Some terminologies

a) Atmospheric windows

• All spectral regions are affected to some extent by absorption in the atmosphere. But for a range of electromagnetic wavelengths 8.5-11 μ (long wave radiation) is which Earth’s atmosphere is largely or partially transparent. These are not absorbed by the atmospheric components and scape to the space. There radiation is called as atmospheric windows.

b) Terrestrial radiation

• The surface of earth becomes hot after receiving SR from sun. As a result, it becomes a source of radiation. The average temp. at the earth surface is around 288oK. It has been estimated that more than 99% of the radiation emitted by earth surface is contained in infrared range from 4 to 100 μ. This outgoing long wave radiation emitted by the earth surface is called terrestrial radiation.

c) Net radiation

• Net radiation (Rn) is the difference between all radiation received at the surface of the earth and all radiation emitted by that surface accounting both short wave (Rs) and long wave length (Rl)

Rn=Received-remitted=Rns-Rnl

I. Net short wave radiation (Rns)= Rs-rRs=Rs(1-r)

Where, r=reflectivity of the surface (For vegetative cover r=0.23; For water r=0.06)

ii. Net Long wave radiation (Rnl)=Rlo-Rli

Where, Rlo= Outgoing long wave & Rli=incoming wavelength

Hence net radiation (Rn) can be expressed as;

Rn=Rns-Rnl=Rs(1-r)- (Rlo-Rli)