Na fa 
water, nocturnal radiatiofi is usually positive even in cloudy weathers 
but occasionally the sky is overcast with clouds sufficiently warm 
and low so that the nocturnal radiation is nesative. 
The following supplementary reading is recommended: 
Admiralty weather manual, Chapter V. 
Petterssen, Introduction to meteorology, Chapter VI. 
Haurwitz, Dynamic meteorology, Chapter V. 
Brunt, Physical and dynamical meteorology, Chapters VI and VII. 
Shaw, Manual of meteorology, Vol.III, Chapters IV and V. 
Solar radiation 
With the sun in the zenith the intensity of solar radiation 
received by unit horizontal surface outside the atmosphere is ecual 
to the solar constant if the earth is at its mean distance from the 
sun. At the eaquinoxes such a point on the ecuator receives radiation 
at an averaze rate over 24 hows which is 1/1 of the noon intensity, 
so that one square centineter receives about 900 calories during the 
Gaye In general the radiation received during a day by a horizgcntal 
surface outside the atmsophere devends on the latitude and on the 
time of year 3 it is zero inside the arctic circle in midwinter and 
reaches a maximum of 1150 cal em-2 at the south vole in midsummer. 
Tn midsummer at the north Hole the value is slichtly less, 1080 ial 
em-2, because the earth is then further from the sun. This difference 
between the northern and southern hemisoheres at corresponding seasons 
will be neglected in what follows, 
In passing through the atmosnhere the intensity of the solar 
beam is decreased by absorption, scattering, and reflection from clouds 
At first a cloudless atmosphere will be considered. 
