Here's a little more carefully drawn picture.  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.

The figure above shows the intertropical convergence zone (the dotted dashed line) south of the Equator.  This happens during the northern hemisphere winter.  A second similar figure was included on a class handout.  It showed the ITCZ north of the Equator.  That is the situation during the northern hemisphere summer. 

The subtropical high pressure centers also move north and south of their nominal locations near 30 latitude.  Movement of these features is part of what causes our summer monsoon


Tucson gets about 12 inches of rain in a normal year.  About half of this comes during the "summer monsoon" season.  The word monsoon refers to a seasonal change in wind direction.  During the summer, subtropical high pressure (the Pacific high) moves north of 30 N latitude.  Winds on the southhern 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.

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 is not as moist because the water is colder than the Gulf of Mexico.  In addition the air 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.

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We're next going to spend a couple of lectures on thunderstorms.  Here's a little bit of an introduction
Thunderstorms come in different sizes and levels of severity.  We will mostly be concerned with ordinary single-cell thunderstorms (also referred to as air mass thunderstorms).  Most summer thunderstorms in Tucson are this type.   An air mass thunderstorm has a vertical updraft.  A cell is just a thunderstorm "unit."

Tilted updrafts are found in severe and supercell thunderstorms.  As we shall see this allows those storms to get bigger, stronger, and last longer.  Supercell thunderstorms have a complex internal structure;  we'll watch a short video at some point that shows a computer simulation of the complex air motions inside a supercell thunderstorm.

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The following somewhat tedious material was intended to prepare you to better appreciate a time lapse video movie of a thunderstorm developing over the Catalina mountains.  I don't expect you to remember all of the details given below.  The figures below are more carefully drawn versions of what was done in class.


Refer back and forth between the lettered points in the figure above and the commentary below.

The numbers in Column A show the temperature of the air in the atmosphere at various altitudes above the ground (note the altitude scale on the right edge of the figure).  On this particular day the air temperature was decreasing at a rate of 8 C per kilometer.  This rate of decrease is referred to as the environmental lapse rate (lapse rate just means rate of decrease with altitude).  Temperature could decrease more quickly than shown here or less rapidly.  Temperature in the atmosphere can even increase with increasing altitude (a temperature inversion).

At Point B, some of the surface air is put into an imaginary container, a parcel.  Then a meterological process of some kind lifts the air to 1 km altitude (in Arizona in the summer, sunlight heats the ground and air in contact with the ground, the warm air becomes bouyant - that's called free convection).  The rising air will expand and cool as it is rising.  Unsaturated (RH is less than 100%) air cools at a rate of 10 C per kilometer.  So the 15 C surface air will have a temperature of 5 C once it arrives at 1 km altitude. 

"Mother Nature" lifts the parcel to 1 km and "then lets go."  At Point C note that the air inside the parcel is slightly colder than the air outside (5 C inside versus 7 C outside).  The air inside the parcel will be denser than the air outside and the parcel will sink back to the ground. 

By 10:30 am the parcel is being lifted to 2 km as shown at Point D.  It is still cooling 10 C for every kilometer of altitude gain.  At 2 km, at Point E  the air has cooled to its dew point temperature and a cloud has formed.  Notice at Point F, the air in the parcel or in the cloud (-5 C) is still colder and denser than the surrounding air (-1 C), so the air will sink back to the ground and the cloud will disappear.  Still no thunderstorm at this point.


At noon, the air is lifted to 3 km.  Because the air became saturated at 2 km, it will cool at a different rate between  2 and 3 km altitude.  It cools at a rate of 6 C/km instead of 10 C/km.  The saturated air cools more slowly because release of latent heat during condensation offsets some of the cooling due to expansion.  The air that arrives at 3km, Point H, is again still colder than the surrounding air and will sink back down to the surface.

By 1:30 pm the air is getting high enough that it has become neutrally bouyant, it has the same temperature and density as the air around it (-17 C inside and -17 C outside).  This is called the level of free convection, Point J in the figure.

If you can, somehow or another,  lift air above the level of free convection it will find itself warmer and less dense than the surrounding air as shown at Point K and will float upward to the top of the troposphere on its own.  This is really the beginning of a thunderstorm.  The thunderstorm will grow upward until it reaches very stable air at the bottom of the stratosphere.

This was followed by a time lapse video tape of actual thunderstorm formation and growth.  I don't have a digital version of that tape, so here is a substitute time lapse of a day's worth of thunderstorm development.