At a much larger scale, temperature differences across the Earth generate the development of the major wind belts. Such wind belts, to some degree, define the climate zones of the world. Air temperature is generally higher at ground level due to heating by the Sun, and decreases with increasing altitude.
This vertical temperature difference creates a significant uplift of air , since warmer air nearer the surface is lighter than colder air above it. The air will be still one day, and the next, powerful gusts of wind can knock down trees. What is going on here? The main cause of wind is a little surprising.
The gases that make up our atmosphere do interesting things as the temperatures change. When gases warm up, the atoms and molecules move faster, spread out, and rise.
When air is colder, the gases get slower and closer together. Colder air sinks. It also happens on a small scale within individual storms. But because Earth is spinning, the air that moves north and south from the equator also turns with the spin of the Earth. Air going north turns to the right. Air traveling south turns to the left. Air flows horizontally at top of the troposphere; horizontal flow is called advection.
The air cools until it descends. Where it reaches the ground, it creates a high pressure zone. Air flowing from areas of high pressure to low pressure creates winds. Warm air can hold more moisture than cold air. Air moving at the bases of the three major convection cells in each hemisphere north and south of the equator creates the global wind belts. Within the troposphere are convection cells Figure below. Air that moves horizontally between high and low pressure zones makes wind.
The greater the pressure difference between the pressure zones the faster the wind moves. When warm air rises and cools in a low pressure zone, it may not be able to hold all the water it contains as vapor. Some water vapor may condense to form clouds or precipitation. When cool air descends, 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 result from air moving between small low and high pressure systems. High and low pressure cells are created by a variety of conditions. Some local winds have very important effects on the weather and climate of some regions. Since water has a very high specific heat, it maintains its temperature well. So water heats and cools more slowly than land. If there is a large temperature difference between the surface of the sea or a large lake and the land next to it, high and low pressure regions form.
This creates local winds. Land and sea breezes create the pleasant climate for which Southern California is known. The effect of land and sea breezes are felt only about 50 to km 30 to 60 miles inland.
This same cooling and warming effect occurs to a smaller degree during day and night, because land warms and cools faster than the ocean. Monsoon winds are larger scale versions of land and sea breezes; they blow from the sea onto the land in summer and from the land onto the sea in winter. Monsoon winds are occur where very hot summer lands are next to the sea. Thunderstorms are common during monsoons Figure below.
In the southwestern United States relatively cool moist air sucked in from the Gulf of Mexico and the Gulf of California meets air that has been heated by scorching desert temperatures. The most important monsoon in the world occurs each year over the Indian subcontinent.
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