Solar Energy Input

We know that the spectral distribution and the intensity of solar radiation incident on the earth's surface are known. Of the enormous'amount of energy that is radiated by the sun (5.6 x l0⁷ cal/min), only about one-half of 1 billionth of that amount is intercepted by the eafth. Not all the solar.radiation can penetrate the earth's atmosphere; however, the amount of solar energy received at the top of atmosphere is constant. This energy is reffered to as solar constant. It is defined as the rate at which solar radiation falls on a unit area is a plane surface, which is oriented peqiendicular to the solar beam, when the earth is at its mean distance from the sun. On an average the value of solar constant is 2 cal/cm²/min.

As the solar radiation &vels through the atmosphere it interkts wiith it and gets diminished in three different ways: by reflection, scamring and absorption. About 30% of the total incoming solar radiation is reflected by clouds and a portion of it is back-scattered and lost in space. About 19% of it is directly absprbed by oxygen, ozone, water, ice crystals and suspended particles.'This absorbed radiation is converted into heat energy and the air is warmed to some extent. The remaining 51 % is absorbed or reflected by earth's surface that is converted to heat. Thus a total of 70% (19% by atmosphere aid 51% by earth) of the radiation absorbed by em and atmosphere is involved in the functioning of our biosphere.

The earth has a variety of surfaces - rough, smooth, ice-covered, or water-covered and areas with differeht types of vegetation. The amount of radiation absorbed or reflected depends upon the nature of surface. features i.e. topography of the area. The percentage of reflectivity of the incident radiation in meteorology is called albedo, which is Reflected radiation

Albedo = (reflected radiation)/(Incident radiation) x l00

Albedo of snow covered landscapes is higher than vegetated landscape or water column. Freshly fallen.sn6w typically has an albedo between 75 to 95%. Ocean waters have low albedo and therefore they appear darker thw the adjacent continental land masses. Rough surfaces have low albedo than smooth surfaces. Also the light coloured surfaces reflect more thandark surfaces. Reflectivity also depends upon the angle of incident radiation. The surfaces that are less perpendicular to sun's rays are more reflective than surfaces that make almost a right angle with the incoming solar radiation.  We have learnt that earth and atmosphere receive solar radiation, absorb a part of it and get warmed up. We also know that during night earth cools down. So where does the energy of radiation absorbed by the earth go? Actually, the absorbed radiation in turn is continually reradiatedfrom the earth as heat in the form of infrared radiation and is sent off to outer space continually. If it had not re-radiated the air temperature would rise steadily day by day.