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What Is A Front?

A front is defined by the transition zone or boundary between two air masses with different characteristics including: temperature, wind direction, density and dew point.

A good comparison would be a red balloon (representing a warm air mass) and a blue balloon (representing a cooler air mass) colliding above a kitchen table.

From this we can see that a front not only defines the horizontal line of collision at the table surface, but it also includes the collision area that extends upward from the table.

This example shows that fronts are described by their interaction near the ground, as well

Types Of Fronts

There are four different types of fronts, differentiated by the type of air masses involved and the direction in which they are moving.

When listening to the weatherman on the radio or watching him/her on TV, the terms they used to describe these air masses are warm fronts, cold fronts, stationary fronts and in some circumstances, occluded fronts.

Warm Fronts

It is not hard to distinguish between the different frontal systems, as the name of the system gives us hints as to what is happening.

A warm front is the leading edge of a warmer air mass coming in to replace a cooler one. 

The general direction in which warm fronts move in North America is from the southwest to the northeast.

Since the air temperature increases as a warm front moves in, the air mass is able to hold more moisture and thus brings warmer and more humid characteristics with it.

The symbol for a warm front as shown on a weather map or a weather channel is a solid line (most often red) with semi-circles pointing in the direction of its movement and towards the cooler air mass.

Precipitation Caused By Warm Fronts
Characteristics Of A Warm Front

-tends to move slowly

-produces less violent weather (light to moderate continuous rain)

-proceeded by cirrus clouds, then altostratus or altocumulus, stratus and in some cases fog (sometimes referred to as the “lowering of the ceiling”)

-brings about clear skies that follow

-gentle slopes (of air)

A Stationary Front

This is a unique front in that it can be produced from either a cold air mass or a warm air mass. Just as the name implies, a stationary front occurs when an air mass stops moving, returning to a cold front or warm front once it is set in motion again.

Since the front is the boundary between two different air masses, it is represented on a map by alternating solid blue and red lines.

On the lines, there are blue triangles pointing towards the warmer air and red semi-circles pointing towards the colder air. Since the air masses involved are of different temperatures, there is a significant change in temperature and/or wind direction when crossing from one side of the front to the other.

Characteristics Of A Stationary Front

 -similar to warm fronts, but even more inactive

-most often, the winds on either side of the front are parallel to each other

Characteristics Of An Occluded Front

 -changes from higher to lower temperatures

-lower dew point temperatures (indication of drier air)

-changes in wind direction

-indicative of dissipating storms (mature storms) 

Signs Of Fronts

When a front moves across an area, not all of the below characteristics must change, but they are definitely clues that a new air mass is moving into an area.

 -sharp changes in temperature

-shifts in wind direction

-changes in moisture content of the air (humidity)

-cloud cover

-precipitation

 

Cold Fronts

Similar to the warm front, a cold front is the leading edge of colder air coming in to replace warmer air.These cold fronts tend to move from northwest to southeast bringing cooler temperatures and dryer air.

The movement of cold air into an area can be quite noticeable as temperatures can drop more than 15 degrees in a short amount of time.

The symbol on a weather map for a cold front is a solid line (usually shown in blue) with triangles pointing in the direction of movement and pointing towards the warmer air.

Characteristics Of Cold Fronts

 -Cold fronts have a tendency to move faster than all other types of fronts

-are associated with some of the most violent weather

-tend to maintain their intensity for very long distances

-cirrus clouds often proceed a cold front

-violent thunderstorms along and ahead of the front can result due to the drop in temperature caused by the cold front

-often followed by a broad band of clouds behind the front

-can also be associated with squall lines or mini thunderstorms located parallel or ahead of the front

-usually bring cooler temperatures, clearing skies and a drastic change in the direction of the wind

-steep slope (of air)

Precipitation Caused By Cold Fronts

As a fast moving, dense cold front runs into a warm air mass, the warm, moist air is pushed upwards quickly.

This sudden intense rising of moist air forms the extremely tall clouds that violent storms stem from.

Accompanied by heavy thunder and lightning, these storms produce heavy rainfall and only last for short periods of time.

An Occluded Front

The main requirement for an occluded front to occur is that a cold front must catch up to a warm front and overtake it.

To the north of this warm front, we also need a third component of cold air in place before the fronts start moving. On a map, an occluded front is shown by a solid line (usually purple) with alternating triangles and semi-circles, all pointing in the direction of travel.

Types Of Occluded Fronts

As the cold front rotates around and overtakes the warm front, two types of occlusions can be formed.

In North America, the most common type of occlusion is called a “cold-front occlusion” which occurs when the air behind an occluded front is colder that the air ahead of it. In this case, the colder air behind the front undercuts the cool air in front of it.

The other type of occlusion is called a warm occlusion and occurs when the air behind the front is warmer than the air ahead of it.

Similar to a warm front, the slightly warmer air behind the front is lifted up over top of the cold air.