Tornadoes often form during violent thunderstorms when there is sufficient (1) instability and (2) wind shear present in the lower atmosphere.
Instability refers to unusually warm and humid conditions in the lower atmosphere and cooler than usual conditions in the upper atmosphere. Wind shear refers to the wind direction changing and the wind speed increasing with height. An example would be a southerly wind of 15 mph at the surface, changing to a southwesterly or westerly wind of 50 mph at 5,000 feet altitude.

Scientists have watched the formation of tornadoes and have determined that this kind of wind shear and instability usually occurs where?

A) within a mesocyclone
B) ahead of a cold front and low pressure system
C) when a warm front is overcome by an arctic front
D) behing a high pressure system moving across the country