When moisture, unstable air and warm air mix together, you could have a big thunderstorm on your hands. That warmer air rises, and it rises quickly if it bumps into cold, sinking air, or against a hillside or mountain. As that warm air goes higher in the atmosphere, it brings any water vapor it’s holding along with it. It cools as it gets higher, and that water vapor condenses into liquid. A cloud begins to form. And it can grow upward into colder levels of the atmosphere, where the temperature is below freezing.
If the cloud continues to be fed by warm air below rising into it, it can develop into a cumulonimbus cloud—a good old thunderstorm.
Credit: NOAA/JPL-Caltech
When that cloud begins to rain, cooler air descends to the surface. That cool air and rain create something called a downdraft. When that cool air descends and hits the ground, it spreads out in all directions, like when you pour a column of water on the floor. This air can be moving fast, so this spreading out can create a gust front. This is a line of sometimes dangerously gusty winds. It’s also called an outflow boundary. A gust front is different from a simple downburst. Downbursts from convective storms are more localized, and are not considered gust fronts.
Credit: NOAA/JPL-Caltech
That gust front can also come in contact with the warm air that is “feeding” the thunderstorm. As the cold gust front goes down and out, the warm air is going over and up. This can create a swirling effect that can produce an ominous-looking shelf cloud. Some people call them roll clouds.
A desert gust front. Credit: NOAA.
The winds from the gust front can be very strong and dangerous. Monitoring gust fronts is important. They can strengthen an existing storm or they can act a triggers for new storms.
In places like Arizona with lots of dirt and dust, these gust fronts can create big dust storms. They look like walls of dust moving across the land.
Credit: NOAA/JPL-Caltech
Gust fronts are particularly dangerous for airplanes. It is hard to navigate through them because as you go toward a gust front, its wind blows against the front of the plane. As the plane flies through to the other side, suddenly the wind is at the back of the plane. For this reason, air traffic controllers pay special attention to gust fronts and tell pilots how to go around them safely.
Gust-front dust cloud, called a Haboob, moving across the Llano Estacado toward Yellow House Canyon near the residential community of Ransom Canyon, Texas. Credit: Leaflet, Wikimedia Commons. (Creative Commons Attribution-Share Alike 3.0).
Derechos
Gust fronts are in some ways similar to derechos, which are rapidly moving thunderstorms. Read all about them here.