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winds, called jet streams, follow rising and falling

paths that have a strong infl uence on weather

patterns (Figure 2).

Weather Is Affected by Changes

in Atmospheric Pressure

Changes in atmospheric pressure also affect

weather. Atmospheric pressure results from mol-

ecules of gases (mostly nitrogen and oxygen)

in the atmosphere zipping around at very high

speeds and hitting and bouncing off everything

they encounter.

A cold front (Figure 1, right) is the leading

edge of an advancing mass of cold air. Because

cold air is denser than warm air, an advancing

cold front stays close to the ground and wedges

underneath less dense warmer air. An approach-

ing cold front produces rapidly moving, tower-

ing clouds called thunderheads.

As a cold front passes through, we may ex-

perience high surface winds and thunderstorms.

After it leaves the area, we usually have cooler

temperatures and a clear sky.

Near the top of the troposphere, hurricane-

force winds circle the earth. These powerful

Weather Is Affected by Moving
Masses of Warm and Cold Air

Weather is the set of short-term atmospheric
conditions—typically those occurring over hours
or days—for a particular area. Examples of at-
mospheric conditions include temperature, pres-
sure, moisture content, precipitation, sunshine,
cloud cover, and wind direction and speed.
Meteorologists use equipment mounted on
weather balloons, aircraft, ships, and satellites,
as well as radar and stationary sensors, to obtain
data on weather variables. They then feed these
data into computer models to draw weather
maps. Other computer models project the
weather for a period of several days by calculat-
ing the probabilities that air masses, winds, and
other factors will change in certain ways.
Much of the weather we experience results
from interactions between the leading edges of
moving masses of warm or cold air. Weather
changes as one air mass replaces or meets
another. The most dramatic changes in weather
occur along a front, the boundary between
two air masses with different temperatures and
densities.
A warm front is the boundary between an
advancing warm air mass and the cooler one it
is replacing (Figure 1, left). Because warm air
is less dense (weighs less per unit of volume)
than cool air, an advancing warm front rises up
over a mass of cool air. As the warm front rises,
its moisture begins condensing into droplets,
forming layers of clouds at different altitudes.
Gradually the clouds thicken, descend to a lower
altitude, and often release their moisture as
rainfall. A moist warm front can bring days of
cloudy skies and drizzle.

Weather Basics: El Niño, Tornadoes, and

Tropical Cyclones (Chapters 4, 7, 11)

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Warm air mass

Warm air mass

Anvil top

Cool

air mass

Cold air mass

Wa

rm

fro

nt

su

rfa

ce

Coldfro

ntsu

rfa

ce

Figure 1 Weather fronts: a warm front (left) arises when an advancing mass of warm air meets and rises up over a
mass of denser cool air. A cold front (right) forms when a moving mass of cold air wedges beneath a mass of less
dense warm air.

Figure 2 A jet stream is a

rapidly flowing air current

that moves west to east

in a wavy pattern. This

figure shows a polar jet

stream and a subtropi-

cal jet stream in winter.

In reality, jet streams are

discontinuous and their

positions vary from day to

day. (Used by permission

from C. Donald Ahrens,

Meteorology Today, 8th

ed. Belmont, Calif.: Brooks/

Cole, 2006)