What are the conditions under which a tornado is likely to occur?

Tornadoes are likely to occur when a storm system drives a cool air mass and a dry air mass into a very warm, moist one, producing thunderstorms. A layer of stable air called a cap holds back storm development at first allowing instability to build, then the strongest storms break the cap and grow at explosive rates. They then encounter wind shear, where the wind speed increases and wind direction rotates counterclockwise (or clockwise in the southern hemisphere) with height. This sets the storms rotating. The rotation in these severe storms, called supercells, can then go on to produce tornadoes