Download Chapter 20.2

Chapter 20.2
Fronts
Formation of Fronts

Air masses have different
temperatures and different
amounts of moisture depending on
where they start.

Also the characteristics can change
as an air mass moves over a region.

When 2 air masses meet, they form
a front, a boundary that separates
two air masses.

Fronts form between any 2
different air masses. They also
usually produce some sort of
precipitation.
Types of Fronts

Fronts are classified by the
temperature of the advancing
front.

4 types of fronts: warm fronts, cold
fronts, stationary fronts, and
occluded fronts.
Warm Fronts

A warm front forms when warm air moves
into an area formerly covered by cooler
air.

On a weather map it will be shown by a
red line with red semicircles that point
toward cooler air.

The slope of a warm front is very gradual.
As warm air rises, it cools to produce
clouds, and precipitation.

A warm front will be signified by cirrus
clouds. The cirrus clouds will change into
cirrostratus clouds and then altostratus
clouds.

This type of front will produce light-tomoderate precipitation over a large area
for a long period of time. A slow increase
of temperature will also take place.
Cold Fronts

A cold front forms when cold,
dense air moves into an area
occupied by warmer air.

On a weather map the cold front
will marked a blue line with blue
triangles that point toward the
warmer air mass.

This front will become steeper and
move faster. Those 2 things make
the weather more violent.

There are usually heavy downpours
and gusty winds.
Stationary Fronts

In stationary fronts the surface
position of the front does not
move.

On a weather map these fronts are
shown by blue triangles on one side
and red semicircles on the other
side.

Sometimes gentle to moderate
precipitation occurs.
Occluded Fronts

When an active cold front
overtakes a warm front an
occluded front forms.

This forces the warm front up. It
may cause light precipitation.
Middle-Latitude Cyclones

Middle-latitude cyclones are large,
low pressure centers that travel
from west to east and cause
stormy weather.

This is the weather producers in
the US.

Most middle-latitude cyclones have
a cold from and a warm front that
moves from the center of the
cyclone.

Look at pg. 569 – explains how
middle-latitude cyclones are made.
The Role Of Airflow Aloft

Most of the time air from high up
in the atmosphere makes a middlelatitude cyclone.

Most of the time cyclones and
anticyclones will be found right
next to one another.