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Thunderstorms

Learning Objectives   


Thunderstorms are convective systems; they are composed of up and down drafts that can be in excess of 50 mph.  By definition thunderstorms contain both thunder and lightning.  They can last hours or minutes, be localized or cover a large area and usually form because of unstable air.  Thunderstorms vary in intensity and associated effects.



Thunderstorms build in stages:
A) Cumulus Stage: warm air rises, cools, condenses forming clouds.  As more warm air rises a convection cell between warm surface air and upper level cool air forms.  The rising and sinking of the warm and cold air forms the up and down drafts.  These air movements continue until a massive cloud is formed that holds lots of moisture.
Lots of vertical development can lead to clouds of 40,000 ft or higher.  As water cools and condenses heat is released adding to the instability of the cloud and further thunderstorm development.  The cumulus stage can last hours.
 

B) Mature Stage: the storm has entered the mature stage once rain begins to fall.  In an ordinary thunderstorm the rain will fall straight down through the up-drafts.  In a super cell thunderstorm the rain and cool down drafts are separated from the warm up drafts.
 

C) Dissipating Stage: Rain acts to cool the up-drafts, the storm is being fueled by temperature differences; eliminate the differences and the storm dissipates.  Once the up drafts weaken the storm rains itself out and dissipates.  In ordinary thunderstorms the falling rain quells the up drafts.  In squall line thunderstorms this may happen as forward movement slows.  In supercell thunderstorms the storm slowly dissipates as temperatures become more evenly distributed between lower and upper atmosphere.


Some Thunderstorm Terminology:
 

Severe Thunderstorms: thunderstorms with more violent up/down drafts (usually in excess of 50 mph and associated effects are more severe. 
These storms can produce hail, spawn tornados and often reach heights over 60,000 ft.


Gust Fronts: an area of gusty high winds in front of a severe thunderstorm caused by downdrafts striking the surface and moving outward along the ground.


Microbursts: localized downdraft that strikes the Earth at a high rate of speed and moves outward, often forming a gust front.  Microbursts can be very destructive and microburst damage is often mistaken for tornado damage.
Multi-Cell Storms: storms that are composed of more than one convective cells. 
These storms form from gust fronts "kicked up" in front of other storms or form along frontal boundaries.



 

1) Ordinary Thunderstorm:

Also called "pop-up" thunderstorms or single cell thunderstorms, the ordinary thunderstorm is usually localized in extent.
In the summer heating at the surface causes warm air to rise, building cloud mass, eventually causing an isolated afternoon thunderstorm.  Sometimes downdrafts from one storm will kick up air ahead of it forming a second storm.



 
 

2) Squall-Line Thunderstorm:

Thunderstorms that form in a line, usually with forward movement.  The storms most commonly occur along a frontal boundary.   The forward motion of the storms cause new storms to be generated in advance of the front.  Individual cells make up the squall line but the cells are closely spaced and continually forming giving the impression that the thunderstorm is a continuous event lasting longer than the ordinary thunderstorm.

Squall line thunderstorms affect a much larger area than an ordinary thunderstorm.  Hail can be associated with squall line thunderstorms  and as well as dangerous microbursts.

The most severe squall line thunderstorms occur along frontal boundaries with the greatest temperature differences.



 

3) Super-Cell Thunderstorms (Mesocyclones):

Very complex storms formed most commonly over the Plains during the Spring when warm moist air from the Gulf of Mexico encounters dry cold air from Canada.  Colder, higher altitude air flows over rising warm humid surface air.  This puts a spin on the air causing a rotating mass.  The central updrafts rotate around the down drafts.  The cold down drafts and warm up drafts are separated from one another so the storm does not easily dissipate and can last hours.  Supercell storms bring heavy rain and can travel over hundreds of miles.  Large hail and  tornadoes are common.
 

Mesocyclones form along frontal boundaries of extreme temperature differences, in areas of warm surface air covered by cold air aloft and in areas where there are upper atmosphere wave disturbances.

 
  
  
 


Effects associated with thunderstorms:
 

Lightning: friction from up and down drafts cause electrons to be stripped off of water drops and ice crystals.   These electrons collect at the bottom of the cloud causing an electrical potential.... negative charges at the bottom of the cloud and positive charges at the top of the cloud.

As the electrical potential builds electrons form a stepped leader, a chain of electrons.  When these electrons near the ground they attract positive ions, when the positive and negative connect a flow of electrons is generated as a lightning bolt.  The electrons flow from the cloud balancing the electrical potential.  The flow is through a series of successive "branches".  The branching nature causes the lightning to appear to flicker.  Lightning can flow cloud to cloud, cloud to ground or within an individual cloud.

Thunder: as the lightning discharges the column of air through which is flows is rapidly heated.  The rapid heating causes expansion.  As the air expands it causes a sonic boom that we hear as thunder.  There can NEVER be thunder without lightning or lightning without thunder.  The lightning may be visible and the thunder not heard or the thunder may be heard and the lightning not visible but the two will always occur together.

Since light travels faster than sound it is true that you can count the seconds between when the lightning is seen and the thunder heard to estimate the distance of the lightning strike.  Every 5 seconds represents one mile of distance.

Hail: Hail forms when updrafts in the thundercloud cause water drops to freeze as they are pushed high into the cloud.  Downdrafts then force the ice into the lower portion of the cloud where more water is adsorbed to its surface.  As the ice with water attached moves back up into the cloud more freezing occurs.  Over many trips up and down through the cloud successive layers of ice are built up forming a hail stone.  The size of the hail stone is determined by the speed of the updraft because the larger the hail becomes the more force is required to keep the hail suspended in the cloud.  Eventually the hail will become too large and heavy to remain suspended in the cloud and it drops to Earth.

The largest hail stone recorded was a 1.67 pound, 5.5 inch diameter stone from Coffeyville Kansas.
To produce a hail stone .5 inches in diameter an updraft of 22 mph is needed.
To produce a hail stone 3 inches in diameter an updraft of 100 mph is needed.

Tornadoes:  a tornado is  a center of low pressure with winds in excess of 200 mph.  Tornadoes are often associated with severe thunderstorms and can last from minutes to hours.  A tornado forms when cold air streaks over warm air in a thunderstorm causing a rolling motion.  The rolling column of air is tilted vertically and a tornado touches down.  A tornado can also form from the center of a mesocyclone.  As the storm rotates a tight center rotation may drop out of the storm as a tornado.

Doppler radar is used to detect tornadoes.  Doppler radar can determine if air is moving away or towards the radar unit and thereby determine if rotation is present.

The Fujita Scale is a measure of tornado intensity.