Weather Fronts

Weather fronts are the leading edges of air masses with different density (e.g. air temperature or humidity). When a front passes over an area, it is marked by changes in temperature, moisture, wind speed and direction, atmospheric pressure, and often a change in the precipitation pattern.

Cold Front

A cold front is defined as the leading edge of a mass of air which is colder than the air in front of it.. The colder air, being denser, wedges under the less dense warmer air, lifting it, and causing the formation of mostly cumuliform (puffy, cotton-ball-like) clouds. The passage of a cold front usually results in changes in wind speed and creates vertical movement of air (turbulence) and can set off atmospheric disturbances such as rain showers, thunderstorms and snowstorms ahead of and immediately behind the moving cold front. The air behind the cold front is generally drier and cooler than that which it is replacing. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel.

Warm Front

A warm front is defined as the leading edge of a mass of warm air. Warm fronts move more slowly than cold fronts and consist of generally stable air, i.e., little vertical air movement, causing the formation of mostly stratiform clouds. Warm fronts usually bring steadier, lighter precipitation in the form of rain, fog or snow which can last from a few hours to several days. On weather maps, the surface location of a warm front is marked with a red line of half circles pointing in the direction of travel.

Occluded Fronts

Occluded fronts (sometimes called Trowal) are formed when a cold front overtakes a warm front, forcing the warm air upwards. The two fronts curve up naturally into the point of occlusion, also known as a triple point. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. Occluded fronts are indicated on a weather map by a purple line with alternating half-circles and triangles pointing in direction of travel. Occluded fronts usually form around mature low pressure areas. They are special forms of a Trowal (Trough of Warm air Aloft), but not all trowals are surface-based occlusions. Many times, trowals end up being analysed as surface troughs that appear inside a dense, cool air mass.

Stationary Front

A stationary front is a boundary between two different air masses, where neither of them is strong enough to replace the other. They tend to remain essentially in the same area for extended periods of time, usually moving in waves. A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts will either dissipate after several days or devolve into shear lines, but can change into a cold or warm front if conditions aloft change. Stationary fronts are generally more numerous in the summer months. Stationary fronts are marked on weather maps with alternating red bumps and blue spikes pointing in opposite directions, indicating no significant movement.