3.Temperature

temperature defn

Temperature is a measure of heat. It is measured by a thermometer in degrees Celsius (Centigrade) or Fahrenheit.

Celsius Scale

: 0°C = melting point of ice; 100°C = boiling point of pure water at normal pressure. Used

internationally, in aviation and science

Fahrenheit Scale:

32°F = melting point; 212°F = boiling point. Used in a few English-speaking countries

Absolute Zero:

The minimum temperature attainable for all substances. Value = −273.15°C = 0K.

temp conversion formulae 3

F = (9C/5) + 32

C = 5/9 × (F − 32)

K = C + 273

F = C what value

Special value — same on both scales:

–40°C = –40°F

instruments for measuring temp

Dry Bulb Thermometer

wet bulb thermometer

Maximum thermometer

minimum thermometer

thermograph

temp vs heat

Temperature measures how hot or cold something is. Heat is the energy that flows from one thing to another because of a temperature difference.

Temperature

• It tells you the average kinetic energy of the molecules: how fast, on average, they are moving.

• It does not depend on how much substance you have.

• It is measured in °C, °F or K, with a thermometer.

2. Heat

• It is energy in transit. Heat always flows from the hotter body to the colder one.

• It depends on the amount of substance as well as its temperature.

• It is measured in joules (or calories

3. Example: the bath and the cup

A cup of boiling water at 100°C and a bath of water at 40°C.

• The cup has the higher temperature.

• The bath holds far more heat, because it has many more molecules, each carrying energy.

• The bath could warm a whole room. The cup could barely warm a bowl.

dry bulb thermometer

Standard mercury thermometer measuring ambient air

temperature.

wet bulb thermometer

Standard mercury thermometer measuring ambient air

temperature.

Maximum Thermnometer

Like a Doctor's thermometer — records maximum

temperature attained. Has a constriction to prevent mercury

from falling back.

Minimum

Thermometer

Uses Alcohol instead of mercury (alcohol doesn't freeze). Has

a dumbbell-shaped iron index. When temperature falls,

alcohol drags index back; index stays at minimum psn

Thermograph

Gives a continuous record of temperature. Uses bimetallic

strips in Upper Air measurements

stevenson screen

surface temp is recorded at a height of 4ft (1.25m) above ground in shade , inside a stevenson screen - a louvred whitepainted wooden box that protects instruments from radiation while allowing free air circulation

types of temp

surface temp

ambient temp

virtual temp

surface temp

Temperature recorded at 4 ft (1.25 m) above ground in shade (inside Stevenson's Screen).

ambient temp

temp of surroundings

Virtual temperature

In a thermodynamic process, the temperature at which dry air parcel would have the same

pressure and density as a moist parcel of air. VT allows use of dry air equation of state for moist

air also. VT is always higher than actual temperature.

heat & temp defn

Heat is energy transferred because of a temperature difference. Internal energy is energy stored in the substance.

temp is the avg KE of all molecules and atoms of substance

**Heat is energy transferred because of a temperature difference; internal energy is energy stored in a substance. Temperature alone does not tell us the total internal energy of a quantity of material. Keep these three ideas separate when explaining atmospheric heating.

Specific Heat

Specific Heat is the heat required to raise the temperature of unit mass of a substance by 1°C

land vs sea specific heat

Aviation Implication: Because land has a much lower specific heat than water ( 0.2 vs 1.0 ), land

heats and cools much faster than the sea. This drives sea breezes, land breezes, and monsoon circulation.

Latent heat defn

Latent Heat is defined as the "amount of heat absorbed or released during change of phase from/to

solid/liquid/vapour".

• Absorbed during: solid → liquid, liquid → vapour

• Released during: vapour → liquid (condensation), liquid → solid (freezing)

Latent heat is "hidden" — it does not change temp , only phase

evaporation vs condensation

Key Point: Evaporation = heat absorbed from surroundings = cooling effect. Condensation = heat

released = warming effect. This is why clouds release heat (Latent Heat Release = LHR) — critical for

thunderstorm development!

Methods for Heat Transfer

Heat is transferred by Conduction, Convection, Radiation, and other methods. In the atmosphere,

Radiation plays the most significant role

conduction

Physical transfer by molecular contact

Important very close to the ground. Heats

lowest air layer in contact with warm surface.

Convection

Convection is the transfer of heat and thermal energy through the movement of a fluid (a liquid or gas)

Free Convection = intense solar heating.

Forced Convection = topography. Air lifts to

higher levels and transfers heat.

Radiaation

Every body radiates at its temperature

Medium neither affected nor required. Solar

radiation heats earth directly.

Advection

Horizontal motion of the atmosphere

transferring heat.

Transfers heat horizontally by winds (e.g.,

warm/cold advection

Turbulence

Irregular eddy motion of the atmosphere causes redistribution of heat

important in the lowest atmospheric layers

Latent heat release

Latent heat absorbed by evaporation is released by condensation or freezing.

released in atmosphere — major heat

source for cloud systems and thunderstorms.

Insolation

is the total amount of radiation received over a particular area

Insolation depends on the obliquity (angle) of sun's rays:

Maximum when sun is overhead (vertical rays cover less area → more energy per unit area)

Least at sunrise and sunset (oblique rays spread over large area)

Maximum insolation in the Tropics (between Tropic of Cancer and Tropic of Capricorn)

Least at the Poles

Laws of Radiation ; what is Black Body?

Objects emit thermal radiation. An ideal blackbody absorbs all incident radiation; real objects

need not behave as perfect blackbodies.

Stefan boltzmanns law

The total amount of energy radiated by a black body is

proportional to the fourth power of its absolute

temperature.

wiens law

the wavelength of the most intense radiation is inversely proportional to the absolute temperature

plancks law

describes distribution of radiated energy with absolute temp

memory aid stefan and wien ?

Stefan = T to the FOURTH (Total energy)

Wien = Wavelength INVERSELY proportional to T (hot = short wave, cold = long wave)

"Hot Sun = Short Wave; Cold Earth = Long Wave" — this is central to the greenhouse effect!

The temperature of the surface of the sun is about

6000 degree celcius

type of solar radiation

Complete solar radiation (including visible, UV, and IR) is responsible for all the heat the earth receives from

the sun as short wave radiation

terrestrial radiation

The earth radiates at its own temperature and loses heat. These are called Terrestrial Radiation. They are

Long Wave Infra-Red (IR) radiation and are invisible.

The earth receives heat as short-wave radiation from the sun and loses heat as long-wave radiation

Nocturnal Radiation

At night, the short-wave radiation from the sun is absent. Only the earth radiates and loses heat. The

radiation emitted by the earth at night is called Nocturnal Radiation.

nocturnal radiation Aviation Significance:

Nocturnal radiation causes surface cooling at night → ground fog formation,

radiation fog, frost → hazardous for takeoff/landing!

solar radiation absorbed by earth surface

51%

solar rdn absorbed by water vapor , dust , ozone

16%

solar radiation absorbed by clouds

3%

total absorbed solar rdn

70%

solar rdn back scattered by air

6%

reflected back by clouds

20%

reflected back by earths surface

4%

total reflected/backscattered rdn

30%

albedo defn

reflected rdn or incident rdn

The 30% of solar radiation reflected back to space by the earth and clouds is the reflecting power of earth,

called Albedo.

In clear weather, about 5/6 of the solar radiation reaches earth surface → about 1/6 (≈17%) is reflected.

Earth surface reflects about 10% in clear conditions.

Snow surface reflects about 80% of incident energy (very high albedo).

sea vs land diurnal variation of temp

Sea vs Land: The sea surface temperature shows a variation of less than 1°C from day to night. Over

land, diurnal variation may average as much as 20°C .

Near the coast, the diurnal variation near the coast may be as large as inland, but with a wind off the sea it

will be small. Sea breezes have a pronounced cooling effect

why diurnal variation small over the sea

Higher specific heat of water (5× that of land)

larger mixing depth - surface water mixes with deeper layers

Evaporation from sea surface moderates temperature changes

4. Diurnal variation is maximum when wind is calm; with strong winds, surface air mixes with air above and

heat spreads through the friction layers (upto 1km above)

Minimum

Temperature what time of day

½ to 1 hour after

sunrise/dawn

Nocturnal cooling continues slightly past sunrise until incoming

solar radiation exceeds outgoing terrestrial radiation. Takes 2–3

hours for heat to transfer from screen level (1.25m)

maximum tempwhen during day

Early afternoon (2–3

hours after noon)

Highest insolation at noon, but 2–3 hours needed for heat to

transfer to screen level. Surface is highest at noon; screen level

maximum occurs 2–3 hours later.

Effect of Clouds on Surface Temperature

Can cut off a fair proportion of solar radiation during the day → reduces daytime heating

A thick layer of stratus cloud can block radiation and reduces a major part going back to space

A thick lower cloud also blocks most of the outgoing terrestrial radiation at night

why cloudy nights are hotter

thelower the cloud the more effectively it reduced the cooling

day with clouds vs night with clouds

reduced insolation → less daytime heating diurnal variation is small

clouds act as blanket - trap outgoing IR

night temp stays higher

Temperature conversion: °F from °C

Temperature conversion: °C from °F

C = 5(F−32)/9

Temperature conversion: K from °C

K = C + 273

Special crossover temperature

-40c = -40 f

Absolute zero

-2731.15c = 0k

Freezing point of water

0c = 32F = 273 k

boiling point of water

100 c = 212 F= 373K

Surface temp measurement height(stevenson screen)

4ft 1.25m

Minimum temperature timing

½ to 1 hr after sunrise

Maximum temp timing

2-3 hrs after noon (early afternoon)

Ocean diurnal Temp variation

< 1 degree celcius

Land diurnal temp variation

upto 20 degree celcius

Specific Heat of water

1 highest

specificheat of ice

0.5

Specific heat of soil / land

0.2

Solar radiation IR component

46

solar radiation Visible component

45%

solar radiation UV component

9%

Solar radiation absorbed by earths surface

51%

Solar radiation Absorbed by water vapour, dust, ozone

16%

solar radiation absorbed by clouds

3%

Total absorbed Solar radiation

70%

Back Scattered by Air

6 %

Reflected by clouds

20%

Reflected by earths surface

4%

Total reflected albedo of earth

30%

clear day radiation reaching the earth

5/6 83%

Snow surface albedo

80%

sun surface temp

6000 degree celcius

Friction layer depth (wind mixing)

upto 1 km above

convection covers more than

70% of earth (water covered )