If you like to look at the radar when observing thunderstorm events, you probably have noticed a long thin line that appears to be a line of rain propagating outward and away from the storms, much like what you see in the images above. This thin line isn’t actually rain, but a gust front. A gust front is a lot like a cold front, except it is on a smaller scale and affects a smaller area. Gust fronts are interesting to observe on radar because you can watch them pulse outward from under a thunderstorm and continue for a long time. While observing the lifespan of a gust front, if conditions in the environment are favorable, you can witness new storms generate in its wake.
Inside a storm, cooled air from evaporating rain becomes dense and sinks down from the storms base, this is called a storms downdraft. Once that air descends from the cloud and hits the surface, it spreads outward from under the thunderstorm. This is where the gust front takes place. As the cold air pushes forward, it forms a boundary between the surrounding warm, humid air on the surface. Just like a small cold front, the colder dense air digs under the warm air, forcing it upward. This is one of the reasons why, if a gust front is strong enough, and the conditions of the environment are favorable, you can watch new storms pop up on the radar during or after its passing.
In a conditionally unstable atmosphere, where the warm, humid air at the surface has a higher temperature than the surrounding air, it will rise. If the explained conditions continue to hold true, the air will continue to rise. In this type of unstable environment, if a gust front is strong and deep enough it can easily lift the warm, humid air ahead of it to the lifting condensation level. At the lifting condensation level, conditions are favorable for the water vapor in that air to condense into cloud droplets. If the air at the surface has enough moisture, it can provide enough water and energy to form a whole new storm. This is especially interesting to watch on radar because new storms tend to form and intensify very quickly after a gust front. They form either a new single cell thunderstorm or continue to generate into a multi-cellular thunderstorm. With each new storm comes a new gust front. Before you know it, you may have a growing cluster of storms popping up everywhere.
When observing gust fronts on radar, if one is approaching your area, you can observe its passing. You can both observe the radar and experience a gust front first hand. When a gust front passes, you will feel the wind pick up in speed and change directions while also experiencing a drop in temperature. It will become very gusty outside and you may even see a few storm features associated with a gust front. One storm feature is a shelf cloud. Shelf clouds are usually present at the leading edge of a storm. These clouds look ominous and form a shape of a shelf or a hood extending from the base of a storm. They are usually formed when moist warm air is forced upward by gust front and settles in a zone of stable air near the base of the storm. Also associated with a gust front is a roll cloud. This type of cloud gets taken up by the upward motion of air being forced over the gust front. It then circulates ahead of the storm on a horizontal axis.
When you experience a gust front passage, you may notice that there isn’t any rain that comes from the front but more so with the storm itself. You may wonder how and why this storm feature is visible on radar, especially if it isn’t a cloud full of rain. When a gust front rolls over the surface, its wind speeds can increase and gust up to 55 knots. A gust front that has strong winds and a thicker layer of cold, dense air is defined as an outflow boundary. These are stronger gust fronts and are much more visible on radar. The stronger the winds the more leaves, dust, dirt, sand and other light debris gets picked up and suspended into the air. The radar beam reflects off the suspended dust and dirt particles giving it a presence on radar. This is similar to how radar can pick up on bugs and insects when they migrate.
If you are a weather watcher and would like to observe the radar for gust fronts, a good radar to use is The National Weather Service radar. This picks up well on gust fronts, better than most website radars. It’s open to the public. All you have to do is go to the National Weather Service website and select the radar closest to your area. If you are not at home and away from the computer, NOAA Weather Radar and Radarscope are good apps to use on your phone. Next time you have some thunderstorms in your area, take a peek at the radar and see if you can spot a gust front. If you do and it is heading in your direction, it would be fun to go outside (if it’s safe to do so) and observe the shift in wind speed and direction when it passes.
For more on severe weather features click here
©2018 Meteorologist Alex Maynard