What is a Gust Front?

In Meteorology there are a plethora of scientific terms, but they can't all be used on air since a majority of the public doesn't know the definition of the words.  So today I'd like to go over a term that you might hear in the severe weather season, its a gust front. 

A gust front is a line, usually ahead of a thunderstorm, with cold air that is rushing out of the storm.  There are two ways to have a gust front; the first is the addition of some dry air into a thunderstorm.  This dry air allows for some rain to evaporate, and since evaporation is a cooling process, the air is cooled rapidly.  This cold air moves down quickly and usually towards the leading edge of the storm.  The second way, comes from falling precipitation, as the rain falls it drags the air downward with it.  This rush of air down allows for a gust front to form.

There are a few things that are usually seen when a gust front forms.  The first can be seen on the ground, the gust front can be observed as a shelf or roll cloud on the leading edge of the storm.  The second goes hand in hand with the first, as the fast moving air moves along the ground it can pick up debris and that can be seen as well, just under the shelf or roll cloud.  The third is seen on radar and works hand in hand with the second indicator.  The debris being picked up as well as insects being blown along by the front, actually show up on the radar.  This can be seen as a thing line ahead of thunderstorms.  The image below is an example of the debris being picked up by a gust front (courtesy of NOAA).  The radar image, above, was taken from this morning (5/7) around 3:00 am CST and shows a few gust front ahead of some thunderstorms, this is indicated by a thin green line (click the picture to get a larger version).