How Thunderstorms Form
Tornadoes occur together with large thunderstorms. Thunderstorms form when warm air quickly rises from the ground and meets cold air in the upper atmosphere. This process, also known as "convection" in meteorology, is a natural process where warm air rises from the surface while the cold air goes down. Although the process doesn't always form a thunderstorm, if convection rapidly occurs, thunderstorms are common. This sudden meeting of cold and warm air forces the warm air to rise even higher, condensing a large amount of moisture in the air. When moisture condenses, it turns into water and falls as rain, hail, snow or sleet.
How Tornadoes Form
During a thunderstorm formation, the wind picks up speed and creates a body of horizontally rotating air. Once the horizontal column of rotating air forms, the sudden updrafts of warm air from the ground force the horizontal column to a vertical position. When this happens, the column of air tightens and gains more speed. This column of fast moving and concentrated rotating wind reaches the ground and forms a tornado.
Tornadoes in Cold Weather
Although thunderstorms commonly form in warm weather, they can also form in cold weather if the right conditions are met. There are cases where a layer of warm air forms on top of cold air on the ground. On top of the layer of warm air is another layer of cold air on the higher atmosphere. This warm air -- sandwiched between two cold air masses -- causes sleet and freezing rain. When snow falls from the top layer of cold air and passes through the warm air, it melts and turns to water; however, this water is still cold and changes into sleet or freezing rain when it enters the cold air on the ground. If the warm air becomes unstable and starts to rise quickly, it can form a thunderstorm even in cold weather. This thunderstorm is known as an "elevated thunderstorm," as it occurs on top of the cold air on the ground. Like the "regular" tornado, the only thing that needs to occur is for the horizontally rotating air to change its direction towards the ground. Meteorologists believe cold weather tornadoes can form, but rarely occur.
Cold Air Funnel
Cold air funnels are somewhat "incomplete tornadoes" that occur above the ground during cold weather. Since the layer of warm air is on top of the cold air covering the ground, the rotating column of air does not touch the ground and dissipates along the layer of warm air. Cold air funnels are not a threat unless they become strong enough to form a tornado.