EG4508: Issues in environmental science

Meteorology and Climate

Dr Mark Cresswell

Introduction, Astronomy, Energy, Radiation & Temperature

Suggested References #1

• Ahrens, C. Donald. (2000) Meteorology today : an introduction to weather, climate, and the environment.

• Harvey, Danny. (2000) Climate and global environmental

change • Burroughs, William James. (2001) Climate change : a

multidisciplinary approach. • Climate change 2001 : The scientific basis / edited by J.T.

Houghton

• McGuffie K and Henderson-Sellers A. (1997). A climate modelling primer. Published by John Wiley, England.

Text Books:

Suggested References #2

• Quarterly Journal of the Royal Meteorological Society

• Monthly Weather Review• Meteorological Applications• Journal of Climatology

Scientific Journals:

SEE UKSCIENCE METEOROLOGY PAGES FOR MORE INFO

Suggested References #3

• KNMI climate explorer: – http://climexp.knmi.nl

• Royal Meteorological Society: – http://www.royal-met-soc.org.uk/

• The Met. Office: – http://www.meto.gov.uk/

• NOAA-ENSO: – http://nsipp.gsfc.nasa.gov/enso/

Internet:

WWW.UKSCIENCE.ORG EGS UNITS EG4508 LINK

General Points

• The atmosphere behaves like a fluid• The atmosphere is a mixture of different

gases, aerosols and particles• The atmosphere remains around the

earth as an envelope because of gravity• Much of the observed motion in the

atmosphere results from solar radiation

Basic Astronomy• For most of the Earth, energy

varies on daily (diurnal) and seasonal (annual) time-scales.

• Changes from daytime to night and progression through the four seasons depends on the configuration of the Earth-Sun orbit

Quantity of solar radiation may vary as a result of solar activity

Solar wind increases in magnitude at times of high sunspot activity

Basic Astronomy• The Earth completes a single rotation about its axis in

approx 24 hours (23.9345 hours!) - this period is known as a day Typical Diurnal Air Temperature

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5

10

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00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00 06:00 12:00 18:00 00:00

Synoptic Hour

Temperature (Celcius)

Basic Astronomy• The Earth completes a single revolution around the Sun in

approx 365 days (365.256 days) - period is known as a yearAverage Daily Maximum Temperature (London)

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5

10

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January February March April May June July August September October November December

Month

mean daily maximum temperature (celsius)

Basic Astronomy

• Energy received at different points on the earth’s surface is not constant

• As we move from the equator to the poles the quantity of energy decreases

• This is due to Earth curvature• The same amount of energy is spread

over a greater area and has to pass through a thicker layer of the atmosphere

Basic Astronomy• Axis about which the earth rotates tilts

Spring

Summer

Autumn

Winter

Basic Astronomy

SUMMER (N. Hemisphere) WINTER (N. Hemisphere)

Basic Astronomy• The Earth does not spin perfectly about its axis - but

tilts to trace a cone in space - caused by the combined Sun and Moon’s gravitational pull and is called precession

• This tilt angle varies - between about 22º and 25º - it is currently 23.5º

Basic Astronomy• In addition to tilt - the elliptical orbit the Earth

takes around the Sun varies also• Mean distance between the Earth and the Sun

is 1AU (1.496 x 108 km). Minimum distance is 0.983AU and the maximum distance is 1.017AU

Basic Astronomy• In addition, the Earth’s path along its ellipse will vary

slightly due to differential gravitation pull. This is known as the eccentricity of the orbit.

Basic Astronomy• The combination of factors such as orbital

eccentricity, precession, tilt angle, distance from the Sun etc greatly affect our climate by varying the quantity of solar energy received

• Collective term is Orbital Forcing

• May have influenced the magnitude and period of past ice ages

Basic Astronomy

• The gravitational pull of the moon affects our tides and also moderates energy levels in the oceans

• Ocean dynamics greatly influences our Earth’s climate system

Composition of the atmosphere

GAS PERCENT* GAS & PARTICLES PERCENT** ppmNitrogen 78.08 Water vapour 0 to 4Oxygen 20.95 Carbon dioxide 0.036 365Argon 0.93 Methane 0.00017 1.7Neon 0.0018 Nitrous oxide 0.00003 0.3Helium 0.0005 Ozone 0.000004 0.04Hydrogen 0.00006 Particles 0.000001 0.01-0.15Xenon 0.000009 Chloroflourocarbons 0.00000002 0.0002* = Percent by volume dry air** = Percent by volume From Ahrens C. D, 2000

Vertical structure of the atmosphere

• Weight is the mass of an object multiplied by the acceleration of gravity

Weight = mass x gravity

• An object’s mass is the quantity of matter in the object

Vertical structure of the atmosphere• The density of air is determined by

the mass of molecules and the amount of space between them

Density = mass/volume

• Density tells us how much matter is in a given space (or volume)

Vertical structure of the atmosphere• Each time an air molecule bounces

against an object it gives a tiny push

• This small pushing force divided by the area on which it pushes is called pressure

Pressure = force/area

Vertical structure of the atmosphere• In meteorology we discuss air

pressure in units of hectopascals (hPa) (previously called millibars mb)

• The average atmospheric pressure at the Earth surface is 1013.25 hPa

• We can sense sudden changes in pressure when our ears ‘pop’ such as that experienced in old aircraft

Relationship between pressure and height

• As we climb in elevation (up a mountain or in a hot air balloon) fewer air molecules are above us:

atmospheric pressure always decreases with increasing height

Relationship between temperature and height

Introduction to the Oceans

• The oceans occupy 71% of the earth’s surface

• Over 60% of global ocean surface is in the southern hemisphere

• Three quarters of the ocean area is between 3,000 and 6,000 metres deep

Structure of the Oceans

• The thermocline is a layer characterised by decreasing temperature and increasing density with depth

• The thermocline is stratified and inhibits vertical mixing

Structure of the Oceans

• Below the thermocline layer is the deep layer of cold, dense water

• Deep layer motion is mostly driven by density variations due to salinity change

Average Ocean Currents for Atlantic and Pacific

Energy: basic laws and theory

• Energy is the ability or capacity to do work on some form of matter

• Energy is transformed when it interacts with matter - e.g. potential energy is transformed into kinetic energy when a brick falls to the ground

• Matter can neither be created nor destroyed - only change form

Energy: basic laws and theory• The energy stored in an object determines

how much work it can do (e.g. water in a dam). This is potential energy

PE = potential energy

m = mass of the object g = acceleration of gravity

h = object’s height above the ground

PE = mgh

Energy: basic laws and theory• A volume of air aloft has more potential

energy than the same volume of air above the surface

• The air aloft has the potential to sink and warm through a greater depth of the atmosphere

• Any moving object possesses energy of motion or kinetic energy

Energy: basic laws and theory• The kinetic energy of an object is

equal to half its mass multiplied by its velocity squared:

KE = ½ mv2

• The faster something moves, the greater its kinetic energy. A strong wind has more kinetic energy than a light breeze

Energy: basic laws and theory• Temperature is a measure of the average

speed of the atoms and molecules, where higher temperatures correspond to faster average speeds

• If a volume of air within a balloon were heated the molecules would move faster and slightly further apart - making the air less dense

• Cooling air slows molecules down and so they crowd together becoming more dense

Energy: basic laws and theory• Heat is energy in the process of being

transferred from one object to another because of the temperature difference between them

Temperature scales

• Hypothetically, the lowest temperature attainable is absolute zero

• Absolute zero is -273.15 ºC• Absolute zero has a value of 0 on a

temperature scale called the Kelvin scale - after Lord Kelvin (1824-1907)

• The Kelvin scale has no negative numbers

Temperature scales• Although Kelvin is the preferred scale

for scientists and physicists there are two more commonly used scales

• Fahrenheit was developed in the 1700s by Daniel Fahrenheit who assigned the value 32 to the temperature at which water freezes and 212 to the temperature at which water boils

Temperature scales• The zero point of the Fahrenheit

scale is the lowest temperature possible when mixing ice, salt and water. The 180 equal divisions between the freezing and boiling points are known as degrees

• A thermometer calibrated with this scale measures temperature in degrees Fahrenheit (ºF)

Temperature scales• The Celsius scale was introduced

in the 18th century. The value of 0 is assigned to the freezing point of water and the value 100 when water boils at sea-level

• An increasing temperature of 1 ºC equals an increase of 1.8 ºF

Specific heat and latent heat

• Liquids such as water require a relatively large amount of heat energy to bring about just a small temperature change

• The heat capacity of a substance is the ratio of the amount of heat energy absorbed by that substance to its corresponding temperature rise

Specific heat and latent heat• The heat capacity of a substance per

unit mass is called specific heat• Specific heat is the amount of heat

needed to raise the temperature of one gram (g) of a substance by one degree Celsius

• 1g of liquid water on a stove would need 1 calorie (cal) to raise its temperature by 1 ºC

Specific heat and latent heat• When water changes its state

(solid to liquid, liquid to gas etc) heat energy will be exchanged

• The heat energy required to change a substance from one state to another is called latent heat

• Evaporation is a cooling process• Condensation is a warming process

Energy transfer in the atmosphere

• Conduction: transfer of heat from molecule to molecule (hot spoon)

• Convection: transfer of heat by the mass movement of a fluid (such as water and air)

• Radiation: Movement of energy as waves - the electromagnetic spectrum

Electromagnetic spectrum with

enhanced detail for visible region of the

spectrum

Note the large range of wavelengths

encompassed in the spectrum - it is over

twenty orders of magnitude!

EMR and the Sun-atmosphere system

• About 50% of incoming solar radiation is lost by the atmosphere: scattered (30%) and absorbed (20%)

• Scattering involves the absorption and re-emission of energy by particles

• Absorption (unlike scattering) involves energy exchange

EMR and the Sun-atmosphere system

• Wavelengths less than and greater than 0.8µm (800nm) are often referred to as shortwave and longwave radiation respectively

• The shortwave solar radiation consists of ultraviolet and visible

• The terrestrial longwave component is known as infrared

EMR and the Sun-atmosphere system

• Just under 50% of the radiation reaching the Earth’s surface is in the visible range

• Components of visible light are referred to as colours

• Each colour behaves differently and white light can be separated out by use of a prism

EMR and the Sun-atmosphere system• The human eye cannot see infrared

radiation• Infrared radiation is absorbed by

water vapour and carbon dioxide in the troposphere

• The atmosphere’s relative transparency to incoming solar (SW) radiation, and ability to absorb/re-emit outgoing infrared (LW) radiation is the natural greenhouse effect

The Earth’s energy balance

• Incoming solar (shortwave) energy should be balanced by outgoing terrestrial (longwave) energy

• Without a balance the Earth would heat up or cool down uncontrollably

• Energy may take a tortuous path from Sun to ground and back to space.

Greenhouse effect

• The natural greenhouse effect maintains a stable climate for life on earth

• Outgoing radiation (longwave) is absorbed by molecules such as water vapour and carbon dioxide

• Energy is then re-emitted in all directions - forming a blanket

GreenhouseEffect