A erecho is a widespread and long-lived, violent convectively induced windstorm that is associated with a fast-moving band of severe thunderstorms usually taking the form of a bow echo. Derechos are usually not associated with a cold front, but a stationary front within a highly buoyant, warm airmass. A warm weather phenomenon, derechos occur mostly in summer, especially July (in the northern hemisphere), but can occur at any time of the year and occur as frequently at night as in the daylight hours. The traditional criterion which has been used to distinguish a derecho from a severe thunderstorm are sustained winds of 58 mph/92 km/h at some point during the storm. High and/or rapidly increasing forward speed, longevity, distinctive appearance on radar (bow echo) and several unique features such as the rear inflow notch and bookend vortex, and presence of two or more downbursts are other common markers for this kind of event.