Development of a thermal circulation
Differences in temperature (such as you might find between a coast and the ocean or between a city and the surrounding country side) can create horizontal pressure differences. The horizontal pressure gradient can then produce a wind flow pattern known as a thermal circulation. 

Because these are generally relatively small scale circulations, the pressure gradient is much stronger than the Coriolis force and the Coriolis force can be ignored.  The Coriolis force is caused by the rotation of the earth and causes winds to turn to the right in the northern hemisphere and to the left in the southern hemisphere.  The Coriolis force explains why winds in the northern hemisphere spin in a counterclockwise direction around low pressure centers and reverse direction in the southern hemisphere.

By applying some of the concepts we learned earlier in the semester we can understand pretty well how thermal circulations develop.



We'll start with this picture of conditions along a sea coast.  At this point the air temperatures and pressures on both sides of the picture are the same.


A beach will often become much warmer than the nearby ocean during the day (the sand gets hot enough that it is painful to walk across in bare feet).  The warm air over the land will expand upward.  Note how the 900 mb level has moved upward in the picture.  We've left the temperature of the water the same as it was in the earlier picture and the 900 mb level above the ocean hasn't changed either.    So on the left side of the figure at the level where we find 910 mb we find 900 mb on right (see the top of the picture below).

Another way of figuring out the upper level pressure pattern is to recall that pressure decreases relatively slowly with increasing altitude in warm low density air.  There is only a 90 mb drop between the ground the green line on the left side of the picture above.  Pressure decreases more rapidly with altitude (a 100 mb drop) in the cooler higher density air on the right side.  We end up with the same upper level pressure pattern (910 mb on the left and 900 mb on the right).



These upper level pressure differences cause air above the ground to start to blow from left to right.


Once the air aloft begins to move it will change the surface pressure pattern.  The air leaving the top left side of the picture will lower the surface pressure (from 1000 mb to 990 mb).  Adding air at upper right side of the picture will increase the surface pressure (from 1000 mb to 1010 mb).  Now we have a pressure difference at the surface and the surface winds will begin to blow from right to left.

Sea and land breezes


You can complete the circulation loop by adding rising air above the surface low pressure at left and sinking air above the surface high at right.  The surface winds which blow from the ocean onto land are called a sea breeze (meteorologists specify where the wind is coming from).  Since this air is likely to be moist, cloud formation is likely when the air rises over the warm ground.  Rising air expands and cools.  If you cool moist air to its dew point, clouds form.

shortcut
It is pretty easy to figure the directions of the winds in a thermal circulation without going through a long-winded development like this.  Just remember that warm air rises.  Draw in a rising air arrow above the warm part of the picture, then complete the loop.

At night the ground cools more quickly than the ocean and becomes colder than the water.  Rising air is found over the warmer ocean water (sea below).  The thermal circulation pattern reverses direction.  Surface winds blow from the land out over the ocean.  This is referred to as a land breeze.




Country breeze
Here is an additional situation where a thermal circulation could develop.



Cities are often warmer than the surrounding countryside, especially at night.  This is referred to as the urban heat island effect.  This difference in temperature can create a "country breeze."  This will sometimes carry pollutants from a factory outside the city back into the city or odors from a sewer treatment plant outside of town back into town.



1-cell model of the earth's global scale circulation
We can use the basic concept of a thermal circulation to learn about global scale pressure and wind patterns.  Ordinarily you couldn't apply a small scale phenomena like a thermal circulation to the much larger global scale.  However if we make some simplifying assumptions, particularly if we assume that the earth doesn't rotate or only rotates slowly, we can ignore the Coriolis force, and a thermal circulation could become established. 

Some additional simplifications are also made and are listed below.
 


Because the earth isn't tilted, the incoming sunlight shines on the earth most directly at the equator.  The equator will become hotter than the poles.  By allowing the earth to rotate slowly we spread this warmth out in a belt that circles the globe at the equator rather than concentrating it in a spot on the side of the earth facing the sun.  Because the earth is of uniform composition there aren't any temperature differences created between oceans and continents. 

You can see the wind circulation pattern that would develop.  The term one cell just means there is one complete loop in the northern hemisphere and another in the southern hemisphere. 








Note that you obtain this same kind of temperature and wind pattern
by rotating the country breeze figure by 90 degrees.


3-cell model of the earth's global scale circulation
Next we will remove the assumption concerning the rotation of the earth.  We won't be able to ignore the Coriolis force now.

This isn't something we can easily work out, we need a computer to predict what would happen.  Things are pretty much the same at the equator in the three cell and one cell models: surface low pressure and rising air.  At upper levels the winds begin to blow from the equator toward the poles.  Once headed toward the poles the upper level winds are deflected by the Coriolis force.  There end up being three closed loops in the northern and in the southern hemispheres.  There are surface belts of low pressure at the equator (the equatorial low) and at 60 degrees latitude (the subpolar low). There are belts of high pressure (the subtropical high) at 30 latitude and high pressure centers at the two poles (the polar highs).

We will look at the 3-cell model surface features (pressure belts and winds) in a little more detail because some of what is predicted, even with the unrealistic assumptions, is actually found on the earth.

Surface wind and pressure belts
Here's a map view of the region between 30 S and 30 N latitude.


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.  Then remember that the pressure belts alternate:  there are belts of high pressure at 30 N and 30 S.

Let's start at 30 S.  Winds will begin to blow from High pressure at 30 S toward Low pressure at the equator.  Once the winds start to blow they will turn to the left because of the Coriolis force.  Winds blow from 30 N toward the equator and turn to the right in the northern hemisphere (you need to turn the page upside down and look in the direction the winds are blowing).  These 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 is the cause of the band of clouds that you can often see at or near the equator on a satellite photograph.  If that link doesn't work try this one.

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 which they probably felt was worse).
  
Hurricanes form over warm ocean water in the subtropics between the equator and 30 latitude.  Winds at these latitudes have a strong easterly component and 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.  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.  Note that sinking air is associated with the subtropical high pressure belt so this is a region on the earth where skies are clear (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).




Here's the other surface map, it's a little simpler (it's a redrawn version of what was done in class).  We're just looking from about 30 N to a little bit past 60 N.  Winds blowing north from H pressure at 30 N toward Low pressure at 60 N turn to the right and blow from the SW.  These are the "prevailing westerlies."   The polar easterlies are cold winds coming down from high pressure at the north pole.  The subpolar low pressure belt is found at 60 latitude.  This is also a convergence zone where the cold polar easterly winds and the warmer prevailing westerly winds meet.  The boundary between these two different kinds of air is called the polar front and is often drawn as a stationary front on weather maps.  A strong current of winds called the polar jet stream is found overhead.  Strong middle latitude storms will often form along the polar front.

Ocean currents
The 3-cell model predicts subtropical belts of high pressure near 30 latitude.  What we really find are large circular centers of high pressure.  In the northern hemisphere the Bermuda high is found off the east coast of the US, the Pacific high is positioned off the west coast.  High pressure centers are found east and west of South America in the southern hemisphere.  Since I can't remember their names, you don't have to either.

Circular low pressure centers, the Icelandic low (off the east coast near Iceland and Greenland in the picture below) and the Aleutian low (off the west coast near the southern tip of Alaska), are found near 60 N.
 

The winds that blow around these large scale high pressure centers create some of the major ocean currents of the world.  If you remember that high pressure is positioned off the east and west coast of the US, and that winds blow clockwise around high in the northern hemisphere, you can determine the directions of the ocean currents flowing off the east and west coasts of the US.  The Gulf Stream is a warm current that flows from south to north along the east coast, the California current flows from north to south along the west coast and is a cold current.  A cold current is also found along the west coast of South America; winds blow counterclockwise around high in the southern hemisphere. 

The SW monsoon




Tucson gets about 12 inches of rain in a normal year (we are at about half of normal this year).  About half of this comes during the "summer monsoon" season.  The word monsoon, again, refers to a seasonal change in wind direction.  During the summer subtropical high pressure (the Pacific high) moves north of its normal position near 30 N latitude.  Winds on the southhern side of the subtropical high have an easterly component.   Moist air originating in Mexico and 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.

The close proximity of the Pacific high, with its sinking air motions, is what gives California, Oregon, and Washington dry summers.

In the winter the subtropical high moves south of 30 N latitude.  Winds to the north of the high blow from the west.  Air originating over the Pacific Ocean loses much of its moisture as it crosses mountains in California (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.