At the top of the troposphere, the air travels horizontally from a high pressure zone to a low
pressure zone. Since it is at the top of the troposphere, the air cools as it moves. This cold,
dense air creates the downward flowing limb of the convection cell. Where the sinking air
strikes the ground, air pressure is relatively high. This creates ahigh pressure zone. The
sinking air is relatively cool, since it has traveled across the tropopause.
Air that moves horizontally between high and low pressure cells makes wind. The winds
will race from the high to low zones if the pressure difference between them is large. If the
difference is smaller, the winds will be slower.
Convection in the atmosphere creates the planet’s weather. It’s important to know that
warm air can hold more moisture than cold air. When warm air near the ground rises in
a low pressure zone, it cools. If the air is humid, it may not be able to hold all the water
it contains as vapor. Some water vapor may condense to form clouds or even precipitation.
Where cooler air descends at a high pressure zone, it warms. Since it can then hold more
moisture, the descending air will evaporate water on the ground.
Air moving between large high and low pressure systems creates the global wind belts that
profoundly affect regional climate. Smaller pressure systems create localized winds that
affect the weather and climate of a local area.
Local Winds
Local winds are created when air moves from small high pressure systems to small low
pressure systems. High and low pressure cells are created by a variety of conditions. Some
of these winds have very important effects on the weather and climate of some regions.
Land and Sea Breezes
You learned that water has a very high specific heat: it maintains its temperature well.
This means that water heats and cools more slowly than land. Sometimes there is a large
temperature difference between the surface of the sea (or a large lake) and the land next to
it. This temperature difference causes small high and low pressure regions to form, which
creates local winds.
In the summer, and to a lesser degree in the day, a low pressure cell forms over the warm
land and a high pressure cell forms over the cooler ocean. During warm summer afternoons,
winds calledsea breezesblow from the cooler ocean over the warmer land (Figure15.20).
Sea breezes often have a speed of about 10 to 20 km (6 to 12 miles) per hour and can lower
air temperature much as 5 to 10oC (9 to 18oF). The effect of land and sea breezes is felt only
about 50 to 100 km (30 to 60 miles) inland.
The opposite occurs in the winter, the land is colder than the nearby water due to its lower
specific heat. The cold land cools the air above it. This causes the air to become dense