220 CHAPTER 9 Global Atmospheric Changes
Light strikes
vertically Light strikes
at an angleSmall area of
illuminationLarger area of
illumination->ÀÊÌiÃÌÞÊ>Ê>ÌÌÕiÊUÊ}ÕÀiÊ°ÎÊ
Earth‘s(^) orbit
Vernal equinox,
March 21 or 22
Spring in Northern
Hemisphere, autumn
in Southern Hemisphere
Winter solstice,
December 21 or 22
Winter in Northern
Hemisphere, summer
in Southern Hemisphere
Summer solstice,
June 21 or 22
Summer in Northern
Hemisphere, winter
in Southern Hemisphere
Autumnal equinox,
September 22 or 23
Autumn in Northern
Hemisphere, spring
in Southern Hemisphere
*À}ÀiÃÃÊvÊÃi>ÃÃÊUÊ}ÕÀiÊ°{Ê
Earth’s inclination on its axis
remains the same as it travels
around the sun. Thus, the sun’s
rays hit the Northern Hemisphere
obliquely during its winter
months and more directly during
its summer. In the Southern
Hemisphere, the sun’s rays are
oblique during its winter, which
corresponds to the Northern
Hemisphere’s summer. At the
equator, the sun’s rays are
approximately vertical on
March 21 or 22 and
September 22 or 23.
moves toward the poles, the light hits the surface more
and more obliquely (represented by the lamp on the
right), spreading the same amount of insolation over
larger and larger areas. Because the sun’s energy does not
reach all places uniformly, temperature varies locally.
Earth’s inclination on its axis (23.5 degrees from a
line drawn perpendicular to the orbital plane) deter-
mines the seasons. During half the year (March 21 or 22
to September 22 or 23) the Northern Hemisphere tilts
toward the sun, and during the other half (September
22 or 23 to March 21 or 22) it tilts away from the sun
(}ÕÀiÊ°{). The Southern Hemisphere tilts the oppo-
site way, so that summer in the Northern Hemisphere
corresponds to winter in the Southern Hemisphere.
Precipitation
Precipitation refers to any form of water, such as rain,
snow, sleet, or hail, that falls from the atmosphere. Differ-
ences in precipitation depend on three factors:
- The amount of water vapor in the atmosphere.
Equatorial uplift of warm, moisture-laden air produces
heavy rainfall in some areas of the tropics. High
surface water temperatures cause vast quantities of
water to evaporate from tropical parts of the ocean.
Prevailing winds blow the resulting moist air over
landmasses. The moist air continues to rise as it is
heated by the sun-warmed land surface. As the air
rises, it cools, which decreases its moisture-holding
ability. When the air reaches its saturation point—
when it can’t hold any additional water vapor—clouds
form and water is released as precipitation.
absorbed and runs the hydrologic
cycle, carbon cycle, and other
biogeochemical cycles; drives
winds and ocean currents; pow-
ers photosynthesis; and warms
the planet. Ultimately, all this en-
ergy returns to space as long-wave
infrared radiation (heat).Temperature Changes with Latitude and Season
Earth’s roughly spherical shape and the tilt of its axis
produce a great deal of variation in the exposure of the
surface to solar energy (}ÕÀiÊ°Î). Sunlight that shines
vertically near the equator (represented by the desk lamp
on the left) is concentrated on Earth’s surface. As one
infrared radiation
Electromagnetic
radiation with
wavelengths longer
than those of visible
light but shorter
than microwaves;
perceived as invisible
waves of heat.