It depends. In the oversimplified scenario presented by the media, tornadoes form along a boundary between warm and cold air masses. However, tornadoes may also be caused by the boundary between masses of contrasting moisture content, rather than temperature. In other cases, they may form within a warm, moist air mass, away from any boundaries.
The colliding air masses in Tornado Alley are warm, moist air from the Gulf of Mexico, cool air from Canada, and dry air from the Rockies. This collision is just part of the recipe for tornadoes.
Stationary Front
Air masses are not involved in typhoons because these are tropical storms, as opposed to mid-latitude cyclones. The air in these storms is homogeneously tropical.
Usually, a stationary front has two air masses. It becomes a stationary front when two different air masses are too weak to replace the other. A wide variety of weather can be found along a stationary front.
Tornadoes in the United States are generally associated with collisions of air masses. In mmany cases there is a warm, moist air mass that originates over the Gulf of Mexico. This meets a cool air mass from Canada, a warm, dry air mass from the Rocky Mountains, or both.
The colliding air masses in Tornado Alley are warm, moist air from the Gulf of Mexico, cool air from Canada, and dry air from the Rockies. This collision is just part of the recipe for tornadoes.
A Tornado A Front.
The cool air sinks, while the warm air rises. If it does so with enough force and torque, a tornado or hurricane will form.
Stationary Front
Air masses are not involved in typhoons because these are tropical storms, as opposed to mid-latitude cyclones. The air in these storms is homogeneously tropical.
When two air masses collide, the formation is called a front. Fronts can be warm, cold, stationary, or occluded, depending on the characteristics of the air masses involved.
Most often a warm, moist air mass collides with a cool air mass, a cold air mass, or both. However, such a collision alone will only form thunderstorms. Other factors are needed for those storms to produce tornadoes.
In Tornado Alley Warm moist air from the Gulf of Mexico meets with dry air masses from the Rockies and/or cool air masses from Canada. These collisions can produce violent thunderstorms that can sometimes produce tornadoes. See the related question below for more detail.
Air masses of low density tend to rise because they are less strongly affected by gravity than cool air masses. Rising warm air masses is the primary cause of convection on earth. Thunderstorms are a product of convection. Tornadoes are a product of thunderstorms.
A tornado usually requires a warm, moist air mass, most often when it collides with a cool and/or dry air mass.
Usually two, but sometimes it can be three.
If the two air masses are moving together in different directions, you could get a tornado.