 How a sea breeze forms
People who spend a lot of time outdoors, like
divers, often develop a practical appreciation of
weather. They learn to watch the forecasts and read
the sky well enough to stay away from dangerous
weather.
Anyone who is curious about how the world works
is likely to wonder what’s behind the movements of
clouds, fronts and storms that affect their lives.
The “why” of the weather can be as complicated
as you want to make it. Many scientists spend their
careers learning the intricacies of the weather. On
the other hand, just about anyone can understand the
basics of how the weather works with the help of
eight principles.
1. It all begins with
the sun heating some places more than others.
Anyone who’s ever gone to the beach on a hot day
has experienced this fact. As you walk barefoot onto
dry sand late in the afternoon, it’s so hot your
feet burn. You rush into shallow water, which feels
cool. Yet the sun has been shining down all day with
equal intensity on the hot sand and the comfortable
water.
 What’s going on? The top layers of the sand
absorb all the sun’s heat reaching it — you don’t
have to dig far down to find cooler sand. At the
same time, solar radiation penetrates farther into
water than sand or dirt, warming a deeper layer.
Also, various things, such as breaking waves, can
stir up the water, mixing cool water from below with
warm water near the top. In addition, it takes more
heat to warm a given amount of water than the same
amount of sand.
In addition to the unequal heating of different
surfaces that receive the same amounts of sunlight,
more sunlight falls on some parts of the Earth than
on others. This causes the seasons. During the
Northern Hemisphere’s winter, the North Pole is
tilted away from the sun, bringing shorter days and
causing the sun to be lower in the sky. During
summer, the North Pole is tilted toward the sun,
which means the sun is higher in the sky and the
days are longer.
The sun is high in the sky in the tropics all
year. This accounts for the Earth’s middle latitudes
having distinct cold and warm seasons, while the
tropics are warm and the polar regions are cold all
year.
2. Air temper-ature differences on both a small
scale and
a global scale cause the winds.
The air temperature near the Earth’s surface
depends mostly on the surface temperature, because
sunlight hardly heats the air as it passes through
it. This means that the air above cool ground or
water will become cool, and air above warm ground or
water will warm up. Thus, variations in ground or
water temperatures create different air temperatures
around the globe and, to a smaller degree, over land
and water.
Warm air is light and tends to rise, while cold
air is heavy and tends to sink. This is what causes
the winds.
On a local scale, unequal heating causes “sea
breezes” near the oceans or large lakes. As the land
heats up during the day, air heated by the warmed
land begins rising and cooler air flows in to
replace it, creating a cooler breeze from the water
to land. Figure 1 shows how this works.
On a global scale, this means that air tends to
rise in the tropics and sink over cooler parts of
the globe. If the Earth were not rotating, warm air
would be rising in the tropics and flowing at high
altitudes to the north and south, where it would
sink in the polar regions. Cool air from the
northern and southern parts of the globe would flow
across the Earth’s surface to replace the rising air
in the tropics.
3. But the Earth
is rotating, and this causes air that would be
flowing toward and away from the poles to turn. The
effect
of the Earth’s
rotation, known as the Coriolis force, combines
with other forces that drive the winds to create
huge wind spirals known as high- and low-pressure
areas.
On the real, spinning Earth, air that rises in
the tropics descends over the subtropical regions
between about 20 and 30 degrees latitude north and
south. In the middle latitudes, the Coriolis force
distorts the winds so much that it creates high- and
low-pressure areas that follow each other from west
to east with ever-changing weather.
In the Northern Hemisphere, the air flows
counterclockwise around low-pressure areas and
clockwise around high-pressure areas. The flows in
the Southern Hemisphere are in the opposite
directions.
4. The amount of water vapor, or humidity, the
air can “hold” depends on the air’s temperature.
Warm air can “hold” more water vapor than cool air.
Anything that cools the air will cause water vapor
to condense.
 When the air is relatively warm, water
evaporates into it, which means the water becomes
the invisible gas known as water vapor. If the air
is cooled, some of the vapor condenses to form dew,
or the tiny water droplets that make up fog and
clouds. (Fog is merely a cloud that’s nearer the
ground.) When conditions are right, cloud droplets
come together to create small drops, called drizzle,
or larger raindrops.
When the temperature is cold enough, water vapor
can turn directly into ice to create frost on
objects or snow in clouds.
5. The higher
you go, the less the pressure
of the air.
The air’s pressure depends on the weight of air
above the place where you’re measuring the pressure.
The higher you go, the less air there is above you.
Therefore, the pressure decreases.
6. If you lower
the pressure of the air, the air
will cool. If you
increase air
pressure, it
warms the air.
This sounds simple, but it’s one aspect of the
basics of weather that leads to the most confusion.
To avoid confusion, keep in mind that this basic law
of nature applies only when the air’s pressure is
changing. It is not why the air is usually colder at
higher altitudes.
To see how this works, take an ordinary bicycle
pump and inflate a tire. The pump warms up because
you are increasing the air’s pressure each time you
push the plunger. Once you’ve pumped up the bicycle
tire, push down its valve to let the air out. The
air coming from the tire feels cool because its
pressure is decreasing as it rushes from the high
pressure in the tire to the lower pressure outside.
You also need to know that areas of low pressure
are not necessarily cool, and areas of high pressure
are not necessarily warm. Hurricanes are areas of
extreme low pressure, yet their centers are warmer
than the surrounding air. In winter storms the
low-pressure center is cooler than the surrounding
air. The highest air pressures measured at the
Earth’s surface are in very cold air, such as over
Siberia.
What do we mean, then, when we say lowering the
air’s pressure cools it and increasing the pressure
warms the air? This is important to weather because
as a bubble of air — meteorologists like to use the
term “parcel of air” — rises, its pressure drops to
match the pressure of the surrounding air. This
cools the rising air. On the other hand, if air is
descending, its pressure increases to match the
pressure of the surrounding air, and the air warms.
7. Rising air
causes clouds
and precipitation. Sinking air tends to clear
the sky.
As rising air cools, the water vapor in the air
begins condensing into the tiny drops of water that
make up clouds, or if it cools enough, the water
vapor turns into ice crystals, which make up colder
clouds.
When air is sinking, its pressure increases as
it descends into the higher-pressure air at lower
altitudes. This causes the air to warm. If there are
any clouds, they begin to evaporate as the air
warms. The warming air, of course, will also keep
clouds from forming.
8. In areas of low pressure at the surface, air
is
rising. In areas of high pressure at the
surface, air is sinking. As a result, low pressure
is usually associated with clouds and precipitation,
while high pressure usually brings clearer skies.
See Figure 2.
At the Earth’s surface, air spirals into areas
of low air pressure where it rises. As the air
rises, it cools, and the water vapor in it begins
condensing to form a widespread area of clouds and
precipitation. Air that rises in low-pressure areas
flows in the upper atmosphere until it eventually
begins sinking to form areas of high pressure at the
surface. Figure 2 shows this.
While the sinking air keeps clouds from forming,
it doesn’t always bring completely clear skies.
Sometimes, the high pressure can trap pollution,
which can create low visibility in haze.
These eight rules, of course, won’t enable you
to understand everything about the weather, but with
them you are ready to see how storms and other
weather phenomena work. As you watch weather reports
on TV or read them in a newspaper, you’ll have a
better understanding of how global weather patterns
affect your local weather.
For instance, warming the air at the Earth’s
surface is one way to make it rise, but it’s not the
only way. Thunderstorms and hurricanes depend on
warm, humid air rising from the surface. This is why
thunderstorms are most common in the spring and
summer when the ground is being warmed. It’s why
hurricanes form only over warm water.
Other storms are much more complicated, with
various factors causing air to rise in areas of low
pressure. These are related to the movements of
upper atmospheric winds. Upper-air winds, in turn,
can be traced back to the unequal heating of the
Earth’s surface and the global-scale winds that this
sets into motion.
The next time you slip from the sun-heated deck
of a dive boat into the cooler water, you will be
experiencing, on a small scale, the unequal heating
of the Earth’s surface by the sun — the force that
sets all weather, including the fearsome storms,
into motion.
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