Page 267 - NS-2 Textbook

262 NAUTICAL SCIENCES

SUN-----~--__
UP

~14=----SUN

AT SAME LATITUDE,
SUN IS LOWER IN THE SKY DURING WINTER

Earth's inclination causes the Sun's position in relation to the equator to be higher in summer and lower in winter.

begins to slow down. Because of this fact, the part of
Earth receiving the most direct rays from the Sun will
vary over a year from 23.5 degrees north (tropic of Can-
cer) to 23.5 degrees south (tropic of Capricorn), as Earth SUMMER
proceeds in its orbit around the Sun.
Our seasonal weather variations are the result of the
angle with which the SW1'S rays strike Earth as it re-
£ARTHS
volves around the Sun, not the nearness of Earth to the SURFACE
Sun. In sununer, because of Earth's inclination, the Sun's
rays in the Northern Hemisphere are more direct, even
though at this time Earth is farther away from the Sun.
Thus, the rays are more concentrated and deliver more
energy per lmit area, making the 'weather warm. In win- WINTER
ter in the Northern Hemisphere, Earth is actually closer
to the Sun, but sunlight hits this hemisphere at a greater
angle. Thus, the same amount of sunlight is spread over
DtcL\IBlR 22
a larger area, delivering less energy per unit aTea, so it
gets cooler. The reverse of this process happens in the
The seasonal temperature variations of Earth in mid- and high lati-
Southern Hemisphere. tudes are a result of the angle at which the Sun's rays strike the sur-
face. Note the difference in the surface areas struck by the same
amount of sunshine in summer and winter. Because the amount of
SECONDARY WIND CIRCULATION energy per unit area is higher in summer (top), it gets hotter than
in winter, when the energy per unit area is lower (bottom).
We have discussed the primary circulation of 'winds on
Earth. It is the unequal heating of the planet between the
equator and the poles that causes north-south winds. The slowly, because the water retains part of the heat it
rotation of Earth turns these winds east or west depend- gained during the previous summer. For this reason,
ing upon the hemisphere in which they occur. But winds 'vater areas are cooler than land areas during the sum-
are also affected by the topography of the land and the mer and wanner during the winter. The daily variation
currents of the seas. of temperature over open ,vater is seldom more than 2 or
We know that nearly three-fourths of Earth's surface 3 degrees, but 300 miles inland, it is rarely less than
is water. But not many people realize that three-fourths 15 degrees.
of the world's land surfaces are in the Northern Hemi- The difference between the land and sea tempera-
sphere. In the summer these land surfaces heat very hU'es causes the pressure belts of the prinlary wind cir-
rapidly, 'ivhile the water areas heat very slO\vly. In winter, culation to be broken up into enclosed high- and low-
the land cools rapidly and the water cools comparatively pressure areas, called cellters of actioll. We see, therefore,