Thunderstorms produce a variety of interesting and potential very destructive phenomena such as lightning, strong winds, hail, heavy rainfall, and tornadoes.  Before getting into all of the details here is a brief 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).  The word cell just refers to a single storm unit.  Most summer thunderstorms in Tucson are this type.   An air mass thunderstorm has a vertical updraft.

Tilted updrafts are found in severe and supercell thunderstorms.  We will learn why this allows those storms to get bigger, stronger, and last longer.  Storms with strong updrafts can produce large hailstones.  Sometimes the tilted updraft will begin to rotate, this can produce tornadoes.  Supercell thunderstorms have a complex internal structure.

Thunderstorms often form in clusters or lines, often ahead of cold fronts.  These are multiple cell storm systems and can cover a larger area.

We won't learn much about mesoscale convective systems.  These are much larger, "state size", storm systems that can last 12 to 24 hours.

Air must be lifted above the level of free convection in order for a thunderstorm to be initiated.  The lead up to this often takes the better part of a day.   We attempt to show this in the figure below.  Early in the day air is only getting a little bit of an upward push.  This is shown at Point 1.  The rising air finds itself cooler and denser than the surrounding air and sinks back down toward the ground.  As the day goes on the upward pushes get stronger.  Air might make it to Point 2 by noon but the result is still the same.  Finally by mid-afternoon perhaps, air is lifted above the LFC, finds itself warmer and less dense than the surrounding air, and explodes upward on its own.


Once a thunderstorm develops it then goes through 3 stages.


In the first stage you would only find updrafts inside the cloud (that's all you need to know about this stage, you don't even need to remember its name).

Once precipitation has formed and grown to a certain size, it will begin to fall and drag air downward with it.  This is the beginning of the mature stage where you find both an updraft and a downdraft inside the cloud.  The falling precipitation will also pull in dry air from outside the thunderstorm (this is called entrainment).  Precipitation will mix with this drier air and evaporate.  The evaporation will strengthen the downdraft (the evaporation cools the air and makes it more dense).  The thunderstorm is strongest in the mature stage.  This is when the heaviest rain, strongest winds, and most of the lightning occur.

Eventually the downdraft spreads horizontally throughout the inside of the cloud and begins to interfere with the updraft.  This marks the beginning of the end for this thunderstorm. 


The downdraft eventually fills the interior of the cloud.  In this is the dissipating stage you would only find weak downodrafts throughout the cloud.

Note how the winds from one thunderstorm can cause a region of convergence on one side of the original storm and can lead to the development of new storms.  Preexisting winds refers to winds that were blowing before the thunderstorm formed.  Convergence between the prexisting and the thunderstorm downdraft winds creates rising air that can initiate a new thunderstorm.

The picture below shows some of the features at the base of a thunderstorm.
The cold downdraft air spilling out of a thunderstorm hits the ground and begins to move outward from underneather the thunderstorm.  The leading edge of this outward moving air is called a gust front.  You can think of it as a dust front because the gust front winds often stir up a lot of dust here in the desert southwest (see below).



The gust front in this picture (taken near Winslow, Az) is moving from the right to the left.  Visibility in the dust cloud can drop to near zero which makes this a serious hazard to automobile traffic.  Dust storms like this are sometimes called "haboobs".

There's lots of video on YouTube of an impressive dust storm that occurred in July2011.  Here's an example from Gilbert Arizona.  Another from South Mountain which is, I think, in the Pheonix area.

The following picture shows a shelf cloud.



Warm moist air if lifted by the cold air behind the gust front which is moving from left to right in this picture.  The shelf cloud is very close to the ground, so the warm air must have been very moist because it didn't have to rise and cool much before it became saturated and a cloud formed. 

A narrow intense downdraft is called a microburst.  At the ground microburst winds will sometimes reach 100 MPH (over a limited area); most tornadoes have winds of 100 MPH or less.  Microburst winds can damage homes (especially mobile homes that aren't tied to the ground), uproot trees, and seem to blow over a line of electric power poles at some point every summer in Tucson. 

Microbursts are a serious threat to aircraft especially when they are close to the ground during landing or takeoff.  An inattentive pilot encountering headwinds at Point 1 might cut back on the power.  Very quickly the plane would lose the headwinds (Point 2) and then encounter tailwinds (Point 3).  The plane might lose altitude so quickly that it would crash into the ground before corrective action could be taken.  Microburst associated wind shear was largely responsible for the crash of Delta Airlines Flight 191 while landing at the Dallas Fort Worth airport on Aug. 2, 1985 (click here to watch a simulation of the final approach into the airport, be advised that the video contains some of the actual cockpit communications).

Falling rain could warn of a (wet) microburst.  In other cases, dangerous dry microburst winds might be invisible (the virga, evaporating rain, will cool the air, make the air more dense, and strengthen the downdraft winds).

In the classroom version of this course we do a simple demonstration just to hammer home the idea of what a microburst might look like.

A large plastic tank was filled with water, the water represents air in the atmosphere.  Then a colored mixture of water and glycerin, which is a little denser than water, is poured into the tank.  This represents the cold dense air in a thunderstorm downdraft.  The colored liquid sinks to the bottom of the tank and then spreads out horizontally.  In the atmosphere the cold downdraft air hits the ground and spreads out horizontally.  These are the strong winds that can reach 100 MPH.


Here's a picture of a wet microburst, a narrow intense thunderstorm downdraft and rain. 

Here are a couple of videos from YouTube.  The first video shows a microburst from some distance away.  The second video was taken in the heavy rain and strong winds under a thunderstorm in the microburst.

A microburst is a violent but short-lived and localized event.  A derecho is a less known phenomenon that, like a gust front, produces straight line winds (derecho is Spanish for "straight").  A derecho however is longer lived (30 minutes to a few hours) and can affect a much larger area.  Derecho winds are produced out ahead of a fast moving line of thunderstorms or a mesoscale convective system.  Winds must, by definition, be greater than 58 MPH and can exceed 100 MPH.  The strong winds can be found tens to hundreds of miles ahead of or along the advancing storm. 


The line of thunderstorms will often appear as a "bow echo" on radar.  The sketch above shows development of a single large thunderstorm into a bow echo and eventually into a comma echo.  In this figure the line of storms is moving toward the right.  Derecho winds would be found out in advance of the moving line.  Tornadoes sometimes form in the counterclockwise and clockwise spinning motions found at the top and bottom of the comma echo.

The figure below (source) shows a shelf cloud forming at the front edge of an approaching derecho.  It would be hard in just a photo like this to distinquish this from a shelf cloud forming at the front of a normal gust front.