NCERT Class 11 - Fundamentals of Physical Geography, Chapter 9

Insolation, Heat Budget and Temperature

Insolation is the incoming solar radiation received by the earth. Only a part of it reaches the surface, and the earth loses exactly as much heat as it gains - a balance called the heat budget, which keeps the average temperature stable.

Insolation and what controls it

• Angle of the sun's rays is the most important control - vertical rays concentrate energy on a small area, while slanting rays spread the same energy over a larger area and pass through more atmosphere.
• Length of the day - longer days mean more hours of receipt.
• Transparency of the atmosphere - cloud, dust and water vapour reflect, scatter and absorb radiation.
• Altitude - thinner air at height absorbs and scatters less, so insolation is stronger on mountains even though the air is colder.
• Insolation therefore decreases from the equator towards the poles, which is the basic cause of climatic zones.

The heat budget

• Of every 100 units of incoming radiation, 35 are reflected by clouds, dust and the surface, and this fraction is called the albedo.
• The atmosphere absorbs 14 units and the earth's surface 51 units.
• The earth returns all 51 units - 17 directly to space and 34 to the atmosphere, partly as radiation, partly by conduction and convection, and partly as latent heat released when water vapour condenses.
• Incoming and outgoing radiation therefore balance at 51 plus 49, which is why the earth is neither heating nor cooling continuously in the natural state.

Distribution of temperature

• Lines joining places of equal temperature on a map are called isotherms.
• Latitude is the main control, since it fixes the angle of the sun's rays.
• Altitude - temperature falls about 6.5 degrees for every kilometre of height in the troposphere.
• Distance from the sea - land heats and cools faster than water, so coasts have a small annual range and continental interiors a very large one.
• Ocean currents - warm currents raise and cold currents lower the temperature of the coasts they wash.
• Isotherms bend sharply where they cross from land to sea, and are far straighter in the southern hemisphere because it has much more ocean.

Temperature inversion

• Normally temperature falls with height; when it rises with height instead, the condition is called an inversion of temperature.
• It occurs on long, clear, calm winter nights, when the ground loses heat rapidly and chills the air just above it.
• In hills and valleys, cold dense air drains downhill and collects in the valley floor, so the valley is colder than the slope above it - this is air drainage, and it is why orchards and settlements in hills are often placed on slopes rather than valley bottoms.
• An inversion traps smoke, dust and pollutants near the ground and produces fog and smog, which is why winter air pollution is worst in northern Indian cities.