Page 471 - Environment: The Science Behind the Stories

At sea level, atmospheric pressure averages 14.7 lb/in. Solar energy heats the atmosphere, helps
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or 1013 millibars (mb). Mountain climbers trekking to create seasons, and causes air to circulate
Mount Everest, the world’s highest mountain, can look up
and view their destination from Kala Patthar, a nearby peak, An enormous amount of energy from the sun constantly bom-
at roughly 5.5 km (18,000 ft) in elevation. At this altitude, bards the upper atmosphere—over 1000 watts/m , thousands
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pressure is 500 mb, and half the atmosphere’s air molecules of times more than the total output of electricity generated by
are above the climber whereas half are below. A climber human society. Of this solar energy, about 70% is absorbed
who reaches Everest’s peak at 8.85 km (29,035 ft) in eleva- by the atmosphere and planetary surface, while the rest is
tion, where the “thin air” is just over 300 mb, stands above reflected back into space (see Figure 18.1, p. 503).
two-thirds of the molecules in the atmosphere! When we fly Sunlight is most intense when it shines directly overhead
on a commercial jet airliner at a typical cruising altitude of and meets the planet’s surface at a perpendicular angle. At this
11 km (36,000 ft), we are above 80% of the atmosphere’s angle, sunlight passes through a minimum of energy-absorbing
molecules. atmosphere and Earth’s surface receives a maximum of solar
Another property of air is relative humidity, the ratio of energy per unit area. In contrast, solar energy that approaches
water vapor a given volume of air contains to the maximum Earth’s surface at an oblique angle loses intensity as it trav-
amount it could contain at a given temperature. Average day- erses a longer distance through the atmosphere. This is why,
time relative humidity in June in the desert at Phoenix, Ari- on average, solar radiation is most intense near the equator
zona, is only 31% (meaning that the air contains less than a and weakest near the poles (Figure 17.4).
third of the water vapor possible at its temperature), whereas Because Earth is tilted on its axis (an imaginary line con-
on the tropical island of Guam, relative humidity rarely necting the poles, running perpendicular to the equator) by
drops below 88%. People are sensitive to changes in relative about 23.5 degrees, the Northern and Southern Hemispheres
humidity because we perspire to cool our bodies. When end up being tilted toward the sun for half of each year,
humidity is high, the air is already holding nearly as much resulting in the seasons (Figure 17.5). Regions near the equa-
water vapor as it can, so sweat evaporates slowly and the body tor experience about 12 hours each of sunlight and darkness
cannot cool itself efficiently. This is why high humidity makes per day throughout the year. Near the poles, in contrast, day
it feel hotter than it actually is. Low humidity speeds evapora- length varies greatly between summer and winter, and season-
tion and makes it feel cooler. ality is pronounced.
The temperature of air also varies with location and time. Land and surface water absorb solar energy and then radi-
At the global scale, temperature varies over Earth’s surface ate heat, causing some water to evaporate. Air near Earth’s
because the sun’s rays strike some areas more directly than surface therefore tends to be warmer and moister than air at
others. At more local scales, temperature varies because of higher altitudes. These differences set into motion a process of
topography, plant cover, proximity of water to land, and many convective circulation (Figure 17.6). Warm air, being less dense,
other factors. Sometimes this local variation is striking; a hill- rises and creates vertical currents. As air rises into regions of
side sheltered from wind or sunlight may have a very different lower atmospheric pressure, it expands and cools. Once the
microclimate, or localized pattern of weather conditions, than air cools, it descends and becomes denser, replacing warm
the other side of the hill—and this often influences where cer- air that is rising. The descending air picks up heat and mois-
tain plants and animals occur. ture near ground level and prepares to rise again, continuing

Air absorbs more energy More sunlight Solar energy
due to longer path per unit of
through atmosphere North pole surface area Figure 17.4 Because of
Earth’s curvature, polar
Low angle regions receive less solar
of incoming energy than equatorial
sunlight
regions. One reason is that
Air absorbs less energy sunlight gets spread over a
due to shorter path larger area when striking the
through atmosphere surface at an angle. Another
Sunlight reason is that sunlight
directly Equator Equatorial regions approaching at a lower angle
overhead Less sunlight Solar energy near the poles must traverse
per unit of a longer distance through the
surface area atmosphere, causing more
energy to be absorbed or
reflected. These patterns rep-
Low angle resent year-round averages;
of incoming
sunlight the latitude at which radia-
tion approaches the surface
South pole perpendicularly varies with the
470 Polar regions seasons (see Figure 17.5).

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