Interesting Weather Information

Thursday, March 14, 2013

Thunderstorm Primer - Introduction

My Thunderstorm Primer is a series of posts that will discuss thunderstorms both severe and non-severe along with the associated phenomena of lightning, mesocyclone tornadoes, leading edge vortices and hail in 10 separate posts in addition to this introduction. Here they are:

Introduction: Convective Storms - Background Information
Part 1. (So Called) Air Mass Thunderstorms
Part 2. Mesoscale Convective Systems - Squall Lines
Part 3. Mesoscale Convective Systems - Bow Echoes
Part 4. Mesoscale Convective Systems - The Line Echo Wave Pattern (LEWP)
Part 5. Mesoscale Convective Systems - The Mesoscale Convective Complex (MCC)
Part 6. The Supercell Thunderstorm

Some Terminology
In meteorology convection is the process of transporting heat vertically or nearly so by the movement of a mass of air. The transport of heat horizontally by the  movement of a mass of air is termed advection. So when warm air is arriving from Texas a meteorologist would say "warm advection' is occurring.

Convection is possible because of differential heating of the surface.  Even though TV meteorologists often just use the term "surface heating", the deep convectionthat causes thunderstorms requires the sun's energy be absorbed un-equally by  neighboring surfaces.

A large park will absorb less solar radiation than and adjacent shopping mall with a mega-parking lot. In turn the air above the shopping mall will become warmer and less dense. The air over the park will be cooler and more dense than the air over the mall.The cooler, denser air is pulled by gravity more than the warmer, less dense air. The cooler air then undercuts the warmer forcing it to rise. That's convection.

Before we proceed ....

Fighting Bad Science by Being Persnickety  - Steve's Meteorological Pet Peeve #1 
 Seemingly, poetically, magically clouds appear to float above our heads. But clouds do not float and warm air does not rise. We like to say they do, but sorry they do not because there is no magic in science.

To be fair sometimes saying clouds float is just a shortcut for saying "the clouds moved across the sky held aloft by a gentle updraft". You see it is easier to say they float. They do not.

Nothing moves in this physical world of ours without a force to do the work. Isaac Newton told us this in 1687. Warm air does not rise and clouds do not seemingly defy the force of gravity without forces. Warm air is forced to rise as cooler, denser air, pulled more by gravity than neighboring warm air undercuts it. That updraft keeps the heavier-than-air clouds aloft.

Back to the post...

A convective storm is one driven by unequal heating of the surface. The world is complex and it is not quite  that simple because rarely is there a single source of lift driving storms. In addition storms interact with their environments in many ways and to many degrees.

We can use these two factors, the sources of lift and the amount and type of interaction wit hte storm's environment to explain the forms and severity of the storms we see.

Next, Part 1 of my Thunderstorm Primer discusses what we probably should not call Air Mass Thunderstorms.

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