Solar Radiation, Heat Balance, and Temperature

Solar Radiation

Our earth receives almost all its energy from the sun in short wavelengths that are radiated back to space by the earth. As a result, the earth neither warms up nor does it get cooled over a period of time.

The amount of heat received by different parts of the earth is not equal, so there are pressure differences in the atmosphere, which leads to the transfer of heat from one region to another by the wind.

As the earth is a geoid which is resembling a sphere, the sun’s rays fall obliquely at the top of the atmosphere and the earth intercepts a very small portion of the sun’s energy.  On average, the earth receives 1.94 calories per sq cm per minute at the top of its atmosphere.

Insolation: The energy received by the earth is known as incoming radiation which in short termed insolation. The earth’s surface receives most of its energy in short wavelengths.

Aphelion: The solar output received at the top of the atmosphere varies slightly in a year due to the variation in the distance between the earth and the sun. The earth is farthest from the sun (152 million km) during its revolution around the sun on 4th July. This position of the earth is known as aphelion.

Perihelion: The earth is nearest to the sun (147 million km) on 3rd January, this condition is known as perihelion. Therefore, the annual insolation received by the earth on the 3rd of January is slightly more than the amount received on the 4th of July.

  • However, the effect of this variation in the solar output is marked by other factors like the distribution of land and sea and atmospheric circulation.
  • So the variation in the solar output is marked by other factors like the distribution of land and sea and the atmospheric circulation.

This variation in the solar output does not have a great effect on daily weather changes on the surface of the earth.

Variability of Insolation on the Surface of the Earth

The amount and intensity of the insolation vary during the day, in a season, and in a year. The variation in insolation can be marked by a number of factors such as:

  • The rotation of the earth on its axis.
  • The length of the day
  • The angle of inclination of the rays coming from the sun.
  • The transparency of the atmosphere, but has less influence.
  • The configuration of land in terms of its aspect. It also shows less influence.

The Rotation of Earth on its axis:

The axis of the earth makes an angle of 66½ with the plane of its orbit around the sun which has a greater influence on the amount of insolation received at different latitudes.

The Angle of Inclination of the Rays of the Sun:

  • The angle of inclination depends upon the latitude of a place.
  • The higher the latitude, the less the angle they make with the surface of the earth resulting in slant sun rays.
  • The area covered by slant rays is always greater than vertical rays.
  • The slant rays are required to pass through greater depth of the atmosphere resulting in more absorption, scattering, and diffusion.

The Passes of Solar Radiation through the Atmosphere:

  • The atmosphere is largely transparent to short-wave solar radiation.
  • The 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 infrared radiation.

Scattering:

Very small suspended particles in the troposphere scatter visible spectrum both to space and towards the surface of the earth. The process of scattering adds colour to the sky.

  • The red colour of the rising and setting of the sun and the blue colour of the sky are the result of the scattering of light within the atmosphere.

Spatial Distribution of Insolation on the surface of Earth:

The insolation received at the surface of the earth varies from about 320 watts/m² in the tropic to about 70 watts/m² in the poles.

  • The sub-tropical deserts receive maximum insolation where the cloudiness is the least.
  • Tropics receive comparatively more insolation than the Equator.
  • Generally, at the same latitude, the insolation is more over the continent than over the ocean.
  • The middle and higher latitudes receive less radiation in winter than in summer.

Read more about solstice:  https://blogs.nasa.gov/Watch_the_Skies/tag/summer-solstice/

Heating and Cooling of the Atmosphere

Following are the ways of heating and cooling the atmosphere:

  1. Conduction
  2. Convection
  3. Advection
  4. Radiation
Conduction

The earth transmits the heat to the atmospheric layers near the earth in the long waveform after heating. 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.

The conditions for conduction:

  • Conduction takes place when two bodies of unequal temperature come in contact with each other, there is a flow of energy from the warmer to the cooler body.
  • The transfer of heat continues until both bodies attain the same temperature or even the contact is broken.
  • 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 to the atmosphere. This process of vertical heating of the atmosphere is known as convection. This process occurs only in the troposphere.

Advection

The transfer of heat through the horizontal movement of air is called advection.

  • The horizontal movement of air is relatively more important than the vertical movement.
  • In middle latitudes, most diurnal (day and night) variation in daily weather is caused by advection.
  • In tropical regions, particularly in Northern India during the summer season, the local wind called loo is the outcome of the process of advection.
Terrestrial Radiation

Insolation comes in short waveforms and heats up the surface of the earth. The earth after being heated itself becomes a radiating body and it radiates energy to the atmosphere in a long waveform that heats up the atmosphere from below. This process is called terrestrial radiation.

  • Through the process of terrestrial radiation, the long wave radiation is absorbed by the atmospheric gases, particularly by CO2 and other greenhouse gases. The atmosphere in turn radiates and transmits heat to space.
  • Finally, the amount of heat received from the sun is returned to space, thereby maintaining a constant temperature at the earth’s surface and in the atmosphere.
Heat Budget of Earth

A heat budget is a balance between incoming heat absorbed by the earth and outgoing heat escaping it in the form of radiation.

If the balance is disturbed, then the earth would get progressively warmer or cooler with each passing year.

The earth as a whole does not accumulate or lose heat. It maintains its temperature through terrestrial radiation. 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.

The heat budget of the earth can be understood through the following facts:

The albedo of the earth:

  • Out of 100 units of heat received, while passing through the atmosphere some amount of energy is reflected, scattered, and absorbed, and the remaining reaches the earth’s surface.
  • Roughly 35 units are reflected back to space even before reaching the surface of the earth.
  • Of these, 27 units are reflected back from the top of the clouds and 2 units from the snow and ice-covered areas of the earth
  • The amount of radiation reflected back from various sources is known as the albedo of the earth.

Estimation of Radiation

  • The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface.
  • The earth radiates back 51 units in the form of terrestrial radiation.  Out of these 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere.
  • within the 34 units of atmosphere, 6 units are absorbed directly by the atmosphere, 9 units through convection and turbulence, and 19 units through latent heat of condensation.
  • 48 units absorbed by the atmosphere (14 units from insolation + 34 units from terrestrial radiation) are also radiated back into space.
  • Thus, the total radiation returning from the earth and the atmosphere is respectively 17 + 48 = 65 units, which balances the total of 65 units received from the sun.
  • This whole process of a huge transfer of heat is termed as heat balance of the earth.

Variation in the Net Heat Budget on the Surface of the Earth:

The amount of radiation received by the earth’s surface varies, some part has a surplus radiation balance, while other parts have a deficit.

  • For example, there is a surplus of net radiation balance between 40° N and S, and the region near the poles has a deficit.
  • The surplus heat energy from the tropics is redistributed polewards. 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.
Temperature

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 movements of particles comprising a substance, temperature is the measurement in the terms of degrees of hotness and coldness of a thing or a place.
Factors Controlling Temperature Distribution 

The factors responsible for controlling the distribution of temperature are as follow:

  • The latitude: The temperature of a place depends on the insolation received. The insolation varies according to the latitude, therefore the temperature also varies.
  • The altitude: Indirectly the atmosphere is heated by terrestrial radiation from below. Hence the places near the sea level record higher temperatures and the temperature go lower in the places of higher elevation.
    • So, we can say, temperature generally decreases with increasing height.
    • The rate of decrease in temperature with height is termed as normal lapse rate. It is 6.5° C per 1,000 m.
  • Distance from sea: The sea gets heated slowly and loses heat slowly in comparison to land, as we know land heats up and cools down quickly.  So, the variation in temperature over the sea is less compared to land.
  • Air mass and ocean current: The passage of air masses also affects the temperature. The places that come under the influence of warm air masses experience higher temperature, in comparison to the places with cold air masses.
Distribution of Temperature

The global distribution of temperature can be understood by studying the temperature distribution in January and July.

The distribution of temperature is generally shown through isotherms. Isotherms are lines joining places having an equal temperature. Generally, isotherms are parallel to the latitude. The deviation from this general trend is more seen in January than in July, especially in Northern Hemisphere.

Temporal distribution:
  • The isotherm deviates to the North over the ocean and to the South over the continent in January. This can be seen in the North Atlantic Ocean.
  • Over the land, the temperature decreases sharply and the isotherms bend towards the South in Europe. In the Siberian plain, it is more pronounced.
  • The mean January temperature along 60° E longitude is -20° C both at 80° N and 50° N latitude.
  • The mean monthly temperature for January is over 27°C in equatorial oceans, over 24°C in tropic, 2°C-0°C in the middle latitudes, and -18°C to -48°C in the Eurasian continental interior.
  • In the Southern Hemisphere, the effect of the ocean is notable. The isotherms here are more or less parallel to the latitudes and the variation in the temperature is more gradual than in the Northern Hemisphere. The isotherm of 20°C, 10°C and 0°C runs parallel to 35°S, 45°S, and 60°S latitudes, respectively.
  • In July, the isotherm generally runs parallel to the latitude. The equatorial oceans record warmer temperature of more than 27°C.
  • Over the land, more than 30°C is noticed in the sub-tropical continental region of Asia along the 30°N latitude.
  • The highest range of temperature between January to July is more than 60°C over the North-Eastern part of the Eurasian continent. This range is due to continentality.
  • The least range of temperature is 3°C which is found between 20°S and 15°N Latitude.
Inversion of Temperature

In general, the temperature decreases in the troposphere with height at the rate of 6.4°C per 1000 m. If there is an increase in air temperature with an increase in height, the phenomenon is known as an inversion of temperature. It is also known as a negative lapse rate.

The inversion of temperature occurs near the earth’s surface, or at a greater height in the troposphere.

The types of inversion of temperature are:

  • Ground surface inversion
  • Upper air inversion
  • Inversion in valley

Ground surface inversion: It is an atmospheric condition in which the temperature near the ground increases, instead the decrease with elevation. It promotes stability in the lower layers of the atmosphere.

Under the following geographical condition, the ground surface inversion occurs:

  • Long winter nights
  • Cloudless clear sky
  • Dry air and low relative humidity
  • Calm atmosphere
  • Snow covered surface

Upper air inversion: It occurs when the warm air is transported upward into the cold air due to the circular movement of air. The cause of upper air inversion may be due to the compression of descending air, as it happens in the case of subtropical high-pressure belts.

Inversion in valleys: The inversion takes place in mountains and hills due to air drainage. Cold air at the hills and mountains produced during the night, flows under gravity.

Reference:

NCERT Geography book

Wikipedia

https://earthobservatory.nasa.gov/global-maps/MOD_LSTD_M

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