Transcript Chapter11a

Air Masses and Fronts – I
Air Masses
•
•
A large (thousands of
kms) body of air with
more or less uniform
properties in any
horizontal direction
at any given latitude
♦ Temperature
♦ Humidity
Next time: fronts –
the boundary
between two
different air masses
Origin and formation of air masses
• formed its properties
Source Region: the location where the air mass
•
♦ Large, flat areas where the air stays stagnant long
enough to take on the characteristics of the
underlying surface.
♦ Ideal source regions are areas of high pressure
Types of sources.
♦ Tropical: warm
♦ Polar: cold
♦ Continental: dry
♦ Water (maritime):
humid
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• cA
Classification of air masses
cP
♦ continental polar
♦ cold, dry, stable
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•
•
♦ continental arctic
♦ very cold, very dry,
stable
mP
♦ maritime polar
♦ cool, moist, unstable
mT
♦ maritime tropical
♦ warm, moist, usually
unstable
cT
♦ continental tropical
♦ hot, dry, unstable
surface air
•
•
•
Stability of air masses
Which is
lighter?
♦ Dry air
♦ Moist air
Dry air:
♦ Nitrogen
♦ Oxygen
♦ Argon
Moist air: all
this plus
♦ H2O
The weight of humid air
• Mean molecular mass of dry air:
gas
m
%
mass of the gas mixture
-------------------------------------------N2
28
78%
28x78/100+32x21/100=28.6
O2
32
21%
• Mean molecular mass of wet air.
gas
m
%
mass of the gas mixture
--------------------------------------------Dry air 28.6
90%
28.6x90/100+18x10/100=27.5
H2O
18
10%
• Moist
air is lighter and less dense than dry air at the same
temperature.
• Moist
air rises more readily. Evaporation enhances convection in
the atmosphere.
Air mass source regions and their paths
Continental Polar (cP) and Continental Arctic
(cA)-winter time
• Forming conditions:
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•
♦ Above the northwest part of Canada and Alaska
♦ Long clear winter nights result in radiational cooling.
Properties:
♦ Very cold temperatures;
♦ Little evaporation -> low water content (low specific humidity and dew
point);
♦ Temperature inversion near the ground -> stable atmosphere;
♦ The cold subsiding air results in broad high P centers (anticyclones).
Weather in the winter:
♦ Bitterly cold, dry weather, clear sky;
♦ It is slightly warmer on the west and the east coast because of
compressional heating of the (slightly) sinking air.
♦ The stagnant, stable ground layer can create poor visibility and air
quality -> pollutants are trapped near the ground
cP, cA
Effect of cP air masses on the weather
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Upper level wind flow and surface position of
anticyclones during two cold spells in 1989 and 1990.
Numbers represent minimum temperatures.
Lake-Effect Snow
• Description: increased snow showers on
the downwind side of large unfrozen lakes.
It is a local phenomena.
• Conditions:
♦ The water is
warmer than the
ground (the first half
of the winter).
♦ Cold dry air is moving
over the lake.
• What
happens? – The cold dry air is warmed up at the surface and picks
up moisture. The boundary layer becomes unstable. Warm and moist air
rises and forms cold cumulous clouds. There is a continuous supply of
moisture from the lake. On the downwind side the air is additionally
cooled and/or lifted up by topography -> precipitation.
Lake Effect Clouds – satellite image
Snow Effect on the Great Lakes
cP air masses-summer time
• Forming conditions:
•
•
♦ Above the northwest part
of Canada and Alaska
♦ The melting ice adds
moisture to the air.
Properties:
♦ Cool temperatures (the sun is
never directly overhead);
♦ Increased water content and
humidity;
♦ No temperature inversion
near the ground, surface
instability
Weather in the summer:
♦ Cooler and comfortable temperatures, the added moisture and
atmospheric instability result in fair-weather clouds;
cP air mass moving over the ocean
• Additional moisture is added to the air.
• The warm ocean surface heats up the air -> instability, cloud formation
• The cP air mass loses its characteristics and is given a new designation
Maritime Polar (mP) Air Masses
• Forming conditions:
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♦ It may start over Asia and the frozen polar pacific regions as cP.
♦ As it moves over the Pacific (around Aleutian low) the boundary
layer warms up and accumulates significant amount of moisture.
Properties:
♦ Cold (freezing) and dry air aloft (similar to cP)
♦ Milder temperatures at the ground 40-50 F (no T inversion)
♦ It is conditionally unstable
Weather in the winter:
♦ This air enters the US from the NW coast; is forced over the
mountains
♦ Rain on the
upwind side
♦ Snow high in
the mountains
mP
mP
Pacific aircold but
not VERY
cold
Effect of mP air masses on the weather
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Maritime Tropical (mT) Air Masses
Forming conditions:
♦ Over the subtropical east Pacific Ocean-spends a
long time over the warm ocean water.
Properties:
♦ In winter it is very warm and moist
♦ Increased water content and humidity
♦ The atmosphere is unstable (mT is the most
unstable of all air masses)
Weather in the winter:
♦ As it moves over the colder continent it produces
heavy rain.
♦ Flooding and mudslides in California in the winter.
mT
mT
“The Pineapple Express” –IR image
Continental Tropical (cT) Air Masses
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♦ In the summer over northern Mexico and SW US.
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Forming conditions:
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Properties:
♦ Very dry and hot;
♦ The atmosphere is unstable;
Weather:
♦ Dry and hot.
♦ Practically no precipitation.
♦ Results in heat waves and droughts across the
great planes.
HOT & DRY
cT
Effect of cT air masses on the weather
Motion of Air Masses
•
• the change in weather
The prevailing upper level winds determine the motion
of the air masses.
The direction of the tropospheric jet steams impacts
♦ The zonal component
results in the E-W
motion of the weather
patterns
♦ The meridional
component drags
tropical air masses to
the North and Polar
air masses to the South.
The Jet stream on Sunday
Sunday’s temperatures