Lecture #2:  Clouds and Airmass Thunderstorms (Continued)
Monday, 22 January 2001

Text Reading for Lecture #2

Airmass (single-cell) thunderstorms:  pages 381-384; 395-396


Web Resources

Single-Cell (Airmass) Thunderstorms
Thunderstorms

Single-Cell Storms (Supplemental reading HERE)

(some material taken from Mandy's Severe Weather Climatology Page): 

Most common;  last for less than an hour;  built-in self-destruct mechanism;  occur all year long, but mostly in summer;  can produce strong winds, lightning, hail, and microbursts;  three stages of growth.

Cumulus Stage:     The cumulus stage is the initial stage in the development of thunderstorm formation.  This stage is dominated by updrafts, which are caused by saturated air that "streams" upward throughout the cloud.  As each new surge of warm air rises higher than the last, the height of the cloud increases.  It only takes about 15 minutes for the tops of the cumulus clouds to reach altitudes of 8,000 to 10,000 meters.  Since the updrafts are strong enough to suspend water droplets and ice crystals in the air, precipitation does not occur during the cumulus stage.  When the accumulation of the precipitation in the cloud becomes too heavy for the cloud to support, it begins to fall causing a drag on the air.  This initiates a downdraft.

Mature Stage:  The mature stage begins once the precipitation hits the earth's surface.  This stage is the most active period of a thunderstorm.  Updrafts exist side by side with down drafts during this stage, which causes the cloud to continue to grow.  Before the actual precipitation reaches the ground, the cool down draft leaves the base of the cloud and spreads horizontally at the surface to create the gust front.  The updrafts spread horizontally as the cloud grows to the top of the unstable region (usually located at the base of the warmer stratosphere) and the "anvil top" forms.   This anvil top is made up of mostly ice-laden cirrus clouds which are eventually spread downwind by rapid winds aloft.  Toward the end of the mature stage, the cell reaches it's maximum intensity.  During this time rain is the heaviest, hail and strong surface winds may develop, as well as weak tornadoes.

Dissipating Stage:  Once the downdrafts begin to dominate throughout the thunderstorm cloud the dissipating stage is formed.  Thunderstorm activity ceases as the influx of colder air and the cooling effect of falling precipitation occur.  During this stage, subsiding air replaces the updraft throughout the cloud.  This cuts off the supply of moisture provided by the updraft, causing the cloud to eventually evaporate.

Major Point:  Single-cell storms form in environments where the winds are relatively weak throughout the depth of the atmosphere, or change little in altitude with regard to speed and/or direction:

Because of this structure, the precipitation that forms during the mature stage falls back on top of the updraft, thereby destroying it.  It is for this reason that airmass storms have a built-in self-destruct mechanism and thus relatively short lifetimes (approximately 45 minutes).  Below is a photograph (courtesy of NSSL) of an airmass thunderstorm.

Airmass storms often contain very strong downdrafts and can produce intense, short-lived and very isolated downdrafts known as microbursts.

  From Microburst:   Handbook for Visual Identification

Courtesy University of Illinois

Microbursts are a notable hazard to aviation because of the strong horizontal winds they produce close to the ground.  See the link HERE for some amazing photographs of microbursts. 

  (Photo courtesy of NCAR)  For additional microburst photos, click here.

Related to microbursts are gust fronts, or the leading edge of precipitation-cooled air from a thunderstorm.  They can spread over very large distances and cause straight-line wind damage.  In contrast to microbursts, gust fronts are much larger in scale and last for longer periods of time.

      Photograph copyright Gene Moore, a legendary OU storm chaser!

To understand how microbursts and gust fronts form, and to better understand why airmass storms "self-destruct," it is necessary to spend some time examining the thermodynamics of moisture and phase change in the atmosphere.

Text Reading

Composition of air;  pressure and density:  pages 2-9
Atmospheric pressure, density, and temperature:  The Ideal Gas Law:  pages 210-216
Moisture in the atmosphere:  pages 105-119

Hand-Written Lecture Notes from Class

Page 1, Page 2, Page 3, Page 4