1.1.2.2. Initiation of convection – how thunderstorms start (daytime heating, fronts, etc)

Another way of initiating convection is orographic lift: air moves horizontally over flat terrain, but is forced upwards when it encounters higher terrain (hills, mountains).

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There is another mechanism that can assist in initiating convection without a convergence of surface winds or orographic lift. Upper level forcing happens when winds at high altitudes (5-10 km) diverge, i.e. the air flow fans out. This creates an area of low pressure, which encourages vertical motion, which in turn helps moist warm air rise before reaching convective temperature.

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