AIR MASSES AND FRONTS
An air mass is a large body of air that has similar temperature and relative humidity throughout. They form
over large flat areas where air can settle long enough to take on the characteristics of the surface below. In our
area, cold air masses are often generated over Canada. Generally, warm air masses come up from southern
regions
Fronts
the boundaries between air masses
A front is defined as boundary between two air masses of different temperatures and humidity. A cold front is
defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally
move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead
of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour.
Symbolically, a cold front is represented by a
solid line with triangles along the front pointing
towards the warmer air and in the direction of
movement.
There is typically a noticeable temperature
change from one side of a cold front to the
other. In the map of surface temperatures to the
left, the station east of the front reported a
temperature of 55 degrees Fahrenheit while a
short distance behind the front, the temperature
decreased to 38 degrees. An abrupt temperature
change over a short distance is a good indicator
that a front is located somewhere in between.
Low Pressure Systems and Associated Cold Front
Below is a simple model of a low pressure system or storm, with a cold front extending to the south from the
center of low pressure and a warm front extending to the east ahead of the storm.
At low levels, several air masses of
distinctly different origin may be found in
varying parts of the low pressure system.
The cold front marks the leading edge of a
colder and drier air mass being wrapped
southeastward by north-northwesterly winds
behind the low.
Clouds and precipitation usually develop
along and ahead of the cold front as the
colder air mass lifts the warm moist air
ahead of it.
When a cold front pushes into warm air
Initially, the cold air mass wedges into the warmer air mass ahead of it, (separated from each other by the cold
front). The lighter warm air is lifted upwards by the denser cold air and if enough water vapor condenses,
clouds develop.
Warm Front
transition zone from cold air to warm air
A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm
fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist
than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and more
humid than it was before.
Symbolically, a warm front is represented by
a solid line with semicircles pointing towards
the colder air and in the direction of
movement.
On colored weather maps, a warm front is
drawn with a solid red line. There is typically
a noticeable temperature change from one
side of the warm front to the other. In the map
of surface temperatures below, the station
north of the front reported a temperature of 53
degrees Fahrenheit while a short distance
behind the front, the temperature increased to
71 degrees. An abrupt temperature change
over a short distance is a good indication that
a front is located somewhere in between.
When a warm air overrides over cold air
Initially, a warm air
mass nudges against a
colder air mass ahead of
it, (separated from each
other by the warm
front). The lighter warm
moist air behind the
front is lifted upward
and "overrides" the
colder air.
As the air rises, it
cools, and if enough
water vapor
condenses, widespread
clouds and
precipitation develop.
Answer the questions below:
1. In your own words explain what happens when a cold air mass pushes into a warm air mass ?
2. In your own words explain what happens when a warm air mass pushes into a cold air mass?