Inflow is the flow of a fluid into a large collection of that fluid. In meteorology, inflow normally means the influx (entry) of warmth and moisture from air within the Earth's atmosphere into storm systems. Extratropical cyclones are created by inflow focused along their cold front and warm fronts. Tropical cyclones need a large inflow of warmth and moisture from warm oceans to become very powerful, mainly within the lowest 1 kilometre (0.62 mi) of the atmosphere. Once the thunderstorms and their associated tornadoes do not get the flow of warm and moist air, the storms begin to dissipate.