Weather Moment: Warm Fronts

Warm fronts are, in many ways, the complete opposite of cold fronts.  They often cover a large area and are slow moving, bringing extended periods of precipitation.    They often produce large amounts of rain or snow (depending on season), and may contained a sizable amount of embedded thunderstorms in them.

Formation

A warm front is formed in the same mechanisms as cold fronts; the warm front is the boundary of warmer air moving into cooler air.  Again, like a cold front, the actual temperature of the air does not matter, as long as it is warmer than the air it is moving into.

Structure

Warm fronts have a defined structure as well, and it is significantly different than a cold front’s structure.

Warm Front Structure

As the warm air moves into the colder air, it encounters air that is more dense than it.  Opposite of the actions of the cold air moving into the warm air, the warm front slopes over top of the cold air, as warm air slowly overtakes the cold air, gently being lifted over top of the cold air at the same time as it moves the cold air out.  This produces a front that is very different in appearance when compared to a cold front.  Instead of being narrow and extremely sharp, warm fronts are extremely wide features, sometimes in excess of 500km, that feature a very gradual slope to their fronts.  The passage of a warm front, due to it’s size, can often take 6-12 hours, and in some cases, as long as a day.

Common Weather

In the wintertime, warm fronts make snow.  A lot of snow.  A majority of the snowfall in East Coast North America winter storms comes from the warm front.  In the summer time, warm fronts often bring prolonged periods of light to moderate rain.  Cloudy conditions are widespread with warm fronts. 

In fact, a warm front is easily detectible and can be forecast by a few keen observations out your window.  The clouds associated with a warm front have a very distinct progression: first high cirrus clouds will move in.  They are the ones that are extremely high up and wispy looking.  They will gradually get lower and lower until there is some thicker mid-level cloud.  This cloud no longer looks wispy and is thick enough to mostly block out the sun.  These clouds will continue to get lower and lower until low clouds show up, which are close to the ground, and completely block out anything above them.  Often if you spot this progression over several hours, some sort of precipitation is likely on it’s way.

In the summer time, warm fronts can offer the conditions necessary to produce Mesoscale Convective Systems, which are a cluster (often you can draw a circle around them) of organized thunderstorms that are usually severe and can last 12-18 hours (although likely only 2-3 hours in any one given location).  As well, warm fronts often have thunderstorms embedded in them in the summer months.

The weather behind a warm front is not as definite as the weather behind a cold front.  It may be completely sunny, it may be cloudy, or it really can be anywhere in between.  The one thing for sure is that it’s warmer than it was before, and often it’s more humid as well.

Sorry it’s late, but I hope you enjoyed this!  Next week we’ll talk about low pressure systems and how these [warm and cold] fronts work in relation to the larger picture!

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