Conveyor belts

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So far, you’ve looked in to why depressions happen in the mid-latitudes and what sort of weather they bring. You saw that latent heat is released when cloud droplets condense in the depression and that this is the energy source that powers the storm. But where does the moisture come from to allow more cloud droplets to form? Without a constant input of warm moist air, a depression would soon lose energy and start to decay.

The feed of warm, moist air is brought by an airflow called the warm conveyor belt. It is found in the warm sector air, and brings air in just ahead of the cold front, before rising up the warm front. Some of the warm conveyor belt air rises up the cold front. You can see this illustrated in red in Figure 1.

Figure 1: A depression showing the cold and warm conveyor belts © Dr Peter Inness

Figure 2: The distinctive cloud patterns associated with the cold and warm conveyor belts © Dr Peter Inness

Figure 3: The cold and warm conveyor belt cloud later in the lifecycle of the depression © Dr Peter Inness

Figure 4: The cold conveyor belt cloud at the end of the depression’s lifecycle © Dr Peter Inness

All that is left is the cold conveyor belt cloud. There is no longer an energy source for the depression and it quickly decays and disappears.