Tornadoes depend on three preconditions:

1 at least 2000 J/kg of convective available potential energy CAPE (Figure 7.6), which is possible when the planetary boundary layer (PBL) is warm and humid, and the air aloft is dry and cool,

2 a thin stable layer above the PBL (Figure 7.9), sufficient to prevent the instability being already released by small thunderstorms, and

3 a great increase of wind strength with height.

This combination arises in the American Midwest when warm, moist, low-level winds blow northwards from the Gulf of Mexico and undercut a strong westerly flow which has lost most of its moisture over the Rockies. Once the stable layer between is breached, there is a sudden updraught of the buoyant lower air, creating a massive thunderstorm which can spawn a tornado.

Some tornadoes occur in association with small thunderstorms ahead of a cold front or within a tropical cyclone (Chapter 13). Many tornadoes in Australia are triggered by convection due to vigorous cold fronts.

Figure 7.9 A schematic diagram of the circulations associated with a tornado within its parent thunderstorm.

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