The boundary between a thunderstorm’s outflow and the surrounding air is called a gust front.
Thunderstorm downdrafts carry rain-cooled, denser air toward the ground. That air spreads outward when it reaches the surface, and its leading edge forms a gust front, also called an outflow boundary. The boundary can produce an abrupt wind shift, a pressure jump, cooler temperatures, and strong gusts even when little rain reaches an observer.
On radar, a gust front may appear as a thin arc or “fine line.” From the ground, it can be marked by a shelf cloud, roll cloud, blowing dust, or a sudden blast of wind. Gust fronts may travel far beyond the storm that produced them and can persist for more than a day.
The feature resembles a cold front, but its origin is different: a gust front is generated by thunderstorm outflow rather than a large-scale air-mass boundary. It can also create low-level wind shear hazardous to aircraft and can help trigger new thunderstorms where it meets warm, moist air or another boundary.