Transcript Highs, Lows, and Fronts

AIR MASSES
Effects of Earth
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Earth is a sphere  uneven heating 
convection currents  world-wide wind
patterns
Rotation of Earth  Coriolis Effect  wind
direction changes to the right (northern
hemisphere) and to the left (southern
hemisphere)
Tilt of Earth  seasons change as move
through orbit
Air is usually in motion
(Convection and the Coriolis effect)
BUT
Large portions of air
often remain in nearly the same place
long enough to take on the
temperature and moisture characteristics
of the land or ocean below
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These characteristics vary greatly
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Differences between the land and ocean can
have a significant influence on the properties
of the air above
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When a large portion of the air is nearly
uniform in temperature and humidity, it is
considered an air mass
Classification
Air masses can be classified due to where on the
planet they form 1. The amount of moisture in the air
 continental or
 maritime
(depends on location – over land or ocean)
2. The temperature of the air
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polar (cool)
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tropical (warm)
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Air masses are not stationary
Often move according to prevailing wind
patterns
Air masses can interact as they move and
even collide= weather.
Highs, Lows, and Fronts
Frontal Systems
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“An air mass is a parcel of air that spends
enough time over a region to take on the
temperature and humidity of the region.”
Can remain in the same place for some time
Eventually moves due to worldwide wind
currents
The zone b/w interacting air masses is called
a front
There are 4 general types:
Warm
 Cold
 Stationary
 Occluded
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2.
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4.
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Cold air mass overtakes a warm
air mass
Cold air is more dense
Moves underneath, pushing
warm air upwards
Warm air rises rapidly forming a
cumulus cloud
Vapour condenses
If this occurs long enough, a
cumulonimbus cloud is created
Rain often comes with a cold
front
If the cold front is fast moving
intense thunderstorms may
develop sometimes bringing hail
and tornadoes
Cold Fronts
Warm Fronts
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Warm air mass overtakes a cooler one
Warm air gently rides up over the trailing end of the
cold air mass
Stratus clouds usually form (altostratus and cirrus
clouds form as warm air climbs higher)
Nimbostratus clouds form close to the front bringing
light rain or snow
Precipitation may last longer than with a cold front
Once it passes, the air is left warmer than before the
front arrived
STATIONARY FRONTS
 Two air masses
(warm/cold) in contact
but neither is moving
 Some warm air rises and
condenses creating
stratus clouds and often
rain
 The front may remain
this way for several days
 = dreary, gloomy sky
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OCCLUDED FRONTS
A faster moving cold
front catches up to a
slower moving warm
front
All the warm air is
pushed up, away from
the ground, contacting
the leading cold air mass
Many types of clouds are
present, one after another
Often cause a period of
steady precipitation
Investigating Weather Fronts
High Pressure Systems
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Any region where air is descending (putting more
pressure on the region below) is a HIGH PRESSURE
SYSTEM.
An air mass cooled by conduction and convection when
it forms over cold ground becomes more dense and
settles lower to the ground
Creates more pressure, continues to settle, draws more air
from above
Air is also pushed out toward low pressure areas at the
perimeter in all directions
This air curves to the right due to the Coriolis Effect
= ANTICYCLONE = clockwise rotation of air due to the
high pressure cell
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High pressure systems create a clear sky
(since air is descending not rising)
Air contains little moisture due to cold in
upper atmosphere = Clouds do not form
High pressure systems are usually located
within a single air mass
May be 100’s of km across
Due to size, weather conditions are stable
Prevents most turbulence
Low Pressure Systems
1. Very intense heating of the ground heats
air above it (conduction)
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Air rises, pulling more, less dense air in beneath it
Air curves to the right (Coriolis Effect) so rising air rotates
= CYCLONE (in the N.hemisphere rotates
counterclockwise)
2. Warm air rises in a frontal system creating
low pressure systems also
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Generally smaller than high pressure systems
Likely to occur b/w air masses
Bring unstable weather conditions (cause precipitation
and often storms)
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