Why were winds so important to the early explorers?
When Columbus sailed the ocean blue, and for centuries before and after, ocean travel depended on the wind. Mariners knew how to get where they were going and at what time of the year based on experience with the winds. Winds were named for their usefulness to sailors, such as the trade winds that facilitated commerce between people on opposite shores.
Global Wind Belts
Global winds blow in belts encircling the planet. Notice that the locations of these wind belts correlate with the atmospheric circulation cells. Air blowing at the base of the circulation cells, from high pressure to low pressure, creates the global wind belts.
The global wind belts are enormous and the winds are relatively steady (Figure below).
The major wind belts and the directions that they blow.
The Global Winds
Let’s look at the global wind belts in the Northern Hemisphere.
- In the Hadley cell air should move north to south, but it is deflected to the right by Coriolis. So the air blows from northeast to the southwest. This belt is the trade winds, so called because at the time of sailing ships they were good for trade.
- In the Ferrel cell air should move south to north, but the winds actually blow from the southwest. This belt is the westerly winds or westerlies.
- In the Polar cell, the winds travel from the northeast and are called the polar easterlies.
The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. These names hold for the winds in the wind belts of the Southern Hemisphere as well.
This video lecture discusses the 3-cell model of atmospheric circulation and the resulting global wind belts and surface wind currents (5a): http://www.youtube.com/watch?v=HWFDKdxK75E&feature=related (8:45).
The high and low pressure areas created by the six atmospheric circulation cells also determine in a general way the amount of precipitation a region receives. Rain is common in low pressure regions due to rising air. Air sinking in high pressure areas causes evaporation; these regions are usually dry. These features have a great deal of influence on climate.
The polar front is the junction between the Ferrell and Polar cells. At this low pressure zone, relatively warm, moist air of the Ferrell Cell runs into relatively cold, dry air of the Polar cell. The weather where these two meet is extremely variable, typical of much of North America and Europe.
The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast-flowing river of air at the boundary between the troposphere and the stratosphere. Jet streams form where there is a large temperature difference between two air masses. This explains why the polar jet stream is the world’s most powerful (Figure below).
A cross section of the atmosphere with major circulation cells and jet streams. The polar jet stream is the site of extremely turbulent weather.
Jet streams move seasonally just as the angle of the Sun in the sky moves north and south. The polar jet stream, known as “the jet stream,” moves south in the winter and north in the summer between about 30oN and 50o to 75oN.
jet stream: A fast-flowing river of air high in the atmosphere, where air masses with two very different sets of temperature and humidity characteristics move past each other.
polar front: The meeting zone between cold continental air and warmer subtropical air at around 50oN and 50oS.
- Global winds blow from high to low pressure at the base of the atmospheric circulation cells.
- The winds at the bases of the cells have names: the Hadley cell is the trade winds, the Ferrel Cell is the westerlies, and the polar cell is the polar easterlies.
- Where two cells meet, weather can be extreme, particularly at the polar front.
Use this resource to answer the questions that follow.
1. What is wind?
2. What causes wind?
3. What gets air moving?
4. Explain how air moves?
5. What are prevailing winds?
6. How are winds deflected in the Northern Hemisphere?
7. What are the jet streams?
8. What are trade winds?
9. Explain the doldrums.
10. What are local winds? What causes local winds?
1. What is a jet stream? What is "the" jet stream?
2. Why does a flight across the United States from San Francisco to New York City takes less time than the reverse trip?
3. Where on a circulation cell is there typically precipitation and where is there typically evaporation?