Insolation and Heat Budget Temperature



• The only source of energy for the earth’s atmosphere comes from the sun which has a surface temperature of more than 10,800 degree F. This energy travels through space for a distance of 93 million miles and reaches us as solar energy or radiant energy in the process called Insolation.
• Only that part of the sun’s radiation which reaches the earth is called Insolation. It is the amount of solar energy reaching the earth’s surface per unit time per sq. cm.
• Insolation is measured with the help of Pyranometer.

Variability in Insolation
• The amount of Insolation received on any date at a place on earth is influenced by the following factors:

the rotation of earth on its axis
– the angle of inclination of the sun’s rays
– the length of the day
– the transparency of the atmosphere
– The configuration of land in terms of its aspect.

• The Earth’s axis makes an angle of 66 with the plane of its orbit round the sun, has a greater influence on the amount of Insolation received at different latitudes.
• The second factor that determines the amount of Insolation received is the angle of inclination of the rays. This depends on the latitude of a place.

• The higher the latitude the less is the angle they make with the surface of the earth resulting in slant sun rays. The area covered by vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases.
• Moreover, the slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion.
• For the year as a whole Insolation is greatest at the equator and decreases towards the poles
• Insolation shows the least variation throughout the year at the equator.
• The diurnal range of temperature is the highest in the hot deserts. (Due to clear skies and high radiation).
• The Summer Solstice and the Winter Solstice fall on 21st June and 23rd December, respectively.
• The Autumn Equinox and the Spring Equinox fall on 23rd September and 21st March, respectively.

Passage of Solar Radiation through the Atmosphere

• The radiation from the sun is made up of three parts, the visible ‘white’ light that we see when the sun shines and the less visible ultra-violet and infra-red rays. The visible ‘white’ light is the most intense and has the greatest influence on our climate. The ultra -violet rays affect our skin and cause sun-burn when our bare body is exposed to them for too long a period. The infra-red rays can penetrate even dust and fog and are widely used in photography.
• The atmosphere is largely transparent to short wave solar radiation. The incoming solar radiation passes through the atmosphere before striking the earth’s surface.
• The earth’s surface receives most of its energy in short wavelengths.
• Within the troposphere water vapor, ozone and other gases absorb much of the near infrared radiation.
• Very small-suspended particles in the troposphere scatter visible spectrum both to the space and towards the earth surface. This process adds color to the sky. The red color of the rising and the setting sun and the blue color of the sky are the result of scattering of light within the atmosphere.

Spatial Distribution of Insolation at the Earth’s Surface
• The Insolation received at the surface varies from about 320 Watt/m2 in the tropics to about 70 Watt/m2 in the poles.
• Maximum insolation is received over the subtropical deserts, where the cloudiness is the least.
• Generally, at the same latitude the insolation is more over the continent than over the oceans.
• Equator receives comparatively less insolation than the tropics.
• In winter, the middle and higher latitudes receive less radiation than in summer.

Heating and Cooling Of Atmosphere
• There are different ways of heating and cooling of the atmosphere. The earth after being heated by Insolation transmits the heat to the atmospheric layers near to the earth in long wave form.

• Conduction: The air in contact with the land gets heated slowly and the upper layers in contact with the lower layers also get heated. This process is called conduction. Conduction is important in heating the lower layers of the atmosphere

• Convection: The air in contact with the earth rises vertically on heating in the form of currents and further transmits the heat of the atmosphere. This process of vertical heating of the atmosphere is known as convection. The convective transfer of energy is confined only to the troposphere

• Advection: The transfer of heat through horizontal movement of air is called advection. Horizontal movement of the air is relatively more important than the vertical movement. In middle latitudes, most of diurnal (day and night) variation in daily weather is caused by advection alone. In tropical regions particularly in northern India during summer season local winds called ‘loo’ is the outcome of advection process

• Terrestrial Radiation – The Insolation received by the earth is in short waves forms and heats up its surface. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in long wave form. This energy heats up the atmosphere from below. This process is known as terrestrial radiation.

• Green House Gases – The long wave radiation is absorbed by the atmospheric gases particularly by carbon dioxide and the other green house gases. Thus, the atmosphere is indirectly heated by the earth’s radiation. The atmosphere in turn radiates and transmits heat to the space. Finally the amount of heat received from the sun is returned to space, thereby maintaining constant temperature at the earth’s surface and in the atmosphere.

Heat budget of the Earth
• The earth as a whole does not accumulate or loose heat. It maintains its temperature.
• This can happen only if the amount of heat received in the form of Insolation equals the amount lost by the earth through terrestrial radiation.
• Our earth is heated by the process of radiation. Radiation is the means by which solar radiation reaches the earth and the earth loses energy to outer space. T
• The global radiation has three major components:

a) Incoming short wave solar radiation.
b) The planetary Albedo.
c) Outgoing long wave radiation from the earth’s surface to the space.


Incoming shortwave solar radiation: equals to 100 units

a)      Amount lost to space through scattering and reflection equals to 35% comprises of

·         Clouds = 27%

·         Reflected by ground = 2%

·         Scattered by dust particles = 6%

b)      Heat received by earth equals to 51% comprises of

·         Through direct radiation = 34%

·         Received as diffuse day light = 17%

c)       Absorption by the atmospheric gases and water vapour equals to 14%

Outgoing long-wave terrestrial radiation

 a)      Reflected by earth which was equal to 51 per cent as shown above

·         23% from radiation

·         9% through convection

·         19% through evaporation

b)      48% absorbed in atmosphere moved through radiation back into space.

• Albedo is the reflective quality of a surface with respect to solar radiation.
• Smooth surface increase albedo, whereas rough surface reduce it.
• The albedo of the earth is approximately 0.4 i.e. about 40 % of solar radiation is reflected back into space.
• Albedo is higher for a snow covered surface, and low for dark soil.
• Tropics (23 ½°N and 23 ½°S)- Albedo is between 19 to 38 per cent
• Polar Regions – Albedo as high as 80%.

Variation in the Net Heat Budget at the Earth’s Surface

• There are variations in the amount of radiation received at the earth’s surface. Some part of the earth has surplus radiation balance while the other part has deficit.
• Figure depicts the latitudinal variation in the net radiation balance of the earth – the atmosphere system.

• There is a surplus of net radiation balance between 40 degrees north and south and the regions near the poles have a deficit.
• The surplus heat energy from the tropics is redistributed pole wards and as a result the tropics do not get progressively heated up due to the accumulation of excess heat or the high latitudes get permanently frozen due to excess deficit.

Latitudinal Variations
• Although the earth and its atmosphere as a whole have a radiation balance, there are latitudinal variations. The heat/energy is transferred from the lower latitudes to the higher latitudes through winds and ocean currents.
• In the low latitudes (between 40 N and 40 S) heat gained by short wave radiation is far more than the heat loss by long waves through the earth’s radiation. In contrast in the higher latitudes more heat is lost by outgoing long wave than it is received in short waves.
• In view of the imbalances at high and low latitudes, there is large-scale transfer of heat from tropics to high latitudes by atmospheric and oceanic circulation.



• The interaction of Insolation with the atmosphere and the earth’s surface creates heat which is measured in terms of temperature.
• While heat represents the molecular movement of particles comprising a substance, the temperature is the measurement in degrees of how hot (or cold) a thing (or a place) is.

Factors Controlling Temperature Distribution
The temperature of air at any place is influenced by

(i) The latitude of the place
(ii) The altitude of the place
(iii) Distance from the sea
(iv) The air mass circulation
(v) The presence of warm and cold ocean currents
(vi) Local aspects

Distribution of Temperature
• The global distribution of temperature can well be understood by studying the temperature distribution in January and July.
• The temperature distribution is generally shown on the map with the help of isotherms.
• The Isotherms are lines joining places having equal temperature

Inversion of temperature

• In the troposphere, the temperature decreases from the earth’s surface up to the Tropopause with increasing altitude. This decrease in temperature is known as normal lapse rate.
• The normal lapse rate is 6.4°C per kilometer. But sometimes, under special circumstances, the decrease in temperature with altitude is reversed and the temperature increase with increasing altitude. This phenomenon is known as inversion of temperature. Inversion of temperature generally occurs in valleys.
• Conditions for the Inversion of Temperatures:

a) In winters, on calm and clear nights.
b) When the sky is clear and anticyclone condition prevails.
c) When the earth surface is covered with ice. Snow and frost.

Significance of Temperature Inversion

The inversion of temperature has a direct bearing on weather, climate, economy and society. Some of the influences of inversion of temperature are as under:

• The weather phenomena like, formation of clouds, atmospheric visibility and precipitation are
• The diurnal range of temperature
• The vertical distribution of temperature
• The equable distribution of moisture in the lower level of the atmosphere is affected.
• At the occurrence of dense fog, the sensitive crops, especially citrus fruits, vegetables, sugarcane, wheat and other cereal crops are damaged.
• The air-traffic is also affected by the inversion of temperature. The density, speed, results in the formation of bumpiness in certain layers of the atmosphere. A surface inversion of temperature makes air navigation difficult and hazardous.
• The reduced visibility at the occurrence of dense fog due to inversion of temperature may be hazardous for road and rail traffics.
• The daily life of people especially in the temperate latitudes is adversely affected.