220.127.116.11. 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).
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.