Download 4aAir Masses & Fronts

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The Final FRONTier
Fill in your note outline as you follow along with
fronts….
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A front is the boundary between two air masses
– where the temperature and humidity changes
An approaching front means a change in the
weather
At the front, the colder, denser air mass will slide
under the warmer, less dense air mass
Fronts usually bring precipitation because warm
air is forced to rise, so then it cools, condenses,
and forms clouds
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Types of Fronts
Figure 1: A front
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Cold Front: cold air is advancing (moving forward) and
displacing warmer air and bringing intense rain and
wind
Cold fronts have steep slopes, with the steepest part
near the ground because friction (with the ground)
slows the air mass
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Warm Front: warm air is advancing (moving
forward) and displacing colder air and bringing
light to moderate rain
Warm fronts have gentler slopes
Figure 2: Cold and Warm Fronts
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Stationary Front: neither air mass is being displaced,
and the front does not move
Occluded Front: when a cold front overtakes a warm
front
Formation of Mid-Latitude Low (Cyclone)
• In the area of continental North America from
Alaska/Yukon/Northwest Territories/Nunavut to
Florida, the main weather-producers are called
Mid-Latitude Lows, or Mid-Latitude Cyclones
• Mid-Latitude Lows are large centres of low
pressure that generally travel from west to east
over a period of a few days to over a week
• A low starts as a kink or wave in the polar front,
possibly caused by a kink or wave in the jet
stream
• The wave moves west to east and ripples the
polar front
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Figure 3: Formation of a Mid-Latitude Cyclone
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The polar front is the boundary between cool air
masses in the polar easterlies and warm air
masses in the prevailing westerlies
Cold polar air masses lie to the north of the front
and warm tropical air masses lie to the south of
the front
The front bulges southward on the west side of the
wave, and polar air pushes tropical air southward
(this is a south moving COLD front)
On the east side of the wave the polar front bulges
northward as a WARM front
Between these two fronts is a large pocket of
warm air that forms a region of low pressure called
a “low” or cyclone
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In the northern hemisphere, winds blow
COUNTERCLOCKWISE (anticlockwise) and
toward the centre of a low
This air converges in the low pressure area, rises,
and forms clouds and precipitation
This condensation releases heat energy, which
lowers the pressure at the surface, making the low
even stronger; this makes the winds stronger
Typical weather sequence as a low passes to the
North:
1. A long period of steady precipitation in advance of
the warm front
2. Warming and slow clearing AFTER the warm front
passes; if the air is humid, showers may occur,
especially near the centre of the low
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Showers around the time the cold front passes
Cooling and rapid clearing, with a change toward the
weather characteristic of the newly-arrived cold air
mass
Sequence of clouds and weather as a warm front passes
1. Cirrus and cirrostratus – no precipitation
2. Altostratus – sun (or moon) blocked
3. Heavy nimbostratus – steady rain (or snow)
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Clouds along a warm front are usually stratiform
because the air is stable – only horizontal movement,
no upward movement
Cold fronts are steeper and move faster than warm
fronts, so air is forced upward quickly by the cold front,
and is usually unstable – clouds are usually
cumuliform
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Cumuliform (such as cumulonimbus) clouds grow
quickly around the cold front
Often rain showers turning into heavier precipitation
and even thunderstorms
When a cold front passes, weather changes are abrupt
In a HIGH pressure area (an anticyclone), weather is
usually bright and clear, because dry air is sinking at
the centre (pressure is greatest) – wind moves
outward from the centre
Air moves in a CLOCKWISE direction away from a
high
Small cumulus clouds (fair weather) may form over
heated ground during the day – no precipitation
At night heavy dew, frost, and radiation fog may form
Temperature inversions may also form