Solar Radiation, Heat Balance And Temperature

The Earth receives most of its energy from the Sun. This energy reaches the Earth mainly as shortwave radiation.

The incoming solar radiation received by the Earth is called insolation.

Since the Earth is a geoid or oblate spheroid, the Sun’s rays do not fall equally everywhere. They fall at different angles at different latitudes, causing unequal heating of the Earth’s surface.

Solar Constant

The average amount of solar energy received at the top of the atmosphere is about 1.94 calories per sq. cm per minute.

This is called the solar constant.

Aphelion And Perihelion

The Earth’s distance from the Sun changes during its revolution.

Aphelion

On 4 July, the Earth is farthest from the Sun.

The distance is about 152 million Km.

This position is called Aphelion.

Perihelion

On 3 January, the Earth is nearest to the Sun.

The distance is about 147 million Km.

This position is called Perihelion.

Therefore, the annual insolation received on 3 January is slightly greater than the amount received on 4 July.

Factors Affecting Insolation

The amount of insolation received at the Earth’s surface depends on several factors.

Rotation And Tilt Of Earth

The Earth’s axis is tilted at 23.5° and makes an angle of 66.5° with the plane of its orbit around the Sun.

This tilt influences the amount of insolation received at different latitudes.

Maximum insolation occurs in the Northern Hemisphere around 21 June, known as the Summer Solstice.

Angle Of Inclination Of Sun’s Rays

The Earth is a geoid or oblate spheroid.

The Sun’s rays strike the Earth’s surface at different angles at different latitudes.

The intensity of insolation decreases with an increase in latitude.

At higher latitudes, the Sun’s rays are more slanting.

Vertical rays cover a smaller area and provide more energy per unit area.

Slanting rays cover a larger area and distribute the same energy over a wider surface. As a result, the net energy per unit area decreases.

Atmospheric Composition

The more transparent the atmosphere, the greater the insolation received at the surface.

The equatorial region receives comparatively less insolation than the tropics because of heavy cloud cover.

Maximum insolation is received over subtropical deserts, where cloudiness is minimal.

Configuration Of Land

At the same latitude, insolation is generally greater over continents than over oceans.

This happens because land surfaces heat up faster and absorb radiation more quickly than water bodies.

The Passage Of Solar Radiation Through The Atmosphere

The atmosphere is largely transparent to shortwave solar radiation.

Incoming solar radiation passes through the atmosphere before striking the Earth’s surface.

Within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation.

Very small suspended particles in the troposphere scatter the visible spectrum both into space and toward the Earth’s surface.

This scattering adds colour to the sky.

The red colour of the rising and setting Sun and the blue colour of the sky are caused by scattering of light within the atmosphere.

Heating And Cooling Of The Atmosphere

The atmosphere is mainly heated from below.

The Earth’s surface absorbs solar radiation and then emits longwave terrestrial radiation.

This terrestrial radiation heats the lower atmosphere.

The main processes of heat transfer are:

• Conduction
• Convection
• Radiation
• Advection

Conduction

Conduction is heat transfer through direct contact.

The air touching the warm land surface becomes heated first.

Convection

Convection is vertical transfer of heat through rising and sinking air currents.

Warm air rises and transfers heat upward.

Radiation

Radiation is the transfer of energy without direct contact.

Solar radiation reaches the Earth, and the Earth sends back longwave terrestrial radiation.

Advection

Advection is horizontal transfer of heat by moving air.

In northern India, the hot summer wind Loo is an example of advection.

Lapse Rate

The rate of change of temperature with altitude is called the lapse rate.

When temperature decreases with altitude, the lapse rate is considered positive.

The average rate of temperature decrease is 6.5°C per 1000 m.

This is called the normal lapse rate.

The lapse rate is zero when temperature remains constant with height.

It becomes negative when temperature increases with height. This condition is called temperature inversion.

Environmental Lapse Rate

The Environmental Lapse Rate, or ELR, is the lapse rate of non-rising air.

It varies due to:

• Radiation
• Convection
• Condensation

Temperature usually decreases with altitude because:

• Atmospheric pressure decreases with height, causing air to expand and cool.
• Greenhouse gas concentration decreases with height, reducing heat absorption.

Adiabatic Lapse Rate

The Adiabatic Lapse Rate is the rate of temperature change in a rising or falling air parcel without exchange of heat with its surroundings.

In this process, heat neither enters nor leaves the air parcel.

Adiabatic lapse rate is governed by gas law, where pressure is directly proportional to temperature when volume is constant.

A parcel of air rises when it is less dense than the surrounding air.

It sinks when it becomes denser than the surrounding air.

When an air parcel receives more heat than the surrounding air, its temperature increases. It expands, becomes lighter and begins to rise.

As it rises, atmospheric pressure decreases. This causes the air parcel to expand and cool.

This cooling without heat exchange is called adiabatic cooling.

The decrease in temperature of a rising air parcel is called Adiabatic Temperature Lapse.

The rate is called Adiabatic Lapse Rate.

The rising of air parcels is the first step in the formation of:

• Thunderstorms
• Cyclones
• Tornadoes

When the air parcel cools sufficiently or becomes denser than the surrounding air, it descends toward lower levels of the troposphere.

During descent, atmospheric pressure increases. This compresses the air parcel and raises its temperature adiabatically.

A katabatic wind is a cold downslope wind that flows down mountain slopes due to gravity.

Dry Adiabatic Lapse Rate

The Dry Adiabatic Lapse Rate, or DALR, is the rate of temperature decrease of unsaturated rising air.

For dry air, the lapse rate is about 9.8°C per 1000 m.

It is often approximated as 10°C per Km.

Dry adiabatic lapse rate is generally associated with stable atmospheric conditions.

Wet Or Moist Adiabatic Lapse Rate

When a saturated air parcel rises, condensation occurs.

During condensation, latent heat of condensation is released.

This heat partly offsets cooling, so the air cools more slowly than dry air.

The Moist Adiabatic Lapse Rate varies depending on moisture content.

Its average value is about 4°C to 6°C per 1000 m.

Wet adiabatic lapse rate is associated with unstable atmospheric conditions.

Weather Conditions At Different Adiabatic Lapse Rates

The difference between normal lapse rate, dry adiabatic lapse rate and moist adiabatic lapse rate determines the vertical stability of the atmosphere.

Absolute Stability

When Environmental Lapse Rate is less than Moist Adiabatic Lapse Rate, the atmosphere becomes absolutely stable.

In this condition, the rising air parcel cools faster than the surrounding air.

It becomes denser than the surrounding air and sinks back toward the surface.

Cloud formation is minimal and rainfall is unlikely.

Conditional Stability

When Moist Adiabatic Lapse Rate is less than Environmental Lapse Rate, but Environmental Lapse Rate is less than Dry Adiabatic Lapse Rate, the atmosphere becomes conditionally unstable.

In this condition, the atmosphere may remain stable for unsaturated air but unstable for saturated air.

If condensation occurs, cloud formation and thunderstorms may develop.

Rainfall may or may not occur depending on moisture availability.

Absolute Instability

When Environmental Lapse Rate is greater than Dry Adiabatic Lapse Rate, the atmosphere becomes absolutely unstable.

In this condition, the rising air parcel remains warmer than the surrounding air.

It continues to rise, causing strong convection.

This condition often produces thunderstorms, heavy rainfall and strong atmospheric disturbances.

Terrestrial Radiation

The insolation received by the Earth is mainly in shortwave form and heats the Earth’s surface.

After being heated, the Earth becomes a radiating body.

It emits energy in the form of longwave radiation.

This outgoing energy is called terrestrial radiation.

Terrestrial radiation heats the lower atmosphere from below.

The atmosphere also radiates energy back to space, maintaining a balance between incoming and outgoing radiation.

Heat Budget

Heat Budget means the estimate of heat received from the Sun and heat radiated back into space.

If the total insolation received at the top of the atmosphere is taken as 100 units, then part of it is reflected, scattered and absorbed.

Out of 100 units:

• 35 units are reflected back to space before reaching the Earth’s surface.

This reflected radiation is called albedo.

Out of these 35 units:

• 6 units are scattered into space.
• 27 units are reflected by cloud tops.
• 2 units are reflected by snow and ice-covered areas.

The remaining 65 units are absorbed.

Out of these 65 units:

• 14 units are absorbed by the atmosphere.
• 51 units are absorbed by the Earth’s surface.

The Earth radiates back 51 units as terrestrial radiation.

Out of this:

• 17 units are radiated directly into space.
• 34 units are absorbed by the atmosphere.

The 34 units absorbed by the atmosphere include:

• 6 units absorbed directly by the atmosphere.
• 9 units through convection and turbulence.
• 19 units through latent heat of condensation.

The atmosphere absorbs a total of 48 units:

• 14 units from incoming solar radiation.
• 34 units from terrestrial radiation.

These 48 units are radiated back into space.

Total radiation returning to space:

• 17 units directly from Earth
• 48 units from atmosphere

Total = 65 units

This balances the 65 units absorbed from the Sun.

This balance forms the heat budget of the Earth.

Albedo

Albedo is the percentage of solar radiation reflected back into space by an object or surface.

It is related to shortwave radiation.

It ranges between 0 and 1.

The average albedo of the Earth is about 0.3.

This means:

• About 30% of incoming solar radiation is reflected back into space.
• About 70% is absorbed.

Snow and ice have high albedo.

Variation In The Net Heat Budget At The Earth’s Surface

Radiation received at the Earth’s surface varies from place to place.

Some parts of the Earth experience a surplus radiation balance.

Other parts experience a deficit radiation balance.

The tropics receive surplus heat energy.

This surplus heat is redistributed toward the poles.

This prevents:

• Tropics from becoming excessively hot.
• High latitudes from remaining permanently frozen.

Temperature

The interaction of insolation with the atmosphere and the Earth’s surface produces heat.

This heat is measured as temperature.

Factors Controlling Temperature Distribution

The main factors controlling temperature distribution are:

• Latitude
• Altitude
• Distance from sea
• Air mass
• Ocean currents

Latitude

Temperature generally decreases from the equator toward the poles.

This is because the angle of the Sun’s rays becomes smaller with increasing latitude.

At higher latitudes, rays are more slanting and spread over a larger area.

Altitude

Temperature decreases with height.

The atmosphere is mainly heated from the Earth’s surface through terrestrial radiation.

So, higher places are cooler than lower places.

Example:

• Shimla and Ludhiana lie at nearly the same latitude, but Shimla is cooler because it is at a higher altitude.

Distance From Sea

Land heats and cools faster than water.

Water heats and cools slowly.

Therefore, coastal areas have moderate temperatures, while interior continental areas have more extreme temperatures.

Air Mass And Ocean Currents

Warm air masses increase temperature.

Cold air masses decrease temperature.

Warm ocean currents raise coastal temperatures.

Cold ocean currents reduce coastal temperatures.

Distribution Of Temperature

Temperature distribution is shown with the help of isotherms.

Isotherms

Isotherms are lines joining places having equal temperature.

In the Northern Hemisphere, land area is larger than in the Southern Hemisphere.

Therefore, landmasses and ocean currents strongly influence the pattern of isotherms in the Northern Hemisphere.

January Temperature Distribution

In January, isotherms deviate northward over oceans and southward over continents.

In the North Atlantic Ocean, warm ocean currents such as the Gulf Stream and North Atlantic Drift make the ocean warmer.

Because of this, isotherms bend northward.

Over land, temperature decreases sharply and isotherms bend southward in Europe.

This effect is strongly pronounced in the Siberian Plain.

The mean January temperature along 60°E longitude is about -20°C at both 80°N and 50°N latitudes.

The mean monthly temperature for January is:

• Above 27°C in equatorial oceans.
• Above 24°C in tropical regions.
• Between 20°C and 0°C in middle latitudes.
• Between -18°C and -48°C in the Eurasian continental interior.

In the Southern Hemisphere, the influence of oceans is more pronounced.

Here, isotherms are more or less parallel to latitudes.

The variation in temperature is more gradual compared to the Northern Hemisphere.

The 20°C, 10°C and 0°C isotherms roughly follow 35°S, 45°S and 60°S latitudes respectively.

July Temperature Distribution

In July, isotherms generally run parallel to latitudes.

Equatorial oceans record temperatures above 27°C.

Over land, temperatures of more than 30°C occur in subtropical continental regions of Asia around 30°N latitude.

Along 40°N, the 10°C isotherm is observed.

Around 40°S, temperatures are about 10°C.

Inversion Of Temperature

Normally, temperature decreases with increasing elevation. This is called the normal lapse rate.

When temperature increases with altitude instead of decreasing, it is called temperature inversion.

In this condition:

• Cold air lies near the ground.
• Warm air lies above cold air.

Temperature inversion can occur near the Earth’s surface or in the upper troposphere.

Temperature Inversion In Mountains

Inversion often occurs in hills and mountainous regions due to air drainage.

At night, cold air forms on mountain slopes.

This cold air flows downward under gravity.

Being dense and heavy, it accumulates in valleys and depressions.

Warmer air remains above.

This process is called air drainage.

Air drainage can protect plants from frost damage.

Conditions For Inversion Of Temperature

The main conditions for temperature inversion are:

• Long winter nights
• Clear sky
• Dry air near the ground
• Calm and stable air

Long Winter Nights

Heat absorbed during the day is radiated away at night.

The ground cools faster than the air above.

Temperature inversion is common in polar regions.

Clear Sky

Clouds act like a blanket and prevent heat from escaping.

A clear sky allows greater terrestrial radiation to escape, causing cooling near the surface.

Dry Air Near The Ground

Dry air absorbs less terrestrial radiation.

It allows heat to escape more easily.

Calm And Stable Air

Calm conditions prevent mixing of air layers.

Strong winds mix warm and cold air and destroy the inversion layer.

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