may occur at the same place in the course of time. The
mean minimum and maximum temperatures or rainfalls of a month or a
season are important data. Further, a determination of the frequency of
occurrence of a given condition, or of certain values of that condition,
is important, for periods of a day, month or year, as for example the
frequency of winds according to direction or velocity; or of different
amounts of cloudiness; or of temperature changes of a certain number of
degrees; the number of days with and without rain or snow in any month,
or year, or with rain of a certain amount, &c. The probability of
occurrence of any condition, as of rain in a certain month; or of a
temperature of 32 deg., for example, is also a useful thing to know.

_Solar Climate._--Climate, in so far as it is controlled solely by the
amount of solar radiation which any place receives by reason of its
latitude, is called _solar climate_. Solar climate alone would prevail
if the earth had a homogeneous land surface, and if there were no
atmosphere. For under these conditions, without air or ocean currents,
the distribution of temperature at any place would depend solely on the
amount of energy received from the sun and upon the loss of heat by
radiation. And these two factors would have the same value at all points
on the same latitude circle.

The relative amounts of insolation received at different latitudes and
at different times have been carefully determined. The values all refer
to conditions at the upper limit of the earth's atmosphere, i.e. without
the effect of absorption by the atmosphere. The accompanying figure
(fig. 1), after Davis, shows the distribution of insolation in both
hemispheres at different latitudes and at different times in the year.
The latitudes are given at the left margin and the time of year at the
right margin. The values of insolation are shown by the vertical
distance above the plane of the two margins.

At the equator, where the day is always twelve hours long, there are two
maxima of insolation at the equinoxes, when the sun is vertical at noon,
and two minima at the solstices when the sun is farthest off the
equator. The values do not vary much through the year because the sun is
never very far from the zenith, and day and night are always equal. As
latitude increases, the angle of insolation becomes more oblique and the
intensity decreases, but at the same time the length of day rapidly
increases duri