Today we are going to look at the 3-cell model surface features (pressure belts and winds) in a little more detail.  We're doing this because, even with the unrealistic assumptions of the 3-cell model, much of what is predicted is actually found on the earth.

Here's the region between 45 S and 45 N latitude viewed from above.

There's a lot of information on this picture, but with a little study you should be able to start with a blank sheet of paper and reproduce this figure.  I would suggest starting at the equator.  You need to remember that there is a belt of low pressure found there, the equatorial low (pressure belt).  Then remember that the pressure belts alternate:  low pressure at the equator then belts of high pressure at 30 N and 30 S.

Let's start at 30 S.  Winds will begin to blow northward from High pressure at 30 S toward Low pressure at the equator (also southward toward low pressure at 60 S latitude).  Once the winds start to blow they will turn to the left because of the Coriolis force.  Up at 30 N winds will also blow southward toward the equator.  They are bent to the right (you need to turn the page upside down and look in the direction the winds are blowing).  These winds just north and south of the equator are the Trade Winds (northeasterly trade winds north of the equator and southeasterly trades south of the equator).  They converge at the equator and the air there rises (refer back to the crossectional view of the 3-cell model). This produces a band of clouds that you can often see at or near the equator on a satellite photograph.

The Intertropical Convergence Zone or ITCZ is another name for the equatorial low pressure belt.  This region is also referred to as the doldrums because it is a region where surface winds are often weak.  Sailing ships would sometimes get stranded there hundreds of miles from land.  Fortunately it is a cloudy and rainy region so the sailors wouldn't run out of drinking water (they might well have run out of rum though).
  
Hurricanes form over warm ocean water in the subtropics between the equator and 30 latitude.  Because winds at these latitudes have a strong easterly component, hurricanes, at least early in their development, move from east to west.  Middle latitude storms found between 30 and 60 latitude, where the prevailing westerly wind belt is found, move from west to east.

You find sinking air, clear skies, and weak surface winds associated with the subtropical high pressure belt at 30 N and 30 S.  This is also known as the horse latitudes.  Sailing ships could become stranded there also.  Horses were apparently either thrown overboard (to conserve drinking water) or eaten if food supplies were running low (Wikipedia has a different explanation of the origin of the term "horse latitudes").  Some of the hottest and driest locations on earth are found near 30 latitude (Tucson is located at 32 N latitude, so we are strongly affected by the subtropical high pressure belt).

The winds to the north of 30 N and to the south of 30 S are called the "prevailing westerlies."  They blow from the SW in the northern hemisphere and from the NW in the southern hemisphere. The 30 S to 60 S latitude belt in the southern hemisphere is mostly ocean.  Because there is less friction over the oceans, the prevailing westerlies there can get strong, especially in the winter.  They are sometimes referred to as the "roaring 40s" or the "ferocious 50s" (the 40s and 50s refer to the latitude belt they are found in).

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The 3-cell model assumes that the earth is of uniform composition and not tilted toward or away from the sun.  It predicts belts of high pressure at 30 N and 30 S latitude as shown above at left.  Because the real world has oceans and continents we find centers of high pressure, not belts, located near 30 latitude.  They move north and south of 30 degrees during the year as the N. Pole tilts toward and away from the sun.

This is shown a little more clearly on the figure below (on a handout distributed in class)

The high pressure center off the East Coast of the US is called the Bermuda High.  The Pacific High is found off the west coast.  Don't worry about the names of the Highs off the east and west coasts of South America.

Winds blowing around these centers of high pressure create some of the world's major ocean currents. The California current is a cold southward flowing current found off the west coast of the US.  The Gulf Stream is the warm northward flowing current along the east coast.

We briefly mentioned the El Nino phenomenon in class on Wednesday.  Ocean water in the tropical Eastern Pacific is normally cold.  The water warms as it moves westward.  This temperature pattern is shown above.  You can now better understand why this is true.  Two cold ocean currents, the California current north of the equator and its analog in the southern hemisphere meet at the equator in the eastern Pacific.  That is why the water there is so cold.  During an El Nino event the two cold ocean currents stop short of the equator.  The ocean water temperature pattern basically reverses.  This has a profound effect on weather around the globe.

The following two pictures show how the 3-cell models features move during the season.

This is a winter picture (northern hemisphere winter, the North Pole is tilted away from the sun).  All of the 3-cell model features have moved south of their nominal locations.  The intertropical convergence zone (ITCZ) which is normally at the equator has moved south of the equator.



Here's the summer picture (North Pole tilted toward the sun).  The ITCZ has moved north of the equator.

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The movement of the Pacific High north and south of its nominal position near 30 degrees latitude is part of what causes our summer monsoon in Arizona.

In the winter the Pacific High is found south of 30 N latitude (the bottom of the figure above).  Winds to the north of the high blow from the west.  Air originating over the Pacific Ocean is moist (though the coastal water is cold so this air isn't as moist as it would be if it came off warmer water).  Before reaching Arizona the air must travel over high mountains in California.  The air loses much of its moisture as it does this (remember the rain shadow effect).  The air is pretty dry by the time it reaches Arizona.  Significant winter rains occur in Arizona when storms systems are able to draw moist subtropical air from the southwest Pacific ocean into Arizona.

During the summer, the Pacific High moves north of 30 N latitude.  Winds on the southern side of the subtropical high have an easterly component.   Moist air originating in Mexico and from over warm water in the Gulf of Mexico blows into Arizona.  The sun heats the ground during the day, warm moist air in contact with the ground rises and produces convective thunderstorms.

Tucson gets about 12 inches of rain in a normal year.  About half of this comes during the "summer monsoon" season.  Remember the word monsoon refers to a seasonal change in wind direction.