Section - 01 cont’d

Lesson 2 Temperature and Heat

Temperature

• Air temperature is a measure of the internal energy of a substance.

• In the case of a mixture of gases such as air it is a measure of the kinetic energy.

• In aviation and meteorology the scale used is the Celsius scale

• Freezing point of pure water 0°C

• Boiling point of pure water at standard pressure 100°C

Other Temperature Scales

• Absolute or Kelvin Scale

• This is assumed to start from a condition in which there is no thermal energy contained in the substance.

• This means the molecules are at rest and the substance is at its most dense.

• taken as -273°C.• (T)K = (t)°C +273

• Freezing point 273 K

• Boiling point 373 K

Other temperature scales• In the USA temperatures are still measured

on the Fahrenheit (°F) scale where:– Melting point of ice = 32°F.– Boiling point of water = 212°F.

• The relation between Celcius and Fahrenheit is governed by the following expression.

– Where: C = Temperature in °CF = Temperature in °F

9

32) x 5 (FC

StevensonThermometerScreen

• Shields from direct solar radiation

• Allows free air circulation

• Sensors 1.2m (4ft) above surface

Solar Radiation• The source of heat for the earth is obviously the

sun.

• The incoming solar radiation is of short wave length, about 2 to 3 microns (1 micron = 10-3 mm) and passes with little absorption through the earth’s atmosphere.

Solar Radiation, cont’d

• The ozone layer high in the stratosphere absorbs some of this SW radiation (the harmful ultra-violet) and a little incoming radiation is absorbed by clouds and dust, etc.

• Apart from this, however, the atmosphere is NOT heated by the sun.

• The net incoming short wave radiation heats the earth's surface

Shortwave Insolation

Albedo = The percentage of reflected radiation

Insolation

Radiation Balance of the Earth

• The earth is heated by short wave radiation from the sun. – Called INSOLATION.

• Radiation includes harmful wave lengths:– Ultra-violet.– Cosmic Rays.

• Ozone Layer absorbs most of the above.

Solar radiation distribution

Earth’s Annual Radiation Balance

Heat Transfer Processes

• Short wave radiation passes through the earth’s atmosphere and warms the surface.

• Thus the Earth is heated from below.

Distribution of short wave

radiation

Methods by which energy is distributed in the atmosphere:

• Conduction - Heat transfer by contact

• Convection - Heat transfer by vertical motion

• Radiation - Absorption or radiation of em energy

Methods by which energy is distributed in the atmosphere, cont’d:

• Release of Latent Heat - during condensation

• Advection - horizontal movement

• Turbulence - mixing during advection process

Earth’s Radiation Balance

Solar Radiation Absorption

Diurnal Temperature Variation

Factors Influencing Diurnal Temperature Variation

• Wind.

• Moisture Content.

• Cloud Cover.

• Proximity of Large Bodies of Water.

• Latitude.

• Season.

Factors Influencing Diurnal Temperature Variation

• Wind:– Calm conditions, no cloud = strong radiation=strong

inversion– Wind = mixing with warmer air aloft=almost isothermal

layer close to the surface. – Winter nights provide the best conditions for strong

radiation inversion. – During the day a brisk wind will mix the cooler air aloft

with the warmer surface air and limit the maximum daily temperature.

Factors Influencing Diurnal Temperature Variation, cont’d

• Moisture Content.

– Moisture absorbs LW IR radiation from surface and limits minimum night temperature

Factors Influencing Diurnal Temperature Variation, cont’d

• Cloud Cover.– Thick clouds will also be more effective than

thin in limiting the temperature drop at night– conversely during the day thick clouds will be

more effective at attenuating the incoming solar radiation and so limit the maximum daily temperature.

Factors Influencing Diurnal Temperature Variation. Cont’d

• Proximity of Large Bodies of Water.– These are a storehouse of thermal energy and will limit

temperature drop close to shore line.

• Latitude.– Equator and poles small diurnal variation

– Mid-latitudes large variation

• Season.– Summer - large variation

– winter - small variation

Diurnal Variation con’t

Greenhouse Effect

• Terrestrial long wave radiation is absorbed and re-radiated by the carbon dioxide and water vapour in the atmosphere.

• This is known as the ‘Greenhouse Effect’ :

– the mechanism by which carbon dioxide and to a lesser extent water vapour freely absorb the outgoing long wave IR terrestrial radiation.

Greenhouse Effect

• The total outgoing long wave radiation must still exactly balance the total incoming short wave radiation, otherwise the temperature of the earth as a whole would rapidly change.