Page 472 - Environment: The Science Behind the Stories

March equinox: Equator faces sun Figure 17.5 The seasons
directly; neither pole tilts toward sun; occur because Earth is tilted
23.5 all regions on Earth experience 12 hours on its axis. As Earth revolves
June solstice: Northern of daylight and 12 hours of darkness around the sun, the Northern
Hemisphere tilts toward Hemisphere tilts toward the
sun; summer begins in sun for one half of the year, and
Northern Hemisphere;
winter begins in the Southern Hemisphere tilts
Southern Hemisphere toward the sun for the other half
of the year. In each hemisphere,
summer occurs when the hemi-
sphere receives the most solar
energy because of its tilt toward
the sun.

December solstice:
Northern Hemisphere
tilts away from sun;
winter begins in
Northern Hemisphere;
summer begins in
September equinox: Equator faces sun Southern Hemisphere
directly; neither pole tilts toward sun; all
regions on Earth experience 12 hours of
daylight and 12 hours of darkness

the process. Convective circulation patterns occur in ocean cloudiness, and wind. Weather specifies atmospheric con-
waters (pp. 443–444), in magma beneath Earth’s surface ditions over short time periods, typically hours or days, and
(p. 53), and even in a simmering pot of soup. Convective cir- within relatively small geographic areas. In contrast, climate
culation influences both weather and climate. describes the pattern of atmospheric conditions found across
large geographic regions over long periods of time, typically
The atmosphere drives weather and climate years, decades, or centuries. Mark Twain once noted the dis-
tinction by remarking, “Climate is what we expect; weather is
Weather and climate each involve the physical properties of what we get.” For example, Los Angeles has a “Mediterranean”
the troposphere, such as temperature, pressure, humidity, climate characterized by reliably warm, dry summers and mild,
rainy winters, yet on occasional autumn days, dry Santa Ana
winds blow in from the desert and bring extremely hot weather.
Heat radiates to space

Air masses interact, producing weather
Condensation
Condensation
Cool, dry air Weather can change quickly when air masses with different
and precipitation
and precipitation
physical properties meet. The boundary between air masses CHAPTER 17 • AT m os PHER i C sC i E n CE , Ai R Qu A li T y, A nd Poll u T i on Con TR ol
that differ in temperature and moisture (and therefore den-
sity) is called a front. The boundary along which a mass of
warmer, moister air replaces a mass of colder, drier air is
Air sinks, compresses, Air rises, expands, termed a warm front (Figure 17.7a). Some of the warm, moist
and warms and cools
air along the leading edge of a warm front usually rises over
the cooler air mass that is blocking its progress. As it rises,
the warm air cools and the water vapor within condenses,
forming clouds and light rain. A cold front (Figure 17.7b) is
the boundary along which a colder, drier air mass displaces
Warm, dry air Hot, moist air
a warmer, moister air mass. The colder air, being denser,
tends to wedge beneath the warmer air. The warmer air rises,
Air picks up moisture and heat expands, then cools to form clouds and thunderstorms. Once
(moist surface warmed by sun) a cold front passes through, the sky usually clears, and tem-

Figure 17.6 Convective circulation helps to drive weather. Air perature and humidity drop.
heated near Earth’s surface picks up moisture and rises. Once aloft, Adjacent air masses may also differ in atmospheric
this air cools and moisture condenses, forming clouds and precipita- pressure. A high-pressure system contains air that descends
tion. Cool, drying air begins to descend, compressing and warming because it is cool and then spreads outward as it nears the
in the process. Warm, dry air near the surface begins the cycle anew. ground. High-pressure systems typically bring fair weather. 471

M17_WITH7428_05_SE_C17.indd 471 12/12/14 3:22 PM