Understanding Flying Weather - Sample - Derek Piggot

7/25/2019 Understanding Flying Weather - Sample - Derek Piggot

1/31

Understanding

lying

Weather

Derek

Piggott


2/31


3/31

lsoavailable

from A C B

lack

BE

GINNING G

LIDING Third Ed

ition

Derek Pig

gott

GLID

ING Sevent

h Edition

Derek

Piggott

GLIDING S F

ETY

Derek

Piggott

METEOROLOGY ND

FLIGHT Third

Edition

A Pilot sGuide to Weath

er

Tom Bradbury

UNDERS

T NDING G

LIDING Third Edition

Derek

Piggott


4/31

nderstanding

F

ly

i

n

g

W

e

a

t

h

e

r

e

cond

e

diti

on

erek

Piggott


5/31

Publishe

d

by

A

C

Black (Publishers) L

td

35 BedfordRow,

London W C1 R 4JH

First

edition 1988

Reprinted 1991

Second edition 1996

Reprinted 1999

1988, 996DerekPiggott

ISBN 0

7136

4346

3

All rights

reserved. N o part of

this

publication may

be

reprodu

ced

in

any form or by any

means

grap

hic,

electronic or mechanical,

in cluding photocopying,

recording, tapingor info

rmation storage and retrieval

systems

without

th e

prior

permission

in

writing

of

the

publishers.

Derek Piggott has asserted

his

rights under the Copyright,

Design and Pat

ents

Act 1988, to

be identified as

the


6/31

CONTENTS

Author s

note

7

Introduction 9

The airmass 11

2 The pressure pattern 14

High

and

low

pressure

areas 15

Buys-Ballot s

Law

16

Veering

and backing

16

Interpreting the isobars 17

Wind strength

18

Wind gradient 22

3

Depressions

and anticyclones

23

Depressions

23

Occlusions

31

Anticyclones 3 4

Timing

3 5

Shipping forecasts 3 7

4 The

atmosphere 41

Water vapour

41

Stability and

instability

of the atmosphere

43

5

Clouds

4 9

The

formation of

clouds 4 9


7/31

6 Local topography

59

Sea

breeze

effects

59

High ground

eff

ects

61

7 Soaring

conditions

6

3

Seasonal effects 63

Thermals 64

Visibility

68

Wave

conditions

7

Ridge

lift

an

d hill soaring 75

8 dvice on using so

aring condition

s

76

n powered

aircraft

76

n

gliders and low

speed aircraft 8

dditional hazard

s

for

gli er pilots

converting

to

powered aircraft 85

ltimeter

settings

86

Revision

questions 9

nswers

92

Index 96


8/31

Author

s N

ote

My

aim

in

writing

this

book

has

been

to provide

a

simple

in-

trodu

ction

t

o Met

eorology

for peopl

e learn

ing to fl

y glide

rs or

other kinds o

f airc

raft. Hopef

ully,the

bookshoul

dh

elp

the

re

ader

to

un

derstand

the

current

wea

ther, to

interpre

t

and ma

ke

bet

ter

use o

f

for

ecasts,and t

o recogn

ise the

better days for

flying

and

the

e

ver presen

t haza

rds associat

ed w

ith fly

ing

in

to bad

weathe

r.

Altho

ugh the book

is

larg

ely gl

ider and

soari

ng

o

rientate

d, am

sure

that

all

pilots would

be

safer

and

have

more

fun

from

their

flyin

g if th

ey understo

od mo

re of the

s

ubtleties

ofso

aring condi-

tions On

many

daysany

knowl

edgeable

pilot can u

se

thermal

s

and hill or

wav

e l

ift

to

improv

e the climb

ing

ability

and even

to

increase

t

he

average

cruis

ing sp

eed

and economy

of a li

ght

air-

cra

ft

With

this

in

mind

,

have

i

ncluded

some notes

on how to

mak

e

use

of

thermal

activity

when

flying

a

light

aircraft

and

also some

advic

e for the

les

s experien

ced

gl

ider p

ilot who

m

ay

b

e

h

aving

diffic

ulties

in staying

up.

For a

long

time ha

ve

known

h

ow

difficult

it is fo

r gliderpilots

reading

for t

heir Bro

nze

C

It

usually

involv

es searchin

g throug

h

number

of

books, n

ever kn

owing

what is essent

ial for the

test.

No one met

eorology

book

seems to cover

wh

at

the

pilot

s

r

eally

need

to

know

in

an

easily

understandable

manner. hope

that

this

boo

k will

solveth

eir pro

blems

a

nd

ha

ve

i

ncluded

question

s

and

answ

ers coveri

ng th

at syll

abus.The

se should

also be

useful

to

the

rea

der w

ho

is

star

ting

to study

for a Pr

ivate Pilot

Licence

I

would

l

ike

to

thank

John

Finlater

and To

m Bradbu

ry,

both

v

ery

experi

enced

forecas

ters, for

the

ir enco

uragem

ent and for

their hel

p

i

n checkin

g

m y te

xt

and

sugge

sting

what should

be

in cluded

D

erek Pig

gott


9/31

IN T

RO

U T

ION

Meteo

rology

is a

comple

x

subject

and th

erefore

to simp

lify

it

m eans

discussing

only

the

basic

factors.

Each

is

affected

and

modi-

f

ied bythe

others

so that

all of them

need

to b

e

taken

in

toaccou

nt

wh

en co

nsidering

what

is happen

ing to

the

weat

her.

In

t

rying to

und

erstand the

weathe

r from

a pilot s poi

ntof

vie

w

the

main factors

invol

ved ar

e:

w

here the airm

ass

has come

from an

d the

type of

terrain

over

w

hich

it

h

as tra

velled;

t

he pressure

di

stributio

n and the

signif

icance o

f

the

pres

sure

p

attern;

the

characte

ristics

of depressi

ons,

fron

tsand anti

cyclones

;

4

th

e stabili

ty of

the airma

ss;

5 th

e local

topo

graphy;

6 the

tim

e of year and

th

e

time of da

y.

All

of these

factors and

many

others

have

to

be

considered

by

a

forec

aster if

a

re

asonable

p

rediction

i

s

to b

e mad

e. How

ever,

p

ilots do

not norm

ally ha

ve detail

ed inf

ormation

a

nd mu

st r

ly

mainly

o

n the pr

ofession

al foreca

ster to

interp

ret the

report

s

and

meas

urement

s

availa

ble

to him.

Altho

ugh this

in

formati

on

i

s up-

d

ated

every fe

w h

ours, many of

the

we

ather ch

anges are

rapid,

making

th

e fo

recasts u

ncertain

. Eve

n

a

pat

ch of hig

h cloud

or an

unexpected

shower

can

change the soaring conditions and wreck

an

o

therwise

acc

urate forecas

t.

This makes i

t particu

larly

impor-

tant fo

r

th

e soaring

p

ilot to

ma

ke

a good

assess

ment of the

l

ikely

co

nditions

by look

ing

at

the

forecast

s availa

ble i

n c

onjuncti

on

wi

th

his ownob

servatio

ns.


10/31

T H

E

A IR

M A

S S

An ai

rmass

is

a b

ody

of air in

which

h

orizon

tal chan

ges of tem

perature andhumidity are

slight.

It

may

extend for

many hun

d

reds

of

k

ilometr

es.

Its c

haracte

ristics

are

ac

quired

by

m

oving

ov

er

a d

istant

source

reg

ion f

or lo

ng

peri

od

befo

rebe

ing mo

ved

o

n to affe

ct ot

her areas.

Differe

nt

air

masses

are

separ

ated by

fro

nts whe

re h

orizon

tal cha

nges o

f te

mpera

ture

and hu

midity

m

ay be

sharp

.

A

front is slo

ping su

rface

sep

arating

twoairm

asses

havi

ng

different

temperature

and humidity. Surprisingly

even

slightly

d

ifferen

t

a

irmass

es do

not

easily

mix

tog

ether. Th

e fr

ontal

zone

whe

re the

two airm

asses

meet

is some

times

only

few

mile

s

across

altho

ugh it m

ay

on

occas

ion

stre

tch for

50 miles o

r

m

ore.

The sour

ce of the

airm

assdet

ermine

s

its char

acteris

tics and

in

partic

ular its

temp

erature

. H

oweve

r the path

over

whic

h the

airm

assha

s travell

edd

etermin

es ho

w

m

uch

thea

irmass

is m

odi

fied

before

it

reaches

us.

Unfortunately TVandnewspaper weath

er

map

s

sel

dom

give any

detai

l

on

the

air

masses

.

Po

lar

airmas

ses come

from

the n

orth

and sta

rt cold o

r cool.

Cold

air

c

annot

hold mu

ch

moi

sture

and

therefo

re

pol

ar a

ir-

m

assesare

likely

t

o

be

relativ

ely dry

.

Arc

tic mar

itime Am

).

This

is

pola

r air w

hich has

on

ly h

ad

short sea

tra

ck. T

hewar

ming ove

r the sea mak

es i

t

very un

stable

pr

oducin

g frequ

ent show

ers in th

e n

ortho

f

E

ngland

.

Apo

lar airmas

swhich

ha

s travel

led

over larg

e areas

of

se will

be

called

Po

lar

m

aritime

airm

ass

Pm). By

o

riginat

ing

i

n the

nort

h Polar

mar

itimeair

reach

es us

by mo

ving so

uthwa

rds over

a pro

gressiv

ely

w

armer sea

thus

warm

ing and

m

oisten

ing th

e

ather


11/31

2 IN

TRO U

TION

Fig

Theo

rigin of

the airm

asses.

m

Arctic Marit

ime; P

ra Pola

r

Maritim

e; P Po

lar

Contine

ntal;

T

c Tro

pical

Conti

nental;

rPm

Return

ing

P

olar

Mari

time.

over co

lder sea

which

stab

ilises t

he low

er layers

giv

ing s

tratus

c

louds.

W

hen

this air reac

hes

the

UK

itmay th

en

be

hea

ted

as

i

t

mo

ves ov

erland

. The

lo

w level

stabilit

y isde

stroyed

causi

ng

deep


12/31

I N TRODUCT I ON

3

more

moisture,

it can

be

dry

or

have a

high

humidity, depending

on the route it

has

taken to

reach us.

A

tropical

airmass

which has moved over large areas of sea will

have

picked

up

a

large

amount of moisture. The

relatively

low

sea

temperatures

will

have cooled

the lower

layers

producing stable,

cloudy conditions. Near windward

coasts

low

cloud is

liable to

persist all day.

This kind

of

airmass is

then

known

as Tropical

maritime Tm) and where it travels over land in summer,

the

cloud will break up leaving

rather stable

muggy weather and poor

visibility.

Where tropical air moves in from the

continent

in summer it

will

be

hot,

hazy

and

dry

and

known

as

a

Tropical continental

Tc)

airmass.

On a much smaller scale the airmass is being continuously

modified during the day and night.

Even

a slight change in the

wind direction, so that the air has travelled over more land, will

often result

in

a dramatic change in soaring conditions. The air

will be drier, giving a high

cloud

base and

better

thermals. For

example,

in

England

a

small change

in

the

wind direction of

northerly winds can

result

in marked improvement and better

soaring.

Instead

of

blowing in

almost

directly

from the sea, the

rather

moist cool

air

will have

dried

out by

travelling

over the

length of

the

country before it reaches central

England.

This will

reduce the amount of

cloud

cover and

raise

the

cloud

base.

With such

a small

island

the proximity of the

coastline is always

a limitation for

long distance

glider flights. The coast

line is only

20 miles or so

from

the

Lasham

Gliding Centre

in

Hampshire

(midway between London and Southampton) and with south

or

southwest winds,

cloud

base will remain low all day and thermals

will usually be weak unless good cloud streets

are formed.

20

miles

further

inland at Booker and Dunstable the cloud base will

be

sever l

thousand feet higher, giving far

better

conditions.

Sim-

ilar

conditions occur near

most

coastlines all over the world, and

have

a significant

effect

on

soaring conditions

whenever

the winds

are

coming inland off the

sea, particularly

in summer.

In

winter

the

sea and

land

temperatures

are much closer

to

each other and

sea breeze effects are insignificant.


13/31

THE

PR SSUR

P TTERN

Si

nce

th

e a

ir

it

sel

f has

wei

ght

the

ai

r

n

ea

r

t

he su

rfa

ce

is

com

pressed

by

theweight of

all

theair above

it.

Thegreaterthe

height

t

he le

ss

we

igh

t

of air

ab

ove

t

h t

l

eve

l an

dthe

ref

ore

t

he

low

er

th

e

p

re

ssu

re. B

el

ow

ab

out

5

00

0

f

eet

the

p

res

sur

e is

r

edu

ced

by a

bo

ut

1

mi

llib

ar

fo

r ev

ery

30

f

eet

of

e

xtr

a

h

eig

ht.

T

his

is

a

n

ave

rag

e

fi

gur

an

dis

im

por

tan

t

to re

me

mb

er

for

c

alcu

lat

ing

he

igh

ts

w

hen

u

sin

g

a

n alti

me

ter.

S

ee

fig.

2

.)

He

f

eet

3

5 0

00-

30

000

-

2

0 0

00-

10

,000

-

5

000

-

ight

:::

Pressure

m

etre

s

mill

iba

rs

1

0,0

00

; ; x

250

5,50

0

500

-

3 0

0 0

j ; ; i ; ;

; ; j

7

0 0

-

1

,50

0

- |

| -850


14/31

T H

E PR E S

S U R E

PA T

T E R N

5

Lifti

ng

th

e

air cau

ses co

olingbeca

use t

he at

mosph

eric pres

sure

de

creases

with

heig

ht, and

any

reduc

tion in p

ressure

o

f

gas

resul

ts ina

n expan

sion

so

tha

t

the

same am

ount of

he

at is co

n

taine

d

in

la

rger

volum

e.

T

hisprinc

iple

is

dem

onstrat

edin many

types

of refrigerator

where

gas

is

allowed to expand, lowering

the tempe

rature;

conv

ersely,

with

bicyc

le pum

p,

co

mpress

ing

t

he

air into

smalle

r vol

ume res

ults

in

a

ris

e in

tempe

rature.

Variat

ions of

p

ressur

e

in

theu

pper

atmos

phere cause

gradu

al

asce

nt

or

subsid

ence in

the

low

er

levels

resu

lting

in

chan

ges

in

the

press

ure at

the

surface

.

I

f air

is

heat

ed,

it expa

ndswhen

free todo

so

.This resu

lts in

reduc

tion of

its densit

y and

pre

ssure. U

neven

heati

ng o

f

the

earth

s

s

urface

will there

fore g

ive r

ise

to

pressu

re

varia

tions from

place

to p

lace. W

inds

are

th

e resu

lt

of

th

e

movem

ents o

f

the

a

ir as

it

tr

ies to ev

en

out th

ese

differen

ces in pre

ssure.

H

igh

n

d

ow

pr

essure

re

s

The

pres

sure

dist

ributio

n

d

etermi

nes

the

d

irectio

n a

ndstren

gth of

the

winds.

Without

the

earth s rotation,

air

would

always

tend to

flo

w

dire

ctly

from

any h

igher pre

ssure

region

to ev

en o

ut any

pressu

re d

ifferen

ces. (Th

is do

es happe

n near

the

E

quato

r). How

e

ver, the

E

arth ro

tates and

this

causes

air mov

ing from

high

to

low

er pressu

re

i

nthe

North

ern hemi

sphere

to

be

deflec

ted

t

o

th

e

right

until

bala

nce is

achie

ved

bet

ween

the p

ressure

gradie

nt

force

and the d

eflectin

g for

ce due to

th

ero

tation

of the

earth

. This

is

known

as

the Coriolis

effect,

and

it causes

the

air to

flow in

clockw

ise direc

tion

aroun

d ana

nticycl

one o

r

hig

hpress

ure are

a,

and

in an anticl

ockwis

e

directi

on aro

und dep

ressio

n

or lo

w

pressu

re area.

See

fig

3.

In

t

he

sou

thern

h

emisp

here th

e

rever

se

is true

an

d

th

e a

ir m

oves clo

ckwise

ro

und the

dep

ression

s. Since

the

wi

nds

are the res

ult

of these

pre

ssure pa

tterns,

to unde

rstand

th

em and th

e mov

ements

of the

we

ather syste

ms

we nee

d

t

o

know

more

about

press

ure

syste

ms.

The

readin

gs of

the

loca

l

pressu

re ar

e recor

ded by

stati

ons all

over the

wor

ld bu

t

it

i

s

only

ove

r thel

and ma

sses that

the

re

a

re

en

ough

statio

ns to

form

fair

ly accu

rate p

icture

of the

ever-

cha

nging p

ressure

patt

erns. In re

cent y

ears

th

e

cha

nges

in

pressu

re

e been

p

redicte

d large

ly by

com


15/31

6 T

HE PR ESSU

R E

PATTERN

Fig 3

Th

e

earth s rotation

causes the air toc

irculate

anti-clockwise

rou

nd

a ow pres

sure and clockwi

se rou

nd a highpressure with

the

surface

winds at about

30 degrees to the iso

bars.

it is well worth

while learni

ng to interpret thes

e

charts.

I

t may

b

e surpri

sing to learn that for

all practical purposes

the

winds at 2 3

fe

et abo

ve

the

surfac

e

b

low along these lines of

equal pressure.

Therea

son

fo

r this

is

not simple

to explain

and

the

ave

rage read

er

need

only

remember that

this

is

a

fact.

Near

t

he surface, however, air

is

retarded b

y

s

urface fri

ction

caused

by

trees,

vegetation,structures and topography

so

that

the

wind

s

peed

and therefore

the deflecting

force caused

by th

e

earth s

rotat

ion

are reduced. Th

is

makes

the s

urface

wi

nd flow

at an

angle

a

cross

th

e isobars towards

the low

pressure

,

ofte

n

about

3

degrees

overland and 1 degrees over

th

e

sea

wherethe f

riction

is

less.

Whenthe

isobars are close tog

ether the pressure

gradient and

the

wind

are strong. Whenthe

isobars

are

widely

spaced

and

the

pressure

g

radient is

weak, the

winds

are

light.

Buys-Ballot s

w

Seefig.

4. This

useful

law

states that:

i

n th

e

northern h

emisphere,

if

you stand with

you

r

back

to

the

wind,

the low pressure will

always

be

to your

left

and

high

pressure to your

right. It

is

also

us

eful t

o note

that if

you stan

d with your

back

to the surface

wind,

the upper wind

at about 2000 feet

will be blo

wing about

15 20

degre

es further round

in a clock

wise

direction.


16/31

TH

E P

RESSUR

E

P

ATTERN

7

Fig Buy

s-Ballot

s Law

. Standi

ng back

t

o

t

he wi

nd,

lo

w press

ure

is

alwa

ys to

the left

in th

e northe

rn hem

isphere

).

backin

g anti

-clockw

ise)

veering

clockwise)

Fig

5 V

eering and bac

king.

nterp

reting

theisob

ars

Fig.

6

see

next

page)

shows

a

weather

map

with

only the

isobars

dra

wn

in

. Eac

h

line co

nnects p

laces

with e

qual

mean sea

level

pr

essurea

nd in

t

his case

is

m

arkedw

ith the

pressu

re

in

m

illibars

.

The

ma

in

fe

ature

is th

e

d

epress

ion

centre

d

ov

erNort

hern

Ir

eland

w

ith a w

eak

r

idge

of

high pres

sure o

ver E

urope.


17/31

8

THE

PRESS

URE

P

ATTER

N

F

ig

Interp

reting the isob

ars.

Isob

ars

link

place

s w

ith

e

qual

pressu

re

and a

re

us

ually spa

ced

at

4

mil

libar

interv

als. W

hich directio

n

are the

surfa

ce

an

d

up

per wi

nds at

Bri

stol?

mor

e sout

herly becau

se it

woul

d

b

e flowi

ng

a

litt

le m

ore towa

rds

th

e

centre

of

the low

.

T

ry

the Buy

s-Ballo

ts law

for Bri

stol. I

m

agin

e

your b

ackto

th

ewind,Yes,

the

l

owpress

ure

is toyou

r left.


18/31

THE PRESS

URE

PATTE

R N 9

Fig

High

and

lo

w

press

ure areas anticy

clones

and dep

ressions)

.

at a

ce

rtain

height by

mea

suring this press

ure g

radient.

special

ly

calibrate

d

scale is

used to measu

re

th

e dist

ance

betwe

en

th

eselines

an

d

g

ives h

im an e

stimate of the

wind

s

peeds

at

bo

ut 200

0feet.

This

is know

n as the geo

strophic

wind.)

At this

heigh

t

the

actual

wind

blow

s

al

ong the isob

ars

b

ut

v

aries from the

geo

strophic

wind

in

some circum

stances

an

d this

is

known as the

gr

adient

w

ind. N

ote th t

thi

s is not

hing to do w

ith vert

ical wind

gradien

ts

w

hich are

caused

by t

he friction

of the

g

round onthe w

ind and

cause us

so much

trouble

during

take-of

f and

landing

in

strong

winds.

In fa

ct,

the

grad

ient wi

nd is

the

wind

bove

the

influe

nce

of the s

urface fric

tion.

At low

levels

the

airf

low

is distu

rbed by

t

rees, build

ings and

hills,

so

th t

it

isne

ver steady

. In

strong

w

inds, gusts of

at lea

st the stren

gthof the

up

per wind shou

ld

be

expectedbecausesome

of the momentumof

th t

upper

wind

is

bro

ught down

bythe

effects

of

conv

ectionand

turbulen

ce.

Fig. 7

sh

ows a

low

a

nd

high

p

ressure

region. In the hi

gh

press

ure area

a

n

an

ticyclon

e)

the i

sobars

are

furt

her p r

t

indica

ting

mu

ch lighter

winds.

Wind

s alw

ays blow

clockwi

se

ro

unda

high.


19/31

2 THE

PRES SU R

E PATTERN

a.

m.

stable

air

12

a.m

.

mix ing

7 p.m.

stable

a

ir

a

.m .

12 a.m.

7

p.m.

su

rface

wind

5

Fig 8 Ch

anges in the

surface a

nd upper win

ds during

the d

ay. The

surface winds increa

se a

nd veer as the

thermal activity

c

auses

m

ixing

and decrease

and

back in the e

venings as

the a

ir becomes

stable.

swin

g

a

littl

e

close

r in

d

irection

to

the wi

nd

at

200

0 feet or so.

See

fig.

8.)

A

knowledge

able

D

uty

I

nstructor

at

a g

liding site will

all

ow for thi

s

in

evitable ve

eringand

strength

ening

o

f the

surface

w

ind

whe

nhe s

ites thepositi

on

o

fthe winches

in the e

arly

mo

rning.

For example,

if

the surface wind

is

a

light

westerly

at

8

a.m.

it

w

ill

most likely increase

by 5

knots or so a

nd

sw

ing

to a more

northweste

rly direct

ion later

in the morning.

Ho

wever,

t

his

e

ffect

ca

n be

complete

ly swam

ped by any rapid

chan

ge

in the

weather,

such as the app

roach o

f

a

de

pression.

The

surface wind

m

ay al

so increa

se and

ch

ange

in dir

ection

if

i

t

is

reinforc

ed by

the

sea

br

eeze. Th

e

timing of thi

s

inc

rease wi

ll

depend

on the

distance

the air has

to

come

from

the

coast. For

e

xample, a

bout 25 m

iles fr

om the

c

oastline

an onshor

e wind can

be

expected

to

in

crease

someti

me aft

er mid-day,

bringing in

cooler,m

oister airan

d

prob

ably spoiling

an

y the

rmal ac

tivity. S

ea

breezes

may

also advan

ce inlan

d against

a gene

rally lig

ht bu

t


20/31

THE PRESS

URE P

TTERN

evening

k

atabatic winds

Fig Anabat

ic an

d katabatic

winds.

very close

to the hill

slo

pe.

Anaba

tic winds

r

e

most

evid

ent on

sun-facing and le

e slopes

where the surfac

e gets the grea

test heat

-

ing

Conversely

in

the

evening

and

particularly

on

a

clear

night

the

eart

h

radia

tes

heat ou

t

into

space

coolin

g the groun

d

an

d the a

ir

c

lose

to it n hilly

country

this

cooler

air f

lo ws dow

n the

s

ides

of

any hill

s or mount

ains into the vall

eys.

This is

known asa k t -

b

tic w

ind.


21/31

THE PRESSURE PATTE

R N

a

short distance up or downwind of these

areas the wi

nd can be

verystrong

indeed. Even if it is a light wind

on the g

round

prec

au

tions

alwa

ys

need

to

be taken to tie down gliders properly,

as the

position of

the wave

m

ay change without warning, resultin

g in a

sudden unexpectedly

strong

wind.

Lee

waves

are

explained

in

a

la

ter chapter.

W ind gra

dient

See

fig

10.) The

layers of air

c

lose to

the

ground are slowed down

by the friction betw

een

the

air

and

the ground. Obviously, the

stronger thewind, the grea

ter the change

in

the wind

speed which

is

possible due

to

this

effect.

The friction

will

also

be

greater

if

the

ground is rough or

covered

by

obstructions. If the

air is unstable

and

the area

isin the

lee

of obstructions such as trees o

r buildings,

some of

th

e

faster

moving air

fro

m

above

may be

brought

down

to ground level

caus

ing

m

omentary stronger winds

or gusts.

These r

apid changes in

wind

speed

combin

ed

with

the wind gra

dient

often

make

take-offs and landings critical

in

wind

y weather.

Flying

downinto

a

rapidly

decreasing wind

speed

during

the

final

s

tages

of

an approach causes a sudden loss of

airspeed

and

a

rapid

loss of height which

can be

dange

rous in a gli

der

or light

aircraft.

Th

is effect

is caused

by

the win gradient Turbulence near

the

ground al

ways

in

dicates rapid changes in

wind

speed and direc

tion and ex

tra flying speed is essential to ensure

that there is

adequate

speed

and

control

at

all

times.

F

ig

10

The wind gradien

t. Surface fr

iction

reduces

the wind

s

peed

n

ear the ground.


22/31

DEPRES

S IONS

AND

AN

T ICYCLONES

In thepast few

years,

with

the developme

nts in

radar

and the

use

of

powerful computers and weather

satellites,

the

mechanisms

by

which

the

depre

ssions develop

have

been

studied

in much more

deta

il than was

ever possible

in the past.

It

is

now kn

own that movements occurring

in the upper

atmo

sphere

are the cause

of the high

and

low

pres

sure

systems

forming

near the

surface

whic

h dominate

our

weather.

W

ith

the

air

circulating

round

t

he low

pressure area,

air

mus

t

be

rising

and

being

removed

at

high

level

or

the low

would

fill

in

very quickly.

Similarly,

fora

high

pressure

system to

persist

there

has

t

o be a

des

cent of theair to m

aintain the higher pressure. It

is

these general

m

ovements

up

and do

wn, started

at

high level

w

hich

dictate

where

systems will

form and

which cause the

characteristic weather associated with

them.

b ).

Fig 11 Symbolsused on weather m

aps, a) A warm front,

b )

A cold

front, c) An occlusion.


23/31

4

D

EPRE

SSIO

NS

ND

NT

ICYC

LONE

S

pola

r

reg

ions

meet

s

the

warm

er tr

opica

l air

along

a

fro

nt an

d

dep

ress

ions

form

alo

ng this

b

ound

ary.

It

is

kno

wn as a

wa

rm f

rontif th

e

wa

rmer

air

mas

s

is

mov

ing

fo

rwar

d

ov

er the

su

rface

and

a c

old

fron

t

if

i

t

is

th

e

coole

r

a

ir

mass

moving

forward

over

the

surface.

In

some

circumstances the

m

ovem

ent of

a f

ront

may

be

reve

rsed so

th t a w

arm

fro

nt be-

come

s

a

co

ld on

e.

Th

e maj

ority o

fo

ur

weat

her

sys

tems

fo

rm

out int

he A

tlant

ic

alon

g the

fro

ntal

zon

e

b

etwe

en t

he

Arc

tic

a

ir

m

ass and

thewa

rmer

Trop

ical mar

itime

a

ir

tothe

sout

h.

W

hen

two air

mas

ses

of

diffe

rent den

sity l

iesi

de

by s

ide

they

indu

ce

a str

ong

cu

rren

t

of air

t

o flo

w par

allel

and

onthe

c

old air

s

ide

of the

fron

t at

very

high

a

ltitud

es.

T

his

i

s

kn

own

as a

jet

s

tream

a

ndis

seve

ralm

iles

dee

p

a

nd

trave

ls at

s

peed

s

o

f

100

t

o 2

mi

les an

hour

.

See

fig.

12

a).)

D

istu

rban

ces in

this

up

per

flow

c

ause t

he

jet

strea

m

to ch

ange

di

recti

on a

nd

swin

g

fr

om

side

to s

ide

and

th

is re

sults

in area

s

w

here

thef

lowis

sp

eede

d up

or

slow

edd

own

. S

ee fig

s 12 b

) n

d

c

).) Wh

ere th

ef

low

s

peeds

u

p

b

ecaus

eof

di

verg

ence th

e pre

ssure

drops

sligh

tly indu

cing

an

upw

ard

flow

of

air.

This

con

tinu

ing up

war

d

m

ove

men

t resu

lts

in

a

cha

nge

o

f

pr

ess-

u

re

at

th

e

su

rface

w

hich

form

s the

d

epres

sion.

D

urin

g

the

de-

vel

opme

nt

of a

d

epres

sion

air

is

flow

ing

in

n

ear th

e su

rface

thou

gh not

fa

steno

ugh

to

stop

th

e

depr

essio

n de

epen

ing.

See fig .

a

)

j t

str m

co

ld a

ir

fron

t

w

armair


24/31

a

DEPR

ESSIONS

ND N

TICYCLONE

S 5

upperconver

gence

j

st

surface pres

sure

rises

b )

upp

er diverge

nce

ac

celerating^

fast

surface

pressure

Fig

3 a)

U

pper converg

encecauses an

increase

in

pressure and

a

descending

fl

ow forming

an

area of high

pressu

re

anticyc

lone),

b )

Upper divergence

causes

a drop

in p

ressure and an upw

ard

f

low,

reduc

ing the surf

ace

pressure and

forming

an

area of

low

pressure

dep

ression).

Si

milarly, anticyclo

nes

start with

convergent

flow

slowing

down

the air

flow

a

t heig

ht and causing an

increase in pres

sure

a

nd

a

downwa

rd

mo

vement

of air. This res

ults in an

area of

hi

gher

pressure

andan

outward

flow at low

levels. See

fig. 13 a).

)

h

e life o

adepressio

n


25/31

26

DEP

RESSION S ND

NTICYCLO

NES

See

figs 14

c) nd

d).)

B

ecause

of the eff

ects of the r

otation of

the

earth this

infl

ow

is div

erted i

nto a circu

lation

in

an

anti-clock

wise directio

n.

Wherea

s at

f

irst the

fro

ntal

surfa

cewas

more or l

ess stationary

th

e circu

lation starts a

wave

in

the

front. The

colddense a

ir moves

a

nd starts

to

u

ndercut th

e warmer

tropical

air formi

ng a

cold

front. At the

same

time the

warmer a

ir

is

moved

up ov

er

the

colder

air m

ass forming

a warm

front d)

Se

e figs 14 e) nd f).

) As

the air iscontinuo

usly

ex

tracted

at the

topof the

syste

m

so

the

pressure at

the sur

face continue

s to

d

rop

increa

sing

the ci

rculation and the

wind s

peeds. The nticloc

kwise

cir

culation

is gradually spread

up to highe

r levels

a

nd the upw

ard

movem

entof the w

armer m

oist air

causes

adiabati

c

cooling

a

nd

the

formatio

n

of

clouds rele

asingmor

e

h

eatenergy as

the mois

ture conde

nses in

to wate

r droplets.

The

cold front

moves mo

re quickly

than the w

arm one

and

catches i

t up

fo

rming an occlusi

on

with the w

arm

se

ctor at

th

e

surface

being

reduc

ed in

size.

S

ee figs

1

4 g) nd

h). The w

arm air con

tinues to

be

carrie

d

round

in

a

spiral

by

the

winds

but

the

upper

extractor

effect

has

cea

sed so

tha

t the

circulat

ion

runs

dow

n with the

surface l

ow

pr

essure

area gra

dually filling. Howev

er the

chang

es in

th

eupper

flow caused

by the

last stage

s of the life o

f this

d

epression

often

c

reate a

furtheraccelerati

on of the

flow

in

the

jet

strea

m. T

his sets

of

f the

p

rocess

again form

ing a

new depres

sion.

Som

etimes a whole

familyof dep

ressions f

orms givi

ng

a

per

iod

of almost

continuous

unsettled

or

bad weather.

On other

occa

sions the uppe

r flow w

ill

be

slowed d

own c using

an

increase

in

press

ure

a

nd a g

radual descending

flow

which

starts t

he

form -

tion

of an anticyclon

e a

nd more se

ttled w

eather.

The meand

ering of

th

e je

t

stream

to the

north and south

as

it

flo

ws

ac

ross

the Atlantic

largely

de

termines

the pattern

of our

we

ather and

thepositi

on

an

d inten

sity of the

areas of hig

h and

low

pressure.

a

rm

a

nd co

ld

fro

nts

Both warm and

cold fronts

m

ay vary

in

the w

eather cond

itions


26/31

DEPRESS I ONS AND ANT I CYCLONES 27

fT

a)

(b)

c

(d )


27/31

28

DEPRESSIO

N S ND NTICY

CLONES

Ns

warm sec

to r

Cs

As

20

0

front

rain or sn

ow

Fig

15

A

section

through

one type

of warm front.

4

500

thun

derstorms

and man

y of

the cha

racteristics

of a

cold

front.

Because of

the many

variations

in fronts an

d depres

sions, I have

not a

ttempted

to explain

all the

types of f

ront.

It

is importa

nt to rea

lisethat

it

is

very

rare

inde

ed to

have

bo

th

an active war

m

f

ront

and an

act

ive co

ld

fro

nt

on the same depress

-

ion.

This

means

that

if

there

is

a

large

amount

ofrainfall

onthe

warm front

there will be lit

tle

activity on t

he cold one.

The norma

l warm

front

has a sha

llow s

lope with

the

a

ir

risin

g

very grad

ually over a

dist

ance

of man

y hu

ndreds of miles.

This

lifti

ng p

roduces the

gradually

thickeni

ng layer of cloud wh

ich

results

in

steady ra

in

ne

ar the fron

t. See f

ig. 15.}

Usually

the

h

ighcloud cuts

off

the sun s heating

, but

if

the

high

cloud

arrives

after

the

thermal

activity

hasbecome

well

establish-

ed it is

not

uncommo

m for s

oaring condit

ions to pers

ist for

several

hours

after the sun

ha

s become

partially o

bscured.

Th

e warm

fr

ont

drizzle and

rain sometime

s

s

tart

sporad

ically,

but

once the

cloud

has

thi

ckened an

d continuo

us ra

in

begins

it is

usually the e

nd of

f

lying

for many hours

, ifnot

for

th

ewh

ole day.

Fig. 16

shows

a

ty

pical depression

with war

m and cold

fronts

and

their probable rain

areas.

In

reality

only

one of the

fronts

is

likel

y

t

o

be act

ive. The

first

indication

of

the appr

oaching

bad

w

eather will

be the h

igh cirru

s cloud, possibly

creat

ing a halo

around the sun

or mo

on. The

cloud will

grad

ually thicken to

altostr

atus

and

n

imbostratu

s,

with rain

beginn

ing

5-

10 ho

urs


28/31

DEP

RES

SION

S

ND

N

TICY

CLO

NES

9

Fig

16

W

arm

a

nd

cold

fro

nts

on a

dep

ressi

on,

show

ing

are

as

of

pr

obab

le

rain a

nd th

e sur

face

win

ds.

in

th

e i

soba

rs

at the

fron

t ind

icat

es the

ch

ang

e in

win

d dire

ctio

n.

he w

arm

s

ector

be

twe

en th

e w

arm

and

c

old fro

nts

s typ

ified

by

relat

ively

wa

rm h

umi

d c

ond

ition

s

and

lo

w clou

d.

It

m

ay

pro

duce

driz

zle,

e

spec

ially

in

win

ter,

but ther

e

are ofte

n g

ood

b

reak

s in

the c

loud

al

low

ing

trai

ning

fligh

ts to

c

onti

nue

al

thou

gh

the

re wl

l

be

f

ewif

an

y the

rmal

s.

The

wa

rm

secto

r ma

y als

o h

ave

s

uitab

le cond

ition

s

foru

sabl

e

wav

es to f

orm

in

the

lee

o

f

hills

a

nd

m

oun

tain

s. Le

e wa

ves ar

ee

xpla

ined

late

r.

No

tice th

at

the

col

d

fron

t s a

re

lativ

ely

stee

p

w

edg

e

of

a

ir

sho

velli

ng u

p

th

e w

arm

erair

r

apid

ly.

See

fig 17

.)

The

pa

ssag

e of


29/31

3

EPR

ESSIO

NS

ND

NTIC

YCLO

NES


30/31

lying

Weather

lying Weather

Derek Piggott 's introduction

for gli er and

aeroplane

pilots. In

simple

and

accessible

terms

it

how

atmospheric phenomena can be used

to

flying performance

and

to

make

flying both

and

enjoyable.

into account advances in satellite and computer

the book describes pressure patterns,

cloud

and

how

soaring conditions can be used

to

a s

well

as

detailing

the effects of

depressions,

and local topography.

uncomplicated text

and

diagrams cover the syllabus

the British Gliding Association Bronze 'C Certificate

so

will

be

particularly relevant to those studying for

important

examination.

Piggott M E

was Chief

Flying Instructor a t Lasham

1953

to

1989. He

is very highly respected throughout

gliding world

and

is the author of several

standard

books.


31/31

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Understanding Flying Weather - Sample - Derek Piggot