Chapter 12, Part 2

Fronts

Fronts

A transition zone between two air masses of different densities is called a front.

There is usually a temperature difference across a front as well.

The humidities may differ as well.

Types of Fronts

Stationary Fronts

Do not move.

On the map, semicircles point to warmer air and triangles point to cooler air.

Here, air blows parallel to the fronts.

Cold Fronts

Cold air is replacing warm air.

The triangles indicate the direction the front is moving.

Determining Location of Front

Sharp temperature changes over short distance

Changes in the air’s moisture content (dew pt.)

Shifts in wind direction

Pressure and pressure changes

Clouds and precipitation patterns

Cold Front Example

Typical Airflow at a Cold Front

Cold dense air forces warm air upwards.

Warm moist air rises and condenses into clouds, producing rain showers at the front.

Leading edge of front is steep (1:50 for a 25 knot front).

Regenerated Fronts

Frontolysis – temperature contrast lessens and front weakens

Frontogenisis – temperature contrast increases and front strengthens

Typical Weather at a Cold Front

Warm Fronts

Warm air is replaces cold air.

The semicircles indicate the direction the front is moving.

Front moves slowly (10 knots), about half the speed of an average cold front.

Warm Front Example

Typical Airflow at a Warm Front

Warm air rises over cold air,

creating clouds and rain showers ahead of the front.

Leading edge of front has a gentle slope (1:300).

There is a temperature inversion (frontal inversion) just ahead of the front.

Typical Weather at a Warm Front

Occluded Front

When a cold front catches up to and overtakes a warm front, the boundary is called a occluded front (or occlusion).

On a weather map, there are alternating cold-front triangles and warm-front semicircles pointing in the same direction.

Cold-Occluded Front

The cold air of the cold-front lifts both the warm and cold air ahead of it.

Warm-Occluded Front

The cold air of the cold front rises over the colder air ahead of it.

Typical Weather - Occluded Front

Summary

When two air masses meet, a front is created.

There are four kinds of fronts:

Stationary – not moving

Cold – cold air replaces warm air

Warm – warm air replaces cold air

Occluded – cold front catches up with warm front


	A transition zone between two air masses of different densities is called a front.
	There is usually a temperature difference across a front as well.
	The humidities may differ as well.


	Do not move.
	On the map, semicircles point to warmer air and triangles point to cooler air.
	Here, air blows parallel to the fronts.


	Cold air is replacing warm air.
	The triangles indicate the direction the front is moving.


	Sharp temperature changes over short distance
	Changes in the air’s moisture content (dew pt.)
	Shifts in wind direction
	Pressure and pressure changes
	Clouds and precipitation patterns


	Cold dense air forces warm air upwards.
	Warm moist air rises and condenses into clouds, producing rain showers at the front.
	Leading edge of front is steep (1:50 for a 25 knot front).


	Frontolysis – temperature contrast lessens and front weakens
	Frontogenisis – temperature contrast increases and front strengthens


	Warm air is replaces cold air.
	The semicircles indicate the direction the front is moving.
	Front moves slowly (10 knots), about half the speed of an average cold front.


	Warm air rises over cold air,
	creating clouds and rain showers ahead of the front.
	Leading edge of front has a gentle slope (1:300).
	There is a temperature inversion (frontal inversion) just ahead of the front.


	When a cold front catches up to and overtakes a warm front, the boundary is called a occluded front (or occlusion).
	On a weather map, there are alternating cold-front triangles and warm-front semicircles pointing in the same direction.


	The cold air of the cold-front lifts both the warm and cold air ahead of it.


	The cold air of the cold front rises over the colder air ahead of it.


	When two air masses meet, a front is created.
	There are four kinds of fronts:
		Stationary – not moving
		Cold – cold air replaces warm air
		Warm – warm air replaces cold air
		Occluded – cold front catches up with warm front