The meeting of a warm and cold air mass produces a weather system called a front. Depending on the season and climate zone, either cold or warm air prevails. If cold air dominates, the front is called a cold front, and when warm air takes over, the result is a warm front.
Warm and cold air masses are pushed together by strong winds. Since they have such different characteristics, the merger of warm and cold air creates an unstable atmosphere, and often turbulent weather spawns as a result. Although fronts themselves produce foul weather, the affected local atmosphere returns to normal quickly after fronts move through, and often enjoy a short period of light winds, sunny skies, and dry conditions following a front.
Cold fronts, as the name implies, are fronts comprised of predominantly cold, arctic air. These fronts form when cold air, which can be either moist or dry, takes over warmer and more humid air. Cold fronts that form in northern latitudes are called continental or maritime polar air masses. Warm fronts, in contrast, form in tropical regions. These fronts are referred to as tropical air masses. Typically, a polar air mass that moves through a location brings significantly cooler air. When they are comprised of moist air, they often produce precipitation too; in the wintertime, they are responsible for dumping large amounts of snow and delivering strong, frigid winds. During the transitional seasons of spring and fall, however, polar air masses may leave warmer air temperatures in their wake, since land during those seasons is warmer than ocean waters.
Warm fronts, in contrast to their northern cousins, bring warm and humid air year-round. These fronts are responsible for creating warm-weather atmospheric events such as thunderstorms, tornadoes and hurricanes. These fronts are considerably slower-moving than those that form in northern latitudes, due to their physical composition. Warm fronts consist of lighter, less dense air. It might seem like they should move faster because of this physical property, but the laws of physics prove otherwise. When warm fronts make their way through the atmosphere, they essentially act as feathers on the wind: it is hard for something so lightweight to push aside heavier air, which accounts for their slow speeds. When warm air masses do eventually meet cold air masses, however, the merger may produce a variety of different weather conditions, depending on which front prevails.
When a warm and cold air mass meet, the resulting front generally brings precipitation in some form, bad weather for a short period of time, then moves on. Sometimes, however, fronts stall. This is called a stationary front. Stationary fronts occur when neither air mass has the power to overtake the other. The weather produced by stationary fronts is quite unpredictable: stormy weather often develops towards the front end of the front, while quieter, calmer weather patterns develop towards the rear. Sometimes these fronts are joined by powerful winds, which might pull them in opposing directions. They spin as a result, and are responsible for creating significant storms, such as nor’easters.