Temperature & Heat — Summary Notes

1. Heat and Temperature

Heat

• Heat is the total energy associated with the molecules of a substance/internal energy.
• Heat is energy in the process of being transferred from one object to another because of a temperature difference.
• Heat always flows from a warmer body to a colder body.

Temperature

• Temperature is a measure of the average kinetic energy or average molecular motion of a substance.
• Therefore:
  • Heat → total/internal energy
  • Temperature → average kinetic energy
  • Heat transfer → caused by temperature difference.

2. Methods of Heat Transfer

Conduction

• Air is a poor conductor of heat.
• Therefore, conduction is effective mainly in the shallow layer of air immediately adjacent to the surface.
• During daytime:
  • Ground becomes warmer than the air immediately above it.
  • Heat conducts from the warmer ground to the cooler air.
  • Warm air can subsequently rise and transport heat upward.

Adiabatic process

• Air is a poor conductor, so a rising or sinking air parcel can change temperature with very little heat exchange with its surroundings.
• Such a process is called adiabatic.
• Rising air:
  • expands → cools
• Sinking air:
  • compresses → warms
• Adiabatic processes are important for atmospheric stability and cloud formation.

3. Solar and Terrestrial Radiation

Solar Radiation

• Radiation received from the Sun is primarily shortwave radiation.
• Most solar radiation reaching Earth’s surface has wavelengths less than 2 μm. The Sun’s peak emission is near 0.5 μm, in the visible region.
• Solar radiation is distributed approximately as:

Terrestrial Radiation

• The Earth is much cooler than the Sun.
• Earth emits predominantly longwave infrared/terrestrial radiation. approximately 4–80 μm for terrestrial radiation.
• The Earth’s surface continuously emits longwave radiation.

Easy comparison

4. Wien’s Law

Principle

Wien’s displacement law:$$\lambda_{max}\propto\frac{1}{T}$$

where:

• $\lambda_{max}$ = wavelength of maximum emission
• $T$ = absolute temperature in Kelvin

Remember

• Temperature increases → maximum-emission wavelength becomes shorter.
• Temperature decreases → maximum-emission wavelength becomes longer.
• Therefore:
  • Hot Sun → shortwave
  • Cool Earth → longwave
• Sun’s maximum emission is approximately 0.5 μm.

5. Stefan–Boltzmann Law

The total radiant energy emitted by a body depends strongly on its absolute temperature.$$E\propto T^4$$

or$$E=\sigma T^4$$

where:

• $E$ = emitted radiant energy
• $T$ = absolute temperature in Kelvin
• $\sigma$ = Stefan–Boltzmann constant

Important points

• All objects above absolute zero emit radiation.
• A hotter body emits radiation at a greater rate/intensity.
• Radiation intensity increases with the fourth power of absolute temperature.
• Therefore, the hot Sun emits much more intensely than the cooler Earth.

Exam memory:
Wien → wavelength
Stefan-Boltzmann → intensity/energy

6. Insolation and Heating of the Atmosphere

Insolation

• Insolation refers to incoming solar radiation absorbed by the Earth’s surface.
• Solar shortwave radiation passes through the atmosphere relatively easily.
• The Earth’s surface absorbs this radiation and becomes heated.

How the atmosphere is heated

Sun → Earth’s surface → atmosphere

1. Solar shortwave radiation reaches the surface.
2. Surface absorbs solar energy and heats up.
3. Heat is transferred to adjacent air by conduction.
4. Convection transports heat upward.
5. Earth’s longwave radiation is absorbed by atmospheric gases.
6. Condensation releases latent heat, further warming the atmosphere.

Thus, the atmosphere/troposphere is mainly heated from the ground upward.

Factors controlling insolation

The amount of insolation received depends on:

• Sun’s angular elevation
• Latitude
• Season
• Time of day.

7. Greenhouse Effect

• Water vapour is a selective absorber of radiation.
• It allows some wavelengths to pass while absorbing others.
• Water vapour and CO₂ are relatively transparent to incoming shortwave solar radiation.
• They absorb part of the Earth’s outgoing longwave terrestrial radiation.
• Some of this absorbed energy is re-radiated toward the Earth’s surface.
• This process contributes to the greenhouse effect and helps maintain Earth’s temperature.

8. Sensible Heat and Latent Heat

Heat transferred from the Earth’s surface to the atmosphere occurs through sensible and latent heat.

Latent heat

• Associated mainly with:
  • evaporation
  • condensation
  • sublimation
• Evaporation:
  • absorbs heat from the surface
  • stores it as latent energy in water vapour
• Condensation:
  • releases the stored latent heat into the atmosphere.
• The source states that latent heat accounts for approximately 77% of the heat transferred from the Earth’s surface to the atmosphere.

Sensible heat

• The source gives approximately 23%.
• Associated with processes such as conduction, convection and terrestrial-radiation-related heating.

9. Specific Heat

Definition

Specific heat = amount of heat required to raise the temperature of a substance by 1°C or 1 K.

Relationship

$$\Delta T\propto\frac{1}{\text{Specific Heat}}$$

Therefore:

• Low specific heat → heats quickly and cools quickly
• High specific heat → heats slowly and cools slowly

Land vs Water

The source gives:

• Water ≈ 1 cal/gm/K
• Dry soil ≈ 0.2 cal/gm/K.

Increasing specific heat according to the source

1. Bare rock/stone
2. Concrete
3. Dry soil
4. Wet soil
5. Oceans
6. Snow surfaces

10. Land–Sea Temperature Contrast

Because land has lower specific heat:

Summer — Northern Hemisphere

• Continents heat rapidly.
• Continents become warmer than oceans at corresponding latitudes.

Winter — Northern Hemisphere

• Continents lose heat rapidly.
• Continents become colder than oceans at corresponding latitudes.

Result

• Continental interiors experience larger annual temperature ranges.
• Oceans act as large heat reservoirs.
• Coastal areas are influenced by the thermal characteristics of the land/water on their windward side.

11. Diurnal Variation of Temperature

Definition

$$\text{Diurnal Variation} = T_{max}-T_{min}$$

It is the difference between the maximum and minimum temperature during a 24-hour period.

Greatest diurnal variation

Occurs with:

• Land surfaces
• Particularly arid/high desert regions
• Clear skies
• Calm or light winds
• Dry surfaces/air

Reasons:

• Low specific heat of land → rapid daytime heating and nighttime cooling.
• Clear skies → greater solar heating during daytime.
• Clear skies → greater terrestrial radiation loss at night.
• Dry air supports strong radiational cooling.

Least diurnal variation

Occurs over:

• Large water bodies
• Sea
• Ice caps, particularly with cloudy/windy conditions

The source gives sea-surface diurnal variation as generally less than 1°C.

12. Effect of Wind on Diurnal Variation

Wind causes turbulent mixing between surface air and air above it.

During daytime

• Surface air becomes warm.
• Wind mixes it with cooler air aloft.
• Maximum temperature decreases.

During nighttime

• Surface air becomes cold.
• Wind mixes cold surface air with warmer air aloft.
• Minimum temperature increases.

Therefore:

Wind decreases the diurnal temperature range.

Greatest DV

Clear skies + calm/light winds.

13. Effect of Clouds on Diurnal Variation

Clouds reduce the diurnal temperature range in two ways.

Day

• Clouds reflect some incoming solar radiation back toward space.
• Less solar energy reaches the surface.
• Tmax decreases.

Night

• Clouds absorb outgoing terrestrial longwave radiation.
• They re-radiate part of this energy toward the surface.
• Radiational cooling is reduced.
• Tmin increases.

Overall

$$\boxed{\text{Clouds → lower Tmax + higher Tmin → smaller DV}}$$

Therefore:

• Cloudy nights are warmer than clear nights under otherwise similar calm conditions.
• Clear nights allow more terrestrial radiation to escape directly into space.
• Greatest DV → clear skies + calm conditions.
• Least DV → cloudy/windy conditions, particularly over sea/ice surfaces.

14. Radiation Cooling

Clear, calm night

1. Ground continuously emits terrestrial longwave radiation.
2. With clear skies, much of this radiation escapes to space.
3. Ground temperature falls rapidly.
4. Air touching the cold ground is cooled by conduction.
5. Because air is a poor conductor, cooling initially remains concentrated near the surface.

This is called radiational cooling.

Important

• Hotter surfaces emit radiation at a greater rate.
• Emission rate follows:

$$E\propto T^4$$

• The hotter the ground, the greater its radiational energy loss rate.

15. Radiation Temperature Inversion

A radiation inversion develops during clear, calm nights.

Formation sequence

Clear sky + calm wind
↓
Strong terrestrial radiation from ground
↓
Ground cools rapidly
↓
Air touching ground cools by conduction
↓
Cold air remains near surface
↓
Temperature increases with height
↓
Radiation inversion

Characteristics

• Coldest air is immediately next to the ground.
• Temperature increases with height within the inversion.
• It is a stable condition.
• The inversion is generally strongest near sunrise.
• Above the inversion, temperature normally resumes decreasing with height, with the source giving an average ELR of approximately 2°C/1000 ft.

16. Temperature Inversion

Definition

An inversion occurs when:$$\boxed{\text{Temperature increases with height}}$$

This reverses the normal atmospheric temperature decrease with altitude.

Characteristics

• Represents very stable/absolute stable conditions.
• Vertical mixing is inhibited.
• Acts like a lid.
• Can trap:
  • pollutants
  • low clouds
  • moisture
    beneath the inversion.

Types mentioned

• Radiation/ground inversion
  • Clear, calm nights
  • Caused by surface radiational cooling
• Subsidence inversion
  • Associated with sinking air in high-pressure systems
  • Sinking air undergoes compressional warming.

17. Isothermal Layer

Definition

An isothermal layer is a layer in which:$$\boxed{\text{Temperature remains constant with height}}$$

Therefore:$$\boxed{\text{Lapse rate}=0}$$

Characteristics

• Stable atmospheric condition.
• Strongly resists vertical movement.
• Isothermal: temperature constant with height.
• Inversion: temperature increases with height.

ISA

The source states that in the ISA:

• Temperature remains approximately −56.5°C
• From 11 km / 36,090 ft
• Up to 20 km / 65,617 ft
• The lower boundary is the tropopause.

18. Temperature Lapse Rate

The source gives the average tropospheric environmental lapse rate as approximately:$$\boxed{2^\circ C/1000\ ft}$$

More specifically, one entry gives approximately:$$1.98^\circ C/1000\ ft$$

Compare

19. Daily Maximum and Minimum Temperature

Minimum temperature

• Usually occurs approximately 30 minutes to 1 hour after sunrise.
• Cooling continues after sunrise because outgoing terrestrial radiation can initially remain greater than incoming solar radiation.
• This is a result of the temperature/radiation lag effect.

Maximum temperature

• The source states approximately 1500 LMT.
• For India, the source notes approximately 1400.

The maximum does not occur exactly at noon because surface heating continues after maximum solar elevation due to the lag effect.

20. Coastal Temperature Variation

At coastal aerodromes, wind direction is particularly important to temperature variation because of alternating land and sea breezes.

Day

• Sea breeze blows onshore.
• Brings cooler maritime air.
• Moderates the maximum temperature.

Night

• Land breeze blows offshore.
• Transports continental air toward the sea.

Key point

• Onshore flow from cooler water tends to minimize the diurnal range.
• Calm conditions generally allow greater diurnal variation.
• Stronger wind/mixing tends to reduce the range.

21. Temperature Measurement

Radiosonde

For upper-air temperature observations:

• A radiosonde is a balloon-borne instrument.
• It continuously transmits:
  • temperature
  • pressure
  • humidity
• As it ascends, it provides a vertical atmospheric profile called a sounding.

Aircraft temperature

Aircraft outside-air-temperature measurements can be affected by:

• aerodynamic/compressibility heating
• lag effects.

Tephigram

• Used to plot and analyse radiosonde data.
• It is not itself a temperature-measuring instrument.

22. Stevenson Screen

Purpose

A Stevenson screen is a louvred wooden shelter used to house meteorological thermometers.

Functions

• Protects instruments from direct solar radiation.
• Protects against precipitation.
• Allows adequate air circulation.
• Helps obtain a representative ambient air-temperature measurement.
• Keeps instruments away from direct influence of ground heating.

Height

Approximately:$$\boxed{4\ ft\approx1.22-1.25\ m}$$

above the ground.

Instruments

The source mentions:

• Mercury thermometers
• Wet-bulb thermometers.

Door orientation

The source states that the door is normally arranged away from direct sunlight, often toward the North in the Northern Hemisphere, so direct sunlight does not strike the instruments during observation.

23. Minimum Thermometer

• A minimum thermometer uses alcohol.
• Alcohol has a much lower freezing point than mercury.
• Alcohol freezing point given in the source:

$$-130^\circ C\approx-202^\circ F$$

• Mercury freezing point:

$$-39^\circ C\approx-38^\circ F$$

Therefore, alcohol is suitable for measuring very low temperatures.

Function

• Records the lowest temperature reached during a period.

24. Temperature Scales

Celsius and Fahrenheit

Conversion

$$F=\frac95C+32$$$$C=(F-32)\frac59$$

Important fixed points

Difference between freezing and boiling points:$$212-32=180^\circ F$$

Coincidence point

$$\boxed{-40^\circ C=-40^\circ F}$$

This is the only point where Celsius and Fahrenheit scales have the same numerical value.

25. Kelvin Scale

The Kelvin scale is the absolute temperature scale.

Conversion

$$\boxed{K=^\circ C+273}$$

Important values

Kelvin and Celsius have equal-sized temperature intervals, but their zero points are different.

26. Temperature Conversion Examples

Fahrenheit → Kelvin

First:$$C=(F-32)\times\frac59$$

Then:$$K=C+273$$

Example: 68°F

$$C=(68-32)\times\frac59=20^\circ C$$$$K=20+273=293K$$

Therefore:$$\boxed{68^\circ F=293K}$$

Example: 82°F

$$C=(82-32)\times\frac59\approx27.8^\circ C$$$$K=27.8+273=300.8K$$

The source notes that this may be treated as approximately 300 K depending on the available options.

27. Dew Point and Condensation Level

Dew Point

The dew point is the temperature to which air must be cooled, at constant pressure and water-vapour content, for saturation to occur.

At dew point:

• Air becomes saturated.
• Relative humidity reaches 100%.
• Further cooling can result in condensation.

Rising air

• Unsaturated air rising upward cools adiabatically.
• When the parcel cools to its dew point, saturation occurs.
• The level at which this happens is the condensation level.

Importance

• Dew point gives an indication of the air’s actual water-vapour content.

28. Albedo

Definition

Albedo is the reflecting power of a surface.

It is the ratio/percentage of:$$\frac{\text{reflected solar radiation}} {\text{incoming solar radiation}}$$

Earth

• Average combined albedo of Earth and atmosphere, including clouds: approximately 30%.

Snow

• Very high albedo.
• Source gives approximately 75–95% for fresh snow.
• Approximately 80% is given as a representative value.

Effect of high albedo

• More solar radiation is reflected.
• Less solar energy is absorbed.
• Surface heating is reduced.
• Snow therefore contributes to very cold surface temperatures.

29. Clear-Day Solar Transmission

On a clear day:

• The atmosphere is relatively transparent to incoming shortwave solar radiation.
• A high proportion reaches the Earth’s surface.
• The source gives approximately 85%, or about 5/6, reaching the surface under clear conditions.

This incoming solar energy heats the surface and is referred to as insolation.

30. Snow, Vegetation and Surface Type

Surface characteristics strongly influence temperature variation.

Bare rock/concrete

• Low specific heat.
• Heat rapidly.
• Cool rapidly.
• Large temperature variation.

Water

• High specific heat.
• Slow heating and cooling.
• Small diurnal variation.

Vegetation

• Smaller temperature range than barren surfaces.
• Some available energy is used for evaporation of water rather than directly heating the air.

Snow

• High reflectivity/albedo.
• Considerable energy is associated with melting.
• Therefore, temperature variation is relatively small.

31. High-Yield Comparison Tables

Radiation Laws

Memory trick

Wien = Wavelength
Stefan = Strength/Intensity

Cloud vs Clear Sky

Wind vs Calm

Land vs Sea

32. Must-Remember Numerical Values

33. Quick Exam Revision — One-Liners

• Wien’s law: Hotter body → shorter wavelength of maximum radiation.
• Stefan-Boltzmann: Radiation intensity $\propto T^4$.
• Sun: Mainly shortwave radiation.
• Earth: Mainly longwave terrestrial radiation.
• Insolation: Solar radiation absorbed by Earth’s surface.
• Atmosphere: Mainly heated from the ground upward.
• Air: Poor conductor of heat.
• Conduction: Molecular/direct-contact heat transfer.
• Convection: Heat transfer by mass movement of fluid.
• Latent heat: Energy involved in change of state without temperature change.
• Dew point: Temperature at which air becomes saturated.
• Condensation level: Level where rising air cools to its dew point.
• Specific heat: Heat required to raise temperature by 1°C/1 K.
• Low specific heat: Rapid heating and cooling.
• Land: Low specific heat → large temperature change.
• Water: High specific heat → small temperature change.
• Maximum DV: Land, especially high deserts, clear skies and calm/light winds.
• Minimum DV: Sea/water surfaces.
• Wind: Reduces DV.
• Clouds: Reduce DV.
• Clouds by day: Lower Tmax.
• Clouds by night: Raise Tmin.
• Clear calm night: Strong radiational cooling.
• Radiation inversion: Temperature increases with height.
• Inversion: Stable atmosphere.
• Isothermal: Temperature constant with height.
• Isothermal lapse rate: Zero.
• Average tropospheric ELR: About 2°C/1000 ft.
• Radiosonde: Upper-air temperature, pressure and humidity measurements.
• Tephigram: Used to plot/analyse radiosonde data.
• Stevenson screen: Protects thermometers and permits ventilation.
• Stevenson screen height: About 4 ft / 1.25 m.
• Minimum thermometer: Alcohol.
• Kelvin: Absolute temperature scale.
• C → K: $K=C+273$.
• C → F: $F=\frac95C+32$.
• F → C: $C=(F-32)\frac59$.
• −40°C = −40°F.
• Snow: High albedo → strong reflection → low absorption/heating.
• Earth + atmosphere albedo: About 30%.
• Water vapour: Selectively absorbs longwave terrestrial radiation.
• Greenhouse effect: Atmospheric absorption/re-radiation of terrestrial longwave energy.