Frontal Systems

Fronts

-A front is a boundary between two air masses. Fronts are classified and named according to which air mass is replacing the other.

Warm Front

Side view of a warm front. Image by Climateandweather.

• Occurs when warm air is displacing cold air i.e cold air is receding.
• The air behind a warm front is warm and moist, while the air ahead of a warm front is cooler and less moist.
• Similar to the cold front, there will a shift in wind direction as the front passes and a change in pressure tendency.
• Warm fronts have a more gentle slope than cold fronts, which often leads to a gradual rise of air.
• This gradual rise of air favors the development of widespread, continuous precipitation, which often occurs along and ahead of the front.
• Although they can trigger thunderstorms, warm fronts are more likely to be associated with large regions of gentle ascent (stratiform clouds and light to moderate continuous rain).
• Warm fronts are associated with a frontal inversion (warm air overrunning cooler air).
• Warm fronts are represented on a weather map by a solid red line with semi-circles pointing in the direction of its movement as pictured above.

Cold Front

Another side view of a cold front. Image by ClimateandWeather

• A front is called a cold front if the cold air mass is displacing the warm air mass.
• The air behind a cold front is colder and typically drier than the air ahead of it, which is generally warm and moist.
• There is typically a shift in wind direction as the front passes, along with a change in pressure tendency (pressure falls prior to the front arriving and rises after it passes).
• Cold fronts tend to be associated with cirrus well ahead of the front, strong thunderstorms along and ahead of the front, and a broad area of clouds immediately behind the front.
• Cold fronts usually bring cooler weather, clearing skies, and a sharp change in wind direction.
• Cold fronts can be associated with squall lines.
• Cold fronts have a steep slope, which causes air to be forced upward along its leading edge.
• This is why there is sometimes a band of showers and/or thunderstorms that line up along the leading edge of the cold front.
• Cold fronts are represented on a weather map by a solid blue line with triangles pointing in the direction of its movement as pictured above.

Occluded Front

Diagrams showing the formation of an occluded front:

Fast cold front catches up with a slow warm front to form an occluded front. Image by ECN

The diagram shows a fast cold front catching up with a slow warm front to create an occluded front.

Side view of an occluded front. Image by CUNY.

• Generally, cold fronts move faster than warm fronts.
• Sometimes in a storm system the cold front will “catch up” to the warm front.
• An occluded front forms as the cold air behind the cold front meets the cold air ahead of the warm front.
• Which ever air mass is the coldest undercuts the other.
• The boundary between the two cold air masses is called an occluded front.
• Occluded fronts are represented on weather maps by a solid purple line with alternating triangles and semi-circles, pointing in the direction of its movement as pictured above.
• Occluded fronts are linked with areas of low pressure called depressions.