uthern
latitudes have higher values of insolation because the earth is then
nearer the sun.

[Illustration: From Davis's _Elementary Meteorology_.
FIG. 1.--Distribution of Insolation over the Earth's Surface.]

At the equinox the days are equal everywhere, but the noon sun is lower
and lower with increasing latitude in both hemispheres until the rays
are tangent to the earth's surface at the poles (except for the effect
of refraction). Therefore, the values of insolation diminish from a
maximum at the equator to a minimum at both poles.

The effect of the earth's atmosphere is to weaken the sun's rays. The
more nearly vertical the sun, the less the thickness of atmosphere
traversed by the rays. The values of insolation at the earth's surface,
after passage through the atmosphere, have been calculated. They vary
much with the condition of the air as to dust, clouds, water vapour, &c.
As a rule, even when the sky is clear, about one-half of the solar
radiation is lost during the day by atmospheric absorption. The great
weakening of insolation at the pole, where the sun is very low, is
especially noticeable. The following table (after Angot) shows the
effect of the earth's atmosphere (coefficient of transmission 0.7) upon
the value of insolation received at sea-level.

_Values of Daily Insolation at the Upper Limit of the Earth's
Atmosphere and at Sea-Level_.

+-----------------+----------------------------+--------------------------+
| | Upper Limit of Atmosphere. | Earth's Surface. |
| Lat. +----------+-------+---------+---------+-------+--------+
| | Equator. |40 deg.| N. Pole.| Equator.|40 deg.|N. Pole.|
+-----------------+----------+-------+---------+---------+-------+--------+
| Winter solstice | 948 | 360 | 0 | 552 | 124 | 0 |
| Equinoxes | 1000 | 773 | 0 | 612 | 411 | 0 |
| Summer solstice | 888 | 1115 | 1210 | 517 | 660 | 494 |
+-----------------+----------+-------+---------+---------+-------+--------+

The following table gives, according to W. Zenker, the relative
thickness of the atmosphere at different altitudes of the sun, and also
the amount of transmitted insolation:

_Relative Distances traversed by Solar Rays through the Atmosphere,
and Intensities of Radiation per Unit Areas_.

+------------------+----+----+----+----+----+----+----+----+----+-----