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CQ# 42: What happens when
two air masses meet?
air mass- a large body of air that has similar
temperature, humidity and pressure throughout it.
front- the area where two air masses meet and do
not mix (from military language meaning a
battle area where opposing armies meet to fight)
**When 2 air masses meet, the more dense air mass
will sink beneath the less dense air mass**
Types of fronts
A front always causes a change in weather because the air
masses that meet do not mix and their different densities
and air pressure produce wind, clouds and temperature
changes.
There are 4 types of fronts:
a) cold front
c) stationary front
b) warm front
d) occluded front
Cold Front
If a cold air mass runs into a slowly moving warm air
mass, the denser cold air slides under the lighter
warm air. The warm air is pushed upward.
What will happen if the warm air mass
contains only a little water vapor?
 Warm air with little water vapor may be accompanied
by ONLY cloudy skies.
 If there is a lot of water vapor in the warm air, heavy
rain(thunderstorms)or snow may fall.
After a cold front passes….
 skies clear up and leave cooler temperatures.
Warm Fronts
At a warm front, moving warm air collides with a cold
air mass. Cold air is more dense than warm air, so the
warm air moves over the cold air.
What type of weather do we see with a
warm front ?
 If warm air is humid, showers and rain fall along the
front. In winter warm fronts can bring snow.
 If warm air is dry, scattered clouds form.
 Because warm fronts move more slowly than cold
fronts, weather may be more foggy or rainy for several
days.
 After a warm front passes, the weather is likely to be
warm and humid.
Stationary Fronts
 Where cold and warm air masses meet, but neither has
the force to move the other.
Weather along a stationary front
 Water vapor in the warm air condenses into rain, snow
or clouds.
 Depending on wind direction the front will start
moving again becoming either a warm or cold front.
Occluded Fronts
 When 2 cold air masses trap a warm air mass between them;
the warm air mass doesn’t touch the ground
 The warm air rises as the two cold air masses come together. The cold front
usually overtakes the warm air.
 Temperature near the ground becomes cooler, and weather may turn cloudy,
rainy or snowy.
Animations:
 http://www.phschool.com/webcodes10/index.cfm?wc
prefix=cfp&wcsuffix=4031&fuseaction=home.gotoWeb
Code&x=0&y=0
Checkpoint:
1. What is a front?
2. What type of front is formed when moving warm air
collides with or pushes into a cold air mass?
3. What type of front is formed when a cold moving air mass
collides with or pushes into a warm air mass?
4.What type of front forms when two air masses meet and
neither one can move?
5. Why do most fronts cause changeable weather, rain or
cloud formation?
6. Why is the warmer air mass always pushed upward?
7. What happens when 2 air masses meet?
8. Draw a picture in color, of a cold front.
9. Draw a picture in color, of a warm front.
10. Draw a picture in color, of a stationary front.