.25 mb set on the altimeter sub scale with an aircraft stationary on the airfield the

altimeter will read:

" a) QNE

b) QNH

c) QFE

d) QFF

8. The aircraft altimeter will read zero at aerodrome level with which pressure setting set on the

altimeter sub scale:

a) QFF

b) QNHc) QNE

d) QFE

9. You are passed an altimeter setting of '29.53' You would then set your altimeter subscale to:

a) QFF

b) 1013

"c) 1000

d) QFE

10. The aerodrome QFE is:

a) the reading on the altimeter on an aerodrome when the aerodrome barometric pressure

is set on the sub scale

b) the reading on the altimeter on touchdown at an aerodrome when 1013.2 is set on the

sub scale

c) the reading on the altimeter on an aerodrome when the sea level barometric pressure is

set on the sub scaleJ' d) the aerodrome barometric pressure.




! 1. When an altimeter sub scale is set to the aerodrome QFE, the altimeter reads:

a) the elevation of the aerodrome at the aerodrome reference point

b) zero at the aerodrome reference point

c) the pressure altitude et the aerodrome reference point

d) the appropriate altitude of the aircraft

12. The aerodrome QNH is the aerodrome barometric pressure:

a) corrected to mean sea level assuming standard atmospheric conditions exist

b) corrected to mean sea level, assuming isothermal conditions exist

c) corrected for temperature and adjusted to MSL assuming standard atmosphere

conditions exist

d) corrected to MSL using ambient temperature.

13. A line drawn on a chart joining places having the same barometric pressure at the same level and

at the same time is :

a) an isotherrn

b) an isallobar

c) a contour

, d) an isobar

14. An isobar on a meteorological chart joins all places having the same:

a) QFE

{ b) QFF

c) QNH

d) QNE

15. Pressure will~~--~-~--·-~withncrease of height and will be about=- ---~~-~~-t 10000 f1 and ~~~~~~~~~~at 30000 ft.

a) Increase 800 mb 400 mb

b) Decrease 700 mb 300 mb

c) increase 200 mb 800 mb

d) Decrease 500 mb 200 mb

16. An airfield in England is 100m above sea level, QFF is 1030hPa, temperature at the surface is

~15°C. What is the valueofQNH?

..\ a) Impossible to determine

b) Less than I030hPa

c) Same as QFF

d) More than lOJOhPa

2 -11 © Oxford Aviation Services limited



METEOROLOGY

ANSWERS


I C 9 C

2 D 10 D

3 C 11 B

4 A 12 A

5 C 13 D

6 C 14 B

7 A 15 B

8 D 16 B

2 -12

PRESSURE




CHAPTER THREE - DENSITY

Contents

Page

3.1 INTRODUCTION 3 - I

3.2 EFFECT OF CHANGES OF PRESSURE ON DENSITY .3- I

3.3 EFFECT OF CHANGE OF TEMPERATURE ON DENSITY 3 - I

3.4 A SIMPLE MATHEMATICAL TREATMENT ... 3 - 2

3.5 EFFECT OF CHANGE OF ALTITUDE ON DENSITY . 3 - 2

3.6 EFFECT OF CHANGE OF LATITUDE ON DENSITY - 2

3.7 EFFECT OF CHANGES IN DENSITY ON AIRCRAFT OPERATIONS. 3-3

DENSITY QUESTIONS 3 - 5

© Ox fo rd Av ia tion Se rv ices L im i ted



METEOROLOGY

3.1 INTRODUCTION

DENSITY

a) Grammes per cubic metre.

Density may be defined as mass per unit volume and may be expressed as:

b) A percentage of the standard surface density - relative density.

3.2 EFFECT OF CHANGES OF PRESSURE ON DENSITY

c) The altitude in the standard atmosphere 10which the observed density corresponds

- density altitude.

As pressure in a container of unit volume is increased, the mass of air will be increased

and therefore the density will rise. Likewise, if the pressure is reduced, the mass of air will

decrease and so will the density

p (rho) = density

We can therefore say that:

DENSITY IS DIRECTLY PROPORTIONAL TO PRESSURE.

In the atmosphere density can be decreased by raising the

volume of air 10 a greater height since we know that

pressure decreases with an increase in altitude. Similarly,

density can be increased by lowering the volume of air to

a lower height.

3.3 EFFECT OF CHANGE OF TEMPERATURE ON

DENSITY

If a volume of air is heated it will expand and the mass of

air contained in unit volume will be less. Thus density

will decrease with an increase in temperarurc and we can

say:

3 - 1

DENSITY IS INVERSELY PROPORTIONAL TO TEMPERATURE.




METEOROLOGY DENSITY

3.4 A SIMPLE MATHEMATICAL TREATMENT

The Fundamental Gas Equation (Boyles + Charles Laws)

says that PV RT (where R = gas constant)

1but p

V

~ RT

andp

RTWhere Pressure

R Gas constant

T Temperature

Density

Note: R for water vapour is 1.6 x that for dry air.

Therefore: p for water vapour is less than for dry air and so p for moist air must be less than p

for dry air

3.5 EFFECT OF CHANGE OF ALTlTUDE ON DENSITY

Although raising and thus expanding the volume of air will decrease its density due to the

reducuon of pressure, at the same time the temperature will decrease and therefore the density

should increase, the one effect cancelling out the other. In fact, there is a greater reduction in

pressure as height increases and the overall effect is for the density to decrease with an

increase of height.

(p = 100% at sea level, 50% at 20,000', 25% at 40,000' and 10% at 60,000')

Density will change by 1% for a 3 degree change in temperature or a 10mb change in pressure.

3.6 EFFECT OF CHANGE OF LATITUDE ON DENSITY

a) at the surface density increases with an increase in latitude.

b) at about 26,000 ft density remains constant with an increase in latitude.

c) above 26,000 ft density decreases with an increase in latitude. (Maximum deviation

from standard occurs at about 50,000 ft.)




METEOROLOGY DENSITY

Figure 3.1. The Effect of Latitude on Density.

Thus aircraft with poor performance at low levels will perform better above the tropopause at

the equator than at the poles.

3.7 EFFECT OF CHANGES IN DENSITY ON AIRCRAFT OPERATIONS

a) Accuracy of aircraft instruments - Mach meters, ASIs.

b) Aircraft and engine performance - low density will reduce lift, increase take off run,

reduce maximum take off weight.

Where L Lift

Coefficient of Lift

Density

v TAS

Wing area




METEOROLOGY DENSITY

Airfields affected would be:

i) High Denver Nairobi Saana

ii) Hot Bahrain Khartoum Singapore

c) Humidity generally has a small effect on density (humidity reduces density), but

must be taken into account at moist tropical airfields, e.g. Bahrain, Singapore.

Figure 3.2. An Illustration of Pressure Decrease with Height in Airmasses with

Different Temperatures and therefore Different Densities




METEOROLOGY DENSITY

Density Questions

Consider the following statements relative to Air Density and select the one which is correct

a) Because air density increases with decrease of temperature, air density must increase

with increase of height in the International Standard Atmosphere (ISA).

b) At any given surface temperature the air density will be greater in anticyclonic

conditions than it will be when the MSL pressure is lower.

c) Air density increases with increase of relative humidity.

d) The effect of change of temperature on the air density is much greaterthan the effect of

change of atmospheric pressure.

2. The tropopause in mid latitudes is:

a) Lower in summer with a lower temperature.

b) Lower in winter with a higher temperature.

c) Lower in summer with a higher temperature.

d) Lower in winter with a lower temperature.

3. Generally as altitude increases:

a) temperature decreases and density increasesb) temperature, pressure and density decreases

c) temperature and pressure increase and density decreases

d) temperature decreases and pressure density increases

4. ln the troposphere:

a) over cold air, the pressure is higher at upper levels than at similar levels over warm air

b) over cold air, the pressure is lower at upper levels than at similar levels over warm air

c) over warm air, the pressure is lower at upper levels than at similar levels over warm air

d) the upper level pressure depends solely on the relative humidity below

5. Density at the surface will be low when:

a) Pressure is high and temperature is high.

b) Pressure is high and temperature is low.

c) Pressure is low and temperature is low.

d) Pressure is low and temperature is high.




METEOROLOGY

ANSWERS

Qucs Answer

I B

2 B

] B

4 B

5 D

3-6

DENSITY

© Oxford Aviation Services limited



CHAPTER FOUR - SYNOPTIC CHARTS

Contents

4. I DEFINITION

4.2 OBSERVATIONS.

4.3 TIMING.

4.4 PLOTTING

4.5 DECODE.

4.6 ANALYSIS

4.7 PROGNOSTIC CHARTS

4.8 EXERCISES

Page

4 - I

4 - I

4-2

...... 4 - 2

.4-3

4-5

.4- 8

.... 4 - 9

© Ox fo rd Av ia ti on Se rv ic es L im it ed



METEOROLOGY SYNOPTIC CHARTS

4. I DEFINITION

Synoptic Meteorology is defined as being concerned with a description of current weather

represented on geographical charts and applied especially to the forecasting of future weather.

4.2 OBSERVAnONS

Weather forecasting has always depended upon accurate observation of rhe weather prevailing

and the availability of that information to all forecasters. Observations made at observing

stations, will be encoded in a universally recognised numerical code (the SYNOP CODE), sent

to a cenrral communication centre (in the UK the National Meteorological Centre (NMC),

Bracknelf) and then re-transmitted to all interested parries in bulletin form.

Figure 4.1. is an example of coded observations from London/Heathrow. You will not berequired to decode such a message, but it is shown for information purposes.

0 . , . . - - , BLOCK NO (UK)

{ : > STATION NO (LHR)

~ CLOUD COVER

. 9 " " WIND VELOCITY (290/15)

VISIBILTYPRESENT WEATHERPAST WEATHER

AMOUNT TYPE & LOW CLOUD ~TYPE OF MEDIUM CLOUD 176'.,..

TYPE OF HIGH CI 01 JD

~ MSL PRESSURE

1777 DRY BULB PRESSURE

DEW POINT TEMP 0 0 9 .

PRESSURE TENDENCY i:>7,j'

RAINFALL ~MAX OR MIN TEMP 17-:;.

AMOUNT TYPE & HEIGHT cI'~

OF LOWEST CLOUD 0 ' " "

AMOUNT TYPE & HEIGHT cl'6'ulOF SECOND CLOUD LAYER ~

Figure 4.1. Heathrow Weather





4.3 TIMING

'Main' observations are made at 0000, 0600,1200 and 1800 UTe: 'intermediate' at 0300,

0900, 1500 and 2100 UTe.

4. 4 PLOTTING

The information fix each observing station is plotted in a standard format of numbers and

symbols around the station on a geographical chart.

Examples of a blank synoptic chart (Figure 4.2) and a station plot (Figure 4.3) are shown:

Figure 4.2. Synoptic Chart.





4.5 DECODE

A full decode of the numbers and symbols follow:

FORM OF HIGH CLOUD

(DensefromCuAnvil)

\

FORM OF MEDIUM CLOUD

AIR (Formed from spreading CuI

~IGHT AND AMOUNT OF MEDIUM CLOUD

---'/ )/ (6/8 at 12,000 ft)

14.( / / MSL PRESSURE (1012.4 mba)

124 / PRESSURE CHANGE IN LAST 3 HRS

I . . . . . (1.5mbs)

15'{+- CH~~~~~~I~;SHANGE

(1.5mb.)

5/15./ .......... ~ FORM OF LOW CLOUD

/ "<, (LargeCu)

AMOUNT OF LOWEST CLOUD (518) HEIGHT OF LOWEST CLOUD

(1,500tt)

Figure 4.4. Station Circle Decode.




: 1 :,~

I; I I I I T !~Ii

~ f:1 1 II ~

j Ii

Figure 4.5 The Station Circle Decode




L !fthere are no symbols in the past weather position then it rneans that observed weather was not

significant.

2. Past weather can have double symbol (W I W2) eg -

.\l

Rain showers in the past 6 hours OR a double precipitation symbol as

distinct from a single symbol:

Rain showers throughout the past 6 hours.

Rain in the past 6 hours.

Rain throughout the past 6 hours (NOTE: not slight continuous rain).

3. lf past weather has a double character but using different symbols e.g.

, 0 * •then the first symbol is the dominant characteristic. Hence the decode for the two examples

above would be respectively:

Rain during the past 6 hours with some drizzle: Snow during the past 6 hours with some rain.

4. Past weather is in the past 6 hours for synoptic times: 0000, 0600,1200,1800 z.

Past weather is in the past 3 hours for synoptic times: 0300, 0900, 1500,2100 z.

Past weather reports for any other times refer to weather in the past hour.

4.6 ANALYSIS

a) Isobars Once the data has been plotted on the chart, the meteorologist will draw in the

isobars, using the plotted values ofQFF, usually for even whole numbers on a chart

ofthis size. Charts covering a greater area, like the North Atlantic, may space the isobars

every four or even eight millibars.

b) Fronts. The positioning of fronts on the chart will require a little marc skill and a

knowledge of the weather changes to be expected at frontal passage. It is common

nowadays for this plotting procedure 10 be completely computerised and the resultingcharts to be despatched by Fax.

Figure 4.6. is an example of a completed (analysed) surface chart.





T,"" __ 1200 _611L __

Figure 4.6. Analysed Chart.




sZ

NN

Ut-

~






4. 8 EXERCISES

We use a number of these synoptic charts in practical exercises in this course and you will need

to be able to deduce the observed weather from the plotted station circles.

A simple exercise using such a chart is appended to this chapter (Chart 85.3). It covers MSL

pressure, pressure tendency and isobar values. More detailed exercises will follow later.

STAnON CIRCLE DECODE EXERCISE (CHART 85/3)

What is the pressure and pressure tendency at the stations listed below and what is the value of

the isobar to the south of each station?

I. 48N 05W

2. SON 06W

3. 56N 04Y2W

4. 4 7 Y 2 N )W

5. 53Y2N 13Y,W

6. 51N 15W

7. 56Y2N 07W

8. 54N lOW

9. 5SV,N 07Y2W



http://slidepdf.com/reader/full/oxford-aviation-jeppesen-meteorology 47/4954 -10 © OxfordAviationServ" Ices Limited



METEOROLOGY

ANSWERS

PRESSURE PRESSURE TENDENCY

1004 1.4 Fal1!Slight rise

1000 0.2 Slight fall/rise

994 0.1 Slight rise/fall

1006 1.2 FaHlSlighl rise

996 0.0 Slight rise/fall

1002 0.8 Fall/Slight rise

992 0.4 Fall/slight rise

990 0.8 Fall

992 0.4 Fall/steady

4 -11

SYNOPTIC CHARTS




CHAPTER FIVE - PRESSURE SYSTEMS

Contents

Page

5.1 INTRODUCTION 5 -

5.2 DEPRESSIONS. 5 - I

5.3 DEPRESSION WEATHER 5-2

5.4 ANTICYCLONES 5-2

5.5 ANTICYCLONIC WEATHER. .5 - 4

5.6 TROUGHS .5 - 5

5.7 TROUGH WEATHER 5-6

5.8 RIDGES -7

5.9 RIDGE WEATHER. .5-

5.10 A RIDGE BETWEEN TWO LOWS .5-8

5.11 COLS. .5 - 8

5.12 COL WEATHER. . . . . . . . . . . . . ........ 5 - 8

5.13 PRESSURE SYSTEMS MOVEMENT. ....... 5- 10

5.14 TERMINOLOGY 5 - II

5.15 BUYS BALLOT'S LAW. 5-11

5.16 PRESSURE GRADIENT. 5 - 12

PRESSURE SYSTEMS QUESTIONS. 5 - 13

© Ox fo rd Av ia ti on Se rv ices L im i ted



METEOROLOGY

5.1 INTRODUCTION

Isobars can form patterns,

which when they are recognized,can help us forecast the weather.

These patterns are called pressure .,,"

distribution systems They

include:

a) Depressions, or lows.

b) Anticyclones, or highs.

c) Troughs.

d) Ridges.

e) Ccls.

f) Secondary depressions

(See Chapter 22)

5.2 DEPRESSIONS

A depression is a region of

comparatively low pressure

shown by more or less circular

and concentric isobars

surrounding the centre, where

pressure is lowest. A depression

is sometimes called a low or acyclone.

PRESSURE SYSTEMS

Figure 5.1. A Depression in the NorthernHemisphere.

~ DIVERGENCE > -:7 )

~

There are two types of

depression, frontal and

non-frontal.

A depression is a region of

converging and rising air as

shown in Figure 5.2. Surface

winds blow anticlockwise arounda low (in the northern

hemisphere) and across the

isobars towards the centre. ,,"L', 1 ' ' - ' ..c...:..L...L.""",;,c_=C£..e...L...L.~.L.CL..L;,.c.J

ASCENT

Fig 5.2. Vertical Cross Section.




METEOROLOGY PRESSURE SYSTEMS

5.3 DEPRESSION WEATHER

Cloud 8/ 8 extending to tropopause and with a low base

Precipitation Can be continuous light to moderate and also heavy showers and

thundcrstonns.

Visibility Poor in precipitation, otherwise good due to ascending air.

Temperature Mild.

Winds Winds arc usually strong- the deeper the depression and the closer the isobars,

the stronger the wind.

5.4 ANTICYCLONES

An anticyclone or high is a region of relatively high pressure shown by more or less circular

isobars similar to a depression but with higher pressure at the centre.

Isobars are more widely spaced than with depressions. There are three types of anticyclone,

warm, cold and temporary cold. They are regions of diverging and descending air. Surface

winds blow clockwise in the N0I1hem Hemisphere and across the isobars away from the centre.

Figure 5.3 An Anticyclone in the Northern

Hemisphere.




METEOROLOGY

Warm Anticyclones

Wann anticyclones are caused by

an excess of air at high level. Thedescending air will be heated by

compression and surface

temperatures wil l rise as a result.

Warm anticyclones normally

occur in lower latitudes.

Cold Anticyclones

These are caused by high density

and low surface temperatures.

As a result, cold anticyclones

occur in Polar and high latitudes

and are more seasonal (Winter)

than warm anticyclones.

Temporary Cold Anticyclones

A temporary cold anticyclone is

produced in the cold air between 50 N

depressions on the polar front.

When eventually the cold air

terminates the series of lows, the

cold anticyclone may be of some

size though not of great depth.

Over the sea, and over the land in

Summer, such an anticyclone will

last only a few days to be replaced

by the subsequent polar frontal

depression.

PRESSURE SYSTEMS

Figure 5.4. Vertical Cross Section.

- ,!

Figure 5.5 A Temporary Cold Anticyclone.





Blocking Anticyclones

Warm anticyclones, which are often an extension of high pressure areas developed in the sub-

tropical regions, may hold up or divert the normal west-east passage of polar front depressions

and persist for several days. The diagram shows how the usual west-east flow becomes morenorth-south, or meridional as the effect of the extension of the Azores High affects the air flow.

There is a decided tendency for blocking highs to persist in certain geographic areas such as 10

to 20W over the North Atlantic. The air within the systems is subsiding down from high levels

and this means that extensive sheets of Stratus or Strata cumulus may develop but there will be

little vertical extent. It is worth noting that a warm anticyclone, in the South, may join up with

a cold anticyclone from the North to create this meridional flow.

Figure 5.3a. High from Azores to Scandinavia.

5.5 ANTICYCLONIC WEATHER

Cloud None except on the edge of the anticyclone.

Precipitation None.

Visibility Generally poorer than with a depression. Autumn/Winter - fog early morning

and night. Summer - haze is possible, otherwise good.

Temperature Dependent on type.

Winds Light.





5.6 TROUGHS

Troughs of [ow pressure are indicated by isobars extending outwards from an area of low

pressure so that the pressure is lower in the trough than on either side.

Figure 5.6. A Trough of Low Pressure.





5.7 TROUGH WEATHER

Cloud Non-frontal: Great vertical development of cloud - CU and CB.

frontal: The cloud will depend on whether cold air is overtaking

warm, when the cloud tends to be as above, or ifwann air is overtaking

cold, in which case the cloud is likely to have much Jess vertical

development.

Precipitation Showers, thunderstorms, hail, with non frontal orcold front; continuous

drizzle, light or moderate rain with warm frontal trough.

Vlsiblllty Fair except in showers, though at a warm frontal trough visibility willbe poor in continuous rain.

Winds Moderate with gusts and squalls.

@CrownCopyright

Figure 5.7. A Frontal Trough Extending from the North





5.8 RIDGES

Ridges of high pressure are indicated by isobars extending outwards from an anticycloneand always rounded, never V-shaped as seen in a trough. They arc also sometimes referred to

as 'wedges'.

Figure 5.8. A Ridge of High Pressure.




5.9 RIDGE WEATHER

Ridge weather is similar to anticyclonic weather.

5.10 A RIDGE BETWEEN TWO LOWS

A ridge often brings a period of good weather between two depressions

5.11 eOLS

Cols are regions of almost level pressure between two highs and two lows. It is an area of

stagnation. This is illustrated in Figure 5.9.

5.12 eOL WEATHER

Col weather is normally settled, but is dependent on changing pressure.

In autumn and winter co Is produce poor visibility and fog, whilst in summer thunderstorms are

common. Figure 5.10 is an example of a weather forecast for a day when a col influenced the

weather over the U.K.





GENERAL SITUATION: Eastern counties of Englal1d and Scotland

will be cloudy and misty at first with some showers.

coast il sllould brighten up, although there is still a chan~y~. The rest ctthe UK will become very

?£Od spells of sunshine. although a scattering of

~~areexpectedfrom late-.-momlngonwards

CHAN ISLES, LONDON. SE ENGLAND. CENT S ENG·LAND, SW

~ ~ ~ ; : ~ , D ~ c r e : : ~~ ~ V : f 1 : : o ~ ~ tU I ~ ; ~ i : da~;;~~wind. Max 73-79f. {23-26c}

MIDLANDS, CENT N ENGLAND, NW ENGLAND, WALES, LAKE

OIST, IOM~N IRELAND: Warm and humid with sunny spells. A40%

chartceof~ryshowers_ Lighlal1dv8nablewinds. Max 73-79f.

(23-26c)

~:v~~~~.P~I~~~~~ ~~~~~nf~ ~~t~I~~~!~U~a~I~~~~~

oullater. Alight north-easterly wind. Max 66-72f. (19-22c)

NE ENGLAND, SE SCOTLAND, EDINBURGH, DUNDEE,

CENTRAL HIGHLANDS, ABERDEEN, MDRAY FIRTH, NE

SCOTLAND: Showers at f irst. SUnr 'iy spells and~aw"yfrom

the coastlater oo. Alight,~ariablewir'ld. MaK64-7Of. (23-26c)

SW SCOTLAND, NW SCOTLAND_._GLASGDW, ARGYLL: Warm L..!!!!!!!!~~~_lf..:~::.J~"'!:~::.Jsunshine end a growing risk of ~~_ A light south to IIlodt drdu: r~ffJj in·C rFin br(Kk~ts). ArfI)WJ:: whisouth-easterlywind. Max 70-751. (21-24c) $pffdinmph. hUl.Vr~1rrmilJlbtvs(incJ.ntnbt~lI)

ORKNEY, SHETLAND: ~y__!gg and low cloud. A light south- ~!!!I!!asterly wind. Max61c(16f)

S_J±_DRTH S~ht north-easterly wind. 'Showers_ Visibility'

bioderate or cssr with !QgJ.'!atches. Slight seas

DOVER STRAIT, ENGLISH CHANNEL, ST GEORGE'S CHAN,

~~~~. S~7ti%~~r n~i~~i~~s~:;~~~. ~~a!~:te~~~c~:r rv-..../ '~._.o-

Slightsaas.

OUTLOOK: Unsettled in the north-west; a good deal of fioe. warm I~~~~~~f-}~~~r,~weather in the south--east tPOLLEN: A moderate count in the south, but high in the north end

west away from coasts

(Pollen forecast from the National Asthma Campaign.)

"/gil Ii 1 4 ' 1 / 1 drift nul hul mglrs K < 7 " " 0 #Ie J lQw _vr'l'Ig.

lAw V i$ utmost SW-fjOlf(lry '!'ltl lA w X wifJ d e _ < t e w p atif riml qNiCkly t M l ' K - a r d s - l..(. ....W i t ltiJ fd ly m o 'l lll1 l

Figure 5.10. Col Weather.





5.13 PRESSURE SYSTEMS MOVEMENT

Weather patterns (pressure systems) vary across the globe. They are mobile in high latitudes

while slow moving in equatorial latitudes. Patterns of isobars which indicate weather will retain(heir general shape while moving, but change their numerical value.

Movement of the systems is the key to accurate forecasting.

The following figures show the movement of weather over a period of four successive days.

Figure 5.11. Maintenance of Shape.




METEOROLOGY

5.14 TERMINOLOGY

Depressions will fill up or decay as pressure rises.

Depressions will deepen as pressure falls.

PRESSURE SYSTEMS

Depressions move rapidly. their average lifetime is 14 days.

Anticyclones will build up as pressure rises

Anticyclones will weaken or collapse as pressure falls.

Anticyclones are very slow moving, they can last for a lengthy period, up to 6 months.

Cols last a few days only and are then absorbed into other systems.

5.15 BUYS BALLOT'S LAW

Changes of shape and intensity are slight intropical regions where pressure is generally low, but

in temperate and polar latitudes changes are much more varied and rapid.

Buys Ballot's Law states that..

In the 19

th

century the Dutch meteorologist Buys Ballot produced a law based on the observationof wind direction and pressure systems.

[f an observer stands with his back to the

wind, the [ower pressure is on his left in

the northern hemisphere, and on his right

in the southern hemisphere.

A corollary of this law is that if you are

experiencing starboard drift in the

northern hemisphere you are heading

towards [ow pressure. This is illustrated

in Figure 5.12.

~

I ~ I I L O W : _ ~ , .S~UREn lf i•• ~: ~!

:J:, "i

-r /~/6...~ . +~~==£IGH PRESSURE

Figure 5.12. A Corollary of Buys Ballot's Law.




METEOROLOGY

5.16 PRESSURE GRADIENT

The pressure gradient is the difference in

pressure between consecutive isobarsdivided by the distance between them, this

is illustrated in Figure 5.13.

Note.

The greater the pressure change fOT a

given distance the faster the wind velocity

Air tries to move from high to low

pressure and this will generate a pressure

gradient force which develops into the

wind velocity that we feel. This will be

discussed in full in chapter 11.

PRESSURE SYSTEMS

Figure 5.13. Pressure Gradient.

rr_."rBGr~dl .. ' fr.mAto 0 102mb. 'n'lII mllH. orO.D2mblml

P,•• u,. Gradi . , fromc,. 0 IoZmb. '"~O", ...... ,a."'rillmI

Figure 5.14. Why Speed Depends on

Gradient.





Pressure Systems Questions

A trough of low pressure is generally associated with:

a) convergence causing increased cloud and precipitation

b) divergence causing increased cloud and precipitation

c) subsidence causing increased cloud and precipitation

d) subsidence causing decreased cloud a':ld precipitation

2. A ridge of high pressure is generally associated with:

a) convergence causing increased cloud and precipitation

b) divergence causing increased cloud and precipitationc) divergence causing cloud to break up and rnore precipitation

d) divergence and subsidence causing clear skies and good weather

3. A small low established within the circulation of another [ow is called

a) a trough

b) a col

c) an anticyclone

d) a secondary depression

4. An area ofindetenninate pressure between two lows and two highs is called:

a) a trough

b) a ridge

c) a col

d) a saddle

5. A trough of low pressure is:

a) a small low established within the circulation of another low

b) an extension or elongation ofa low pressure system alongan axis on each side of which

pressure Increases

c) a centre of pressure surrounded on all sides by higher pressure

d) an area where the pressure is lower than anywhere else in the area





6. If in the southern hemisphere an aircraft in flight at 2000 ft is experiencing starboard drift, the

aircraft is flying towards:

a) an area of high pressureb) an area of low pressure

c) a warm front

d) a depression

7. ln the Southern Hemisphere, the surface winds at Bl; and C2 would be respectively:

a) clockwise across the isobars away from the centre: and anti-clockwise across the isobars

towards the centre.

b) Anti-clockwise across the isobars towards the centre: and clockwise across the isobarsaway from the centre.

c) Anti-clockwise across the isobars away from the centre: and clockwise across the

isobars towards the centre.

d) Clockwise across the isobars towards the centre: and Anti-clockwise across the isobars

away from the centre.

8. Subsidence in an anticyclone produces:

a) saturated air and an inversionb) dry air and an inversion

c) isothermal dry and stable air

d) increased pressure at the surface

9. With an anticyclone over the UK the expected weather is:

a) Thunderstorms in summer, fog in winter.

b) Stratus in summer with drizzle, CU and snow in winter.

c) Clem skies or fair weather CU in summer, fog in winter

d) Clear skies in summer with haze, cold frontal weather in winter.

Refer to appendix A and answer questions IOta 14

10. The pressure systems at A2; 8 I; 82; 83; and C2 are respectively:

a) Depression; Anticyclone; Col; Ridge; and Trough.

b) Ridge: Anticyclone; Col; Trough; and Depression.

c) Trough; Depression; Col; Ridge; and Anticyclone.

d) Ridge: Depression; Col; Trough; and Anticyclone.




METEOROLOGY

11. Two important weather factors at 82 will be:

a) Frontal weather in winter, fog in summer,

b) Clear conditions in summer, thunderstorms in winter.c) Thunderstorms in summer, fog in winter.

d) Fog in summer, thunderstorms in winter.

12. Haze in summer and radiation fog in winter can be expected at:

a) C2

b) 83

c) B!

d) 82

13. In the non-frontal pressure system at B3, the expected weather

a) ST SC with drizzle or light precipitation.

b) Clear skies with moderate winds.

e) CU CB with showers.

d) Light winds and haze with an inversion

A c

5 -15

PRESSURE SYSTEMS

2

3




METEOROLOGY

ANSWERS


I A 8 D

2 D 9 C

3 D 10 D

4 C II C

5 8 12 A

6 A 13 C

7 D

5 -16

PRESSURE SYSTEMS




CHAPTER SIX - ALTIMETRY

Contents

Page

6.1 THE ALTIMETER .6- I

6.2 ALTIMETER SETTTNGS . .6-3

6.3 TERM INOLOGY 6 - 5

6.4 ALTIMETER ERRORS 6 - 5

ALTIMETRY QUESTIONS ... 6 -7

6.5 TERRAIN CLEARANCE. 6-

6.6 MINIMUM FLIGHT LEVEL .6-8

6.7 TRANSITION ALTITUDE. ....... .... 6-9

6.8 TRANSITION LEVEL 6-9

6.9 TRANSITION LAYER. . .. 6 - 9

ALTIMETRY QUESTIONS. 6 - 1 1

© O xfo rd A viatio n Services Limited



METEOROLOGY

6. I THE ALTIMETER

ALTIMETRY

An altimeter is an instrument which measures pressure and causes a needle to move across a dial.

The dial is calibrated in feet rather than pressure as we know that pressure decreases as altitudeincreases.

The instrument is

calibrated in accordance

with the ICAO

International Standard

Atmosphere so that all

altimeters will read the

same altitude for the samepressure. (See previous

notes on the need for the

ISA).

Inaddition, altimeters have

a means of adjusting the

needle setting to take

changes in the surface

atmospheric pressure intoaccount.

Figure 6.1. shows how the

altimeter reading will

change with a change in

pressure.

In Figure 6.2. section A,

the pressure at the

airfield, which is at sea

level, is 1010 mb. The

altimeter reads zcro feet.

In section B, the pressure

at the airfield has fallen to

1000 mb and t he

altimeter, rather than

showing a decrease in

pressure, shows

increase in height.

Figure 6.1. A Simple Altimeter.

A

B

~MSl

Figure 6.2. The Altimeter Responding to Changes in

Pressure.




METEOROLOGY ALTIMETRY

a) When flying at a constant

indicated altitude, outside

air pressure must remain the

same. To achieve this wemust fly along a pressure

level. However, when we

fly to an area of lower

pressure, these pressure

lines will dip, consequently

our true altitude will

decrease. Conversely when

flying into a region of

higher pressure, the pressurelines will rise and our true

altitude will increase.

HIGH TO LOW , L OOK 0l1T BElOWI

Figure 6.3

HIGHER PRESSURE; TRUE ALTITUDE> INDICATED ALTITUDE

LOWER PRESSURE; TRVE AL TITVDE < INDICATED ALT1TVDE

b) Varying temperatures within

the atmosphere havesignificant effects on the

pressure and the shape of the

pressure lines. Cold air will

tend to compact and lower

pressure lines whilst warm air

will expand and raise

pressure lines. Using Figure

6.4 you can see that when

flying to a colder area at a

constant indicated altitude

your true altitude decreases.

Conversely, when flying into

warmer region your true

altitude will increase.

Figure 6.4

COLDER THAN ISA; TRVE ALTITUDE < INDICATED AL T1TVDE

WARMER THAN ISA; TRUE ALTITUDE> INDICATED ALTITUDE

c) There is a need to be able to reset the altimeter to take account of the fall in pressure.

Consequently, if the altimeter is reset when the pressure changes, the altimeter will read

correctly. We may, by altering the altimeter subscale setting, set QFE, QNH or SPS for use

when we fly to ensure more accurate readings.





6.2 ALTIMETER SETTINGS

QFE Airfield pressure, With this pressure set on the altimeter, the instrument will

read zero on the ground, or the height of the aircraft above the airfield

Figure 6.5. Airfield Pressure - QFE,

QNH

This is the airfield pressure converted to MSL in accordance with the ICAO

(SA. The altimeter will then read the height ofthe airfield above MSL, or the

aircraft's height AMSL.

Figure 6.6. Mean Sea Level Pressure - QNH.

Forecast QNH

The lowest forecast QNH within an area. forecast for one hour ahead. The

altimeter will be in error, but as the setting is the lowest forecast, the actual

pressure will always be higher, or at least equal to the forecast QNH, and the

altimeter will read low (or safe) or the correct altitude.





Figure 6.7. Altimeter Setting Regions.

FO UK 70 EGRR 1100600

FOQNH

VALIDITY PERIOD 0070K 01992 02995 03003 04007 05001 RUilON NUMBER

07011 08011 09011 10014 110[4 12019

13020 14015 15017 16987 17998 189K9 R.P_S

19998 20004 21981 22987 23001 24011

25014

Note: The Cotswold area where Kidlington is situated is No.1S on the above decode table.





SPS (Standard Pressure Setting) If the standard pressure of 1013 mb is set on the

altimeter, the instrument will read what is known as pressure altitude height

in the Standard Atmosphere. This is the altimeter setting used when flying

above the transition altitude.

6.3 TERMINOLOGY

Altitude Vertical distance above mean sea level.

Height Vertical distance of a level or point measured from a specific datum, e.g.

height above a surface.

Elevation Height when the datum is MSL.

Flight Level Surface of constant atmospheric pressure measured from the 1013.25 datum

used for vertical separation by specified pressure intervals (usually 500 or 1,000

ft). Flight Level is measured in hundreds of feet.

e.g., FL 350 ~ 35,000 FT.

Figure 6.9 Altimetry Terminology.

6. 4 ALTIMETER ERRORS

Apart from instrument errors, there are two errors of interest meteorologically. They are:

a) Barometric Error - Errors caused by setting a pressure on the subscale other than the

correct one.





INDICATED HEIGHT 4.000 FT

TRUE HEIGHT 3,850 FT

SUBSCALE

SETTING

1010

TRUE MSL PRESSURE 1005 mb

Figure 6.10. Barometric Error.

b) Temperature error - The altimeter is calibrated in accordance with the ICAO [SA. If

the temperature is other than that in the lSA, the altimeter will be in error. Corrected

altitude is calculated by using a navigational computer, or a correction table. HI-LO-HI

will still apply. An example ofa temperature error correction is shown:

ALTIMETER TEMPERATURE ERROR CORRECTION

a) Pressure altimeters are calibrated to indicate true altitude under ISA conditions. Any

deviation from ISA will result in erroneous readings.

b) When temperatures are less than ISA an aircraft will be lower than the altimeter

reading.

c) The error is proportional to the difference between actual and lSA temperature, and the

vertical distance or the aircraft above the altimeter setting datum, i.e. height above

touchdown. The error is approximately 4 ft/I OOO tt for each °C of difference.

d) To ensure adequate obstacle clearance on approach add figure in body of table to

calculated DH/MDH.





ISA TEMP HEIGHT ABOVE TOUCHDOWN OR HEIGHT ABOVE AERODROME IN

DEVIATION FEET

'C200 300 400 500 600 700 800 900 1000

-15 12 18 24 30 36 42 48 54 60

-25 20 30 40 50 60 70 80 90 100

-35 28 42 56 70 84 98 112 126 140

-45 36 54 72 90 108 126 144 162 180

-55 44 66 88 110 132 154 176 198 220

-65 52 78 104 130 156182

208234 260

QUESTIONS ON ALTIMETRY. For all of the following questions assume that 1mb=27ft.

An aircraft is at an airfield with an elevation of 350 fl. The altimeter setting is 1002, but the

actual QNH is 993. What is the altimeter reading? Assume that I mb = 27ft.

2. An aircraft is on an airfield, elevation 190 ft and has an altimeter reading or70 fI with a

setting of 1005. What is the actual QNH?

3. What is the altimeter reading if the setting is 978, rho QNH 993 and the airfield elevation

770ft?

4. The regional pressure setting is 1012, the altimeter setting is 1022 and the indicated altitude

is 4100 ft. Ahead is some high ground shown on the map as being at 3700 ft. Willthe

aircraft clear the high ground, and if so, by how much?







Altimetry Questions

An aircraft is flying at 3000 feet indicated with the altimeter sub scale set to 1020 rub

towards a mountain range with an elevation of 1600 feet. Ifduring theflight

the QNH in thearea falls to 989 mb and the altimeter sub scale is not reset, the expected clearance over the

mountain range will be: (assume 27 feet = I rub)

a) 1400 ft

b) 470 F t

c) 930 ft

d) 563 n

2. When flying towards a depression at a constant indicated altitude, the true altitude will be:

a) Lower than indicated.

b) Higher than indicated.

c) The same as indicated.

d) Lower than indicated at first then the same as indicated later.

3. The name given to the lowest forecast mean sea level pressure in an area is:

a) QFE

b) Regional QNH

c) QFF

d) QNE

4. The Altimeter will always read

a) With 1013setthealtitudeaboveMSL

b) With airfield QNH set the height above the airfield datum

c) The vertical distance above the pressure level set

d) the correct flight level with regional QFE set.

5. An aircraft at airfield P elevation 270 ft has the airfield QNH 1012 mbs correctly set. The

altimeter setting is not changed. Later on landing at airfield Q elevation 450 ft the aircraft

altimeter reads 531 fl. What is the correct QNH at airfield Q? (Assume 27 ft = 1mb)

a) 1014.7 mbs

b) 1009.3 mbs

c) 1015mbs

d) 1009 mbs





6. The altimeter subscale is set to 1030 mbs and the altimeter reads 4500'. QNH is 996 mbs

What is the altitude of the aircraft? (Assume I mb = 27')

a) 3480'b) 3990'

c) 5418'

d) 3582'

7. An aircraft flies over high ground 4730 metres' above rnsl. The track is 1400M and the QNH

995 mbs. The required clearance is a minimum of 1500' What is the minimum flight level

in cloud? (Assume I mb=27')

a) 175

b) 195

c) 190

d) 215

An aircraft, flying at FL 100 at a constant RAS, flies from an area ofwann air into an area of

cold air. The QNH is unchanged. How has the aircraft altitude and TAS changed?

Altitude TAS

a) decreased increased

b) Increased increased

c) decreased decreased

d) Increased decreased

An aircraft flies on a track of356°M over high ground which rises to 4693 metres above msl.

Drift is 10° Port and the regional QNH 993 mbs. The aircraft is required to clear this high

ground by 1500'. What is the minimum quadrantal rules flight level? (Assume [ mb=27')

a) FL 210

b) FL 205

c) FL 190

d) FL 185

[0 QNH at Johannesburg is [025 hPa, elevation is [600m amsl. What is the QFE. (Assume I

mb=8tn)

a) IOOO.8'Pa

b) 830.6 hPa

c) 1002hPa

d) 825 hPa





11. When flying from Paris (QNH 1012) to London (QNH 1015) at FL 100. You neglect to reset

your altimeter but why does your true altitude remain the same throughout the flight.

a) Paris has a higher pressure than Londonb) The air at London is warmer than Paris

c) London is at a lower altitude than Paris

d) The air at Paris is warmer than London

12. An airfield in Holland is 20m below sea level, QFF is 1020 hPa, temperature at the surface is

+30"C. What is the value ofQNH.

a) impossible to determine

b) Less than 1020 hpa

c) Same as QFF

d) More than 1020 hPa




METEOROLOGY

ANSWERS

Ques Answers

I 0

2 A

3 8

4 C

5 0

6 0

7 B

8 C

9 0

10 0

II 0

12 B

6 -14

ALTIMETRY




CHAPTER SEVEN - TEMPERATURE

Contents

Page

7.1 INTRODUCTION .7- I

7.2 MEASUREMENT 7 - I

7.3 INSTRUMENTS. 7-2

7.4 HEATING OF THE ATMOSPHERE 7-4

7.5 TEMPERATURE VARIATION WITH HEIGHT 7 -7

7.6 LAPSE RATE 7-7

7.7 INVERSIONS .7-7

7.8 SURF ACE TEMPERATURE. .7 - 8

TEMPERATURE QUESTIONS 7 -18




METEOROLOGY TEMPERATURE

7.1 INTRODUCTION

One of the important variables in the atmosphere is temperature. The study of temperature

variation. both horizontally and vertically has considerable significance in the study ofmeteorology.

7.2 MEASUREMENT

There arc three scales which may be used to measure temperature though only Celsius and

Kelvin arc used in meteorology. The ligures show the melting point oficc and the boiling point

ofwater (nr STP) in each scale.

a) The FAHRENHEIT scale: +3210 +2 12degrees.

b) The CELSIUS (or Centigrade) scale: 0 10 + 100 degrees.

c) The KELVIN (or Absolute) scale: +273 to +373 degrees.

Conversion factors:

(.56)

(1.8)

K = °CI 273




METEOROLOGY

7.3 INSTRUMENTS

The standard means of measurement on

the ground is a mercury thermometerplaced in a Stevenson Screen. Electrical

resistence thermometers may be used

where the Screen is nor readily accessible

to the observer.

TEMPERATURE

Figure 7.1. The Stevenson Screen.

A Thermograph (similar in its output to a Barograph) will also be found inside the screen. The

Stevenson Screen is a louvred box 4 feet (1.22m) above the ground. This screen, shown in

Figure 7.1, is used worldwide.

Figure 7.2. Thermograph.





Upper air temperatures (Ire taken using a Radiosonde, shown in Figure 7.5, - a device

transmitting continuous readings of temperature, pressure and humidity whilst being carried

aloft beneath a balloon. Rate of climb is 1200 fpm and maximum ceiling between 65,000 and

115,000 n.

,

•

BALLOON

RADAR

REFLECTOR

RADIOSONDE

Figure 7.3. A Radiosonde.

GROUND RADAR

•t , \ ,

~

Aircraft readings, though often the only way in which atmospheric temperature may be measured

over the oceans and other areas far away from meteorological stations, are notus accurate as they

arc affected by compressibility and lag. The electrical thermometer will give a digital readout

oftcmpcruturc and this can be automatically calibrated and transmitted on some modem aircraft.

" ' _ " ' ~ - - = - ,~ ,..,~ ~, ~ . . _ ~~. - - . . . . .

t;t-----_;___.,~ 'C

- - > . /

/

Figure 7.4. Electrical Thermometer




METEOROLOGY

7.4 HEATING OF THEATMOSPHERE

TEMPERATURE

The atmosphere is heated by 5 different processes:

a) Solar Radiation. Radiation [TOm the sun is of Short wave-length ( / 0 . . ) and passes

through the atmosphere almost without heating it at all.

A~ 0.151Q 4 microns (micron

Some solar radiation is

reflected back to the

upper air from cloudtops and from water

surfaces on the earth.

The rest of this radiation

heals the earths surface.

The process whereby the

surface is heated by solar

radiation is called

insolation

b) Terrestrial Radiation.

106m)

microns, peaking < I t 10

The earth radiates heat at all times. It is relatively long wave radiation A"" 4 to 80

Figure 7.5, Solar Radiation.

lt is absorbed and then

retransmitted us heat by

the water vapour and

C02 in the

atmosphere. This

retransmission of heat to

the surrounding air is

the main method by

which the atmosphere is

heated and explains why

the atmosphere reduces

in temperature with an

increase in height. It is

heated from below -

hence there is a lapse

rate.Figure 7.6. Terrestrial Radiation.





c) Conduction. Air lying in contact with the earths surface by day will be heated by

conduction. AI night air in contact with the earths surface will be cooled by

conduction. Because of the air's poor conductivity, the air at a higher level will remain

at the same temperature as during the day and an inversion will result.

Figure 7.7. Conduction.

d) Convection. Air heated by conduction will be less dense and will therefore risco This

will produce up currents called thermals or convection currents. These will takethe warm air to the upper levels, thus helping to heat the upper atmosphere.

Figure 7.B. Convection Currents.





e) Condensation. As the air is lifted it will cool by adiabatic process and the water

vapour in the air will condense out as visible droplets forming cloud. As this occurs

latent heat will be released by the water vapour and this will heat the atmosphere.

WATER VAPOUR RISES

t t ~ t t

Figure 7.9 Latent Heat being released through

Condensation.

Figure 7.10. Heat Processes in the Atmosphere.





7.5 TEMPERATURE VARIA TlON WITH HEIGHT

We have seen thai although our source of

heat is the sun, because of the

atmosphere's virtual transparency to

insolation, it is in fact heated (by long

wave TR) from the surface upwards.

TEMPERATURE

Thus as we move further and further from

the surface we would expect the heating

effects to diminish.

Figure 7.11. Temperature Variation with

Height

7. 6 LAPSE RATE

The rate at which temperature falls with an increase in height is called the Lapse Rate. An ideal

uniform atmosphere would show a constant lapse rate rather like the ISA, which is 1.98°C (2°)

per 1000ft .

7.7 ISOTHERM

If temperature remains constant with height it is called an isothermal layer.

7.8 INVERSIONS

Where the temperature increases with an increase in height, then we have what is called an

inversion. We have already seen that at night we can expect an inversion above the surface, but

this can occur in many different ways.

Radiation, on a night of clear skies, will also result in a temperature inversion above the surface.

This is called a Radiation Inversion.

When we look at cloud formation, we shall see that because of turbulence in the layer closest

to the surface we can have an inversion at a height of 2 or 3 thousand feet.

Quite often, at the tropopause instead of the temp. remaining constant, it may show a slight rise

for a few thousand feet.

At the higher levels of the stratosphere, temp. will show an increase with height (in ISA from

65,617ft temperature increases at a rate of 0.3°/1 OOOft).





In.a high pressure system, air descends at the centre. As the air descends it will be heated

adiabatically (more of this later) and will be warmer than the air at a lower level. This is called

a Subsidence Inversion.

l-IomI

TEMPERATURE

7.9 SURFACE TEMPERATURE

Figure 7.12. Inversions.

The surface air temperature measured in a Stevenson Screen is subject to considerable

variations: Latitude Effect. Seasonal Effect, Diurnal Variation and multiple effects due to cloud

and wind.

a) The angular elevation of the sun.

i) Latitude Effect. At the equator

only a small area is affected by

the suns rays and therefore will

be subject to the greatest

heat/unit area. At the poles the

SUIlS fays will cover a larger area

and there will be the least

heat/unit area. The actualdistance of polar regions from the

sun is only fractionally more than

that from the equator, and the

effect may be ignored. Figure 7.13. The Effect of Latitude.

lOW LATITUDE

SMALL AREA





ii) . Seasonal Effect. On thep~(~~~1;~12

and 23 Septe~~?'~;J\I~~:,y~rnalnd

Autumnal Equinoxes) the sun is

directly overhead the equator andmaximum heating occurs. On 21

June" ;tb~_ sun:i~. overhead the

T J j p i c "'0'[ C ~ r i c - e rand maximum

heating will occur there. 111the

Northern hemisphere the

temperature will increase as the

sun moves north and decrease as it

moves South, reaching minimum

about 23 December

Figure 7.14. The Seasonal Effect.

b) TimeofDay(Diurnal Variation).

i) The sun is at its highest elevation at noon, but for two to three hours after this time,

the earth is receiving more solar radiation than it is giving up as terrestrial radiation

(Thermal Inertia). As a result temperature is highest at about 15:00 (Tmax).

ii) From 15:00 onwards, the temperature falls continuously until a little after sunrise. The

lowest temperature occurs at about 0500 (T min) C.

iii) Diurnal Variation is greatest with clear skies and little wind. DV varies with a number

offactors, but in temperate latitudes is about ± 6 degrees about the mean.

JUST AFTER

SUNRISEMINIMUM

TEMPERATURE

, - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ,: '" ....... DIURNAL VARIATION IN I

, -,.-.,-.,-, -' -,,- CLEAR.CALM CONDITION~

! . . . _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ,[ " , - , , - , , - , , - , - , , - , , - ' - " O i U ' R ' N A - L " V A ' R I ' A T i c i N - w i T H :, -- .. CLOUD COVER OR STRON"

L .. _ ,_ ,, _ ._ . . _ . ._ ._ . ._ _ .~ .I~ .I?._._.._. ._.._._._.._._.._.j

0000 1200 1800 2400

Figure 7.15. Diurnal Variation.





some ofthe solar

1. Cloud cover by

day. By day

radiation isreflected back by

the cloud tops

and T Max is

reduced.

Figure 7.16. Cloud Cover by Day

Figure 7.17. Cloud Cover by Night.

2. Cloud cover by night. By night terrestrial radiation is absorbed and radiated back to

the earth's surface from the clouds. T min is increased.

Note.

The so called greenhouse effect has a similar affect upon temperature as that of cloud

cover by night but is generated differently in that long wave radiation f r 0 1 1 1 the Earth

heats up the large quantities of carbon dioxide trapped in the lower levels of the

atmosphere. This process continues day or night and is said to be leading to an overall

increase in atmospheric temperature.







In summary. wind on cloud cover will cause T max to be reduced and T min 10be increased.

Therefore DV will be reduced

5. DV over sea. As the Specific Heat (SH) of water is unity, compared to othersubstances whose SH is much less, and as the temperature rise is inversely

proportional to the Specific Heat, the temperature rise and fall over the sea is srnall,

generally less than 1°C.

c) Nature of the Surface.

i) Sea. The sea takes a long time to heat (and cool) and as we have seen has a

very small DV.

The difference in DV values between land and sea is the cause of sea breezes,

The minimal DV of sea temperature is the reason why the most common form

of fog, radiation fog, never fonns over the sea.

When the angular elevation of the sun is low, much solar radiation is reflected

back to the atmosphere.

Figure 7.20. Diurnal Variation Over the Sea.




~1 §ill

~f-

'5ill

g >~»,

;: ?~c I'-r oill

:;;

N

, . . :

~CO

u :








e) Origin of air supply. Air tends to retain its temperature and humidity for a

considerable time, therefore air from high latitudes will bring lower temperatures to

UK for example. A southerly wind, however, will normally provide an increase in

temperature.

Figure 7.25 Origin of Air Supply





Temperature Questions

1. The measurement of surface rcrnperature is made:

a) at ground level

b) at approximately 10 metres from ground level

c) at approximately 4 feet above ground level

d) at approximately 4 metres above ground level

2. The purpose of a "Stevenson Screen" is to:

a) maintain a moist atmosphere so that the wei bulb thermometer can function correctly

b) to prevent the mercury freezing in the [ow winter temperatures

c) protect the thermometer from wind, weather and from direct sunshine

d) keep the wet and dry bulb thermometers away from surface extremes of temperature

lf rernperature remains constant with an increase in altitude there- is:

a) an inversion

b) an inversion aloft

c) uniformlapse rule

d) an isothermal layer

4. The surface of the earth is heated by:

a) convection

b) conduction

c) long wave solar radiation

d) short wave solar radiation

5. Cloud cover will reduce diurnal variation of temperature because:

a) incoming solar radiation is reflected back to space and outgoing terrestrial radiation is

reflected back to earth

b) incoming solar radiation is re-radiated back to space and atmospheric heating ·by

convection will stop at the level of the cloud layer

c) the cloud stops the suns rays getting through to the earth and also reduces outgoing

conduction

d) incoming solar radiation is reflected back to space and outgoing terrestrial radiation is

re-radiated from the cloud layer baek to the surface





6. Diurnal variation ofthe surface temperature will:

a) be unaffected by a change of wind speed

b) decrease as wind speed increasesc) increase as wind speed increases

d) be at a minimum in calm conditions

Which of the following surfaces is likely to produce a higher than average diurnal variation of

temperature:

a) rock or concrete

b) water

c) SIlOW

d) vegetation

8. Most accurate temperatures above ground level are obtained by:

a) tephigram

b) aircraft reports

c) temperature probe

d) radio sonde

The method by which energy is transferred from one body to another by contact is called:

a] radiation

b) convection

c) conduction

d) latent hea t

[0. The diurnal variation of temperature is:

a) greater over the sea than overland

b) less over desert areas then over temperate grassland

c) reduced anywhere by the presence of cloud

d) increased anywhere as wind speed increases

1 1 The troposphere is heated largely by:

a) absorption of the sun's short wave radiation

b) radiation of heat from cloud tops and the earth's surface

c) absorption by ozone of the sun's short wave radiation

d) conduction from the surface, convection and the release of latent heat





12 An inversion is one in which:

a) there is no horizontal gradient of temperature

b) there is no change of temperature with height

c) there is an increase of temperature as height increases

d) there is a decrease of temperature as height increases

13. The sun gives OU\ amount of energy with wavelengths.

The earth gives out relatively amounts of energy with reiatively _

wavelengths:

a) Large, large, small, small.

b) Small, small, large. large.

c) Large, large, small, large.

d) Large, small, small, large.

14. With a clear night sky, the temperature change with height by early morning is mostlikely to

show:

a) A steady lapse rare averaging 2 C per 1000 ft.

b) A stable lapse rate o r I C per 1000 ft.c) An inversion above the surface with an isothermal layer above.

d) An inversion [rom Ileal' the surface and a 2 C per 1000 ft lapse rate above.

15. Over continents and oceans, the relative temperature conditions are:

a) Wanner in winter over land, colder in summer over sea

b) Colder in winter over land, warmer in winter over sea.

c) Cold in winter over land and sea,

d) Wanner in summer over land and sea.






CHAPTER EIGHT - HUMII)ITY

Contents

Page

8 - 1

8 - 1

.... 8-1

.. 8-1

8-

8 - 2

8 - 2

..... 8 - 2

8 - 3

8.10 DRY-BULB AND WET-BULB HYGROMETER OR PSYCHROMETER 8 - 4

8.11 DEWPOINT TEMPERATURE .8 - 4

8.12 DIURNAL VARIA nON OF HUMIDITY .... 8 - 5

8. I DEFINITION OF LATENT HEAT

8.2 EVAPORATION .

8.3 SATURATION

8.4 CONDENSA TlON

8.5 FREEZING

8.6 MELTING

8.7 SUBLIMATION

8.8 HUMIDITY MEASUREMENT

8.9 WET BULB TEMPERATURE .

HUMIDITY QUESTIONS. 8-7

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Oxford Aviation Jeppesen-Meteorology