ng the summer, and towards the pole of the hemisphere
which is having its summer the gain in insolation from the latter cause
more than compensates for the loss by the former. The double period of
insolation above noted for the equator prevails as far as about lat. 12
deg. N. and S.; at lat. 15 deg. the two maxima have united in one, and
the same is true of the minima. At the pole there is one maximum at the
summer solstice, and no insolation at all while the sun is below the
horizon. On the 21st of June the equator has a day twelve hours long,
but the sun does not reach the zenith, and the amount of insolation is
therefore less than at the equinox. On the northern tropic, however, the
sun is vertical at noon, and the day is more than twelve hours long.
Hence the amount of insolation received at this latitude is greater than
that received on the equinox at the equator. From the tropic to the pole
the sun stands lower and lower at noon, and the value of insolation
would steadily decrease with latitude if it were not for the increase in
the length of day. Going polewards from the northern tropic on the 21st
of June, the value of insolation increases for a time, because, although
the sun is lower, the number of hours during which it shines is greater.
A maximum value is reached at about lat. 43-1/2 deg. N. The decreasing
altitude of the sun then more than compensates for the increasing length
of day, and the value of insolation diminishes, a minimum being reached
at about lat. 62 deg.. Then the rapidly increasing length of day towards
the pole again brings about an increase in the value of insolation,
until a maximum is reached at the pole which is greater than the value
received at the equator at any time. The length of day is the same on
the Arctic circle as at the pole itself, but while the altitude of the
sun varies during the day on the former, the altitude at the pole
remains 23-1/2 deg. throughout the 24 hours. The result is to give the
pole a maximum. On the 21st of June there are therefore two maxima of
insolation, one at lat. 43-1/2 deg. and one at the north pole. From lat.
43-1/2 deg. N., insolation decreases to zero on the Antarctic circle,
for sunshine falls more and more obliquely, and the day becomes shorter
and shorter. Beyond lat. 66-1/2 deg. S. the night lasts 24 hours. On the
21st of December the conditions in southern latitudes are similar to
those in the northern hemisphere on the 21st of June, but the so