Global+Wind+Patterns

How to draw the global wind belts: Start with a blank earth... I like to put in an equator for reference.

All along the equator, there is a belt of LOW pressure, both sides. Notice the circulation pattern, counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

We call this the ITCZ or Intertropical Convergence Zone, it also has a nickname, the Doldrums, because here the air is always rising, so there is no cross breeze. It is a place sailors used to get stuck. Because the pressure here is always low, it is pretty much always cloudy and rainy. This is where the belt of rain forest is located.

On either side of the ITCZ is a belt of wind that blows from the east to the west, known as the trade winds. They are called the northeast trades in the northern hemisphere and the southeast trades in the southern hemisphere.

On the other side of the trade winds, there is a belt of high pressure known as the STH, or the Subtropical High Pressure Belt.

Note the pattern of circulation of the high pressure, clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. The circulation of the STH fuels the trade winds and also the next wind belt, the westerly winds. The STH also has a nickname, it is the horse latitudes, because the air here is descending, or falling and there is no cross breeze right under the high, and this is another place sailors used to get stuck. They used to throw horses overboard here, thinking that if they just lightened the load, their ships would sail. This belt of high pressure coincides with the desert belts of the world, because high pressure is falling air, so it cannot rise to form clouds.

The next belt of wind is the westerly winds. These winds blow from the west to the east from about 30° to 60° both north and south. This is the belt we live in and what most of the US is within.

The next pressure belt is the Subpolar Low, low pressure on the poleward side of the westerlies.

Note the pattern of circulation of the low pressure - it helps to fuel the westerlies and it also helps to fuel the next wind belt.

The next wind belt is the Polar Easterlies. This is a belt of wind that blows from the east to the west and is located on the poleward side of the subpolar low.

The last pressure belt is the Polar High, both poles have high pressure because of the cold. Notice the pattern of circulation, which helps to fuel the polar easterlies.

There is a river of high speed wind which is located between the polar easterlies and the westerlies. It is the Polar Jet Stream. It moves in waves, which is why I draw it as a squiggle, those waves are called Rosby Waves and they bring us our winter weather when the jet stream comes down over us.

There is also a Tropical Jet Stream, which also moves in waves, but doesn't usually affect us much. It is located usually below the westerlies, between the trade winds and the westerlies. This El Nino year means that the tropical jet will be bringing us more moisture than normal.

The last thing to add to the global wind patterns, is what are known as Hadley Cells. At the equator, the ITCZ is a belt of low pressure. Low pressure is rising air. What goes up must come down. The air at the equator rises, and it moves across the atmosphere, then falls about 25° both north and south to create the STH.

Now that you have learned where everything is, there are a few things to note. There is curvature to all of the wind belts because of the Coriolis effect. I draw straight lines for beginners, because I know if we were worried about all of the curves, then this would be harder. Nothing stays where we drew it. The winds shift with the seasons, so everything shifts north and south. The movement of the ITCZ bring rain when it comes and drought when it leaves. For us particularly, the Hawaiian High - the STH closest to us, shifts north in the summer, which blocks storms, and is why we have a dry summer, then it shifts south in the winter which allows the jet stream to come back our way and bring us rain and cool weather.

Understanding the wind patterns on the planet will help you understand the climate system and the biomes, and even the distribution of soil, so it is a very important part of chapter five.

Here is the picture from the textbook: This is how everything curves. Here is a link to a live wind map based on satellite data: [|Earth Wind Map] which I think is kind of neat to look at once you are done to see where everything is in reality, because of course it is not as perfect as the textbook either. This is an El Nino year too, which makes everything different too.