Fronts
 Air masses eventually move around due to worldwide wind
currents, and overtake one another.
 The zone between the air masses is called a front.
 Meteorologists define four general types of fronts:
-cold, warm, stationary, and occluded.
Cold Fronts
 A moving cold air mass overtakes a warmer air mass.
 Because the cold air is more dense than the warm air, it moves in
below the warm air and pushes the warm air upwards.
 This causes a cumulus or cumulonimbus cloud to form.
 Rain, thunderstorms, and tornadoes result from cold fronts.
 Precipitation and storms form before the front, but do not last long.
Symbol on Map
(at ground level)
Warm Fronts
 A warm air mass overtakes a cooler one.
 The warm air gently rides up over the trailing end of the cold air
mass.
 Stratus and nimbostratus clouds form and can bring light rain or
snow lasting longer than it does with a cold front.
 After the front passes, it leaves warmer temperatures than before it
arrived.
Symbol on Map
(at ground level)
Stationary Fronts
 Cold and warm air masses are in contact, but
neither is moving.
 Warm air drifts up and condenses to form
stratus clouds and often rain.
 The front may remain stationary for several days.
 The result is a dreary, gloomy sky.
Occluded Fronts
 A cold air mass catches up with a slower-moving warm front.
 Many types of clouds are present, one after the other.
 Occluded fronts often cause a period of steady precipitation.
Symbol on Map
(at ground level)
Science 10 Highs & Lows
High Pressure System
Low Pressure System
HIGH PRESSURE SYSTEM
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An air mass that forms over cold ground
is a high pressure system.
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The cold air is dense, and settles lower
to the ground.
Dense air creates more pressure, as the
system draws more air in from above.
Air at the bottom curves out to the right,
due to the Coriolis Effect.
The overall result is a clockwise rotation
of air called an anticyclone.
Cool descending air has little moisture,
so highs create clear skies.
Located within a single air mass, they
may be hundreds of km across.
They cause weather conditions to stay
the same for several days.
LOW PRESSURE SYSTEM

Very intense heating of the ground can
create a low pressure system.

The heated air rises, leaving less dense
air below.
 As the air rises, it pulls more air beneath
it.
 The Coriolis Effect causes the air to curve
to the right.
 This results in a counterclockwise rotation
of air known as a cyclone.
 Warm rising air usually contains moisture
which forms clouds and precipitation.
 Lows are generally smaller in size than
highs, and more likely to occur between air
masses.
 They bring unstable conditions, which
cause changing weather, precipitation, and
often storms.
Air Pressure and Winds at the Earth’s Surface
Which of the following best describes surface wind circulation around the centre of a Highpressure system (as seen from above)?
(Draw arrows around the high.)
a) Counter-clockwise and spiralling outward
b) Counter-clockwise and spiralling inward
c) Clockwise and spiralling outward
H
d) Clockwise and spiralling inward
Which of the following best describes the surface wind circulation around the centre of a Lowpressure system (as seen from above)?
(Draw arrows around the low.)
a) clockwise and spiralling outward
b) Counter-clockwise and spiralling inward
L
c) Clockwise and spiralling outward
d) Clockwise and spiralling inward
Weather Conditions within Highs and Lows (At the Center)
Vertical motions lead to temperature changes in the rising or sinking air. The temperature
changes occur because air warms when it is compressed and cools when it expands.
(That is why a bicycle pump heats up as it compresses air and why air coming out of an
aerosol spray can cools as it expands.)
Circle the Correct Words:
In the open atmosphere, air pressure decreases with height. Consequently, air expands and
cools when (ascending) (descending). Air is compressed and warms when (ascending)
(descending).
In a Low, air generally exhibits ascending motion. The rising air experiences (increasing)
(decreasing) atmospheric pressure. The ascending air (expands) (is compressed) and its
temperature (increases) (decreases).
In a High, air displays descending motion. The sinking air experiences
(increasing)(decreasing) atmospheric pressure. Consequently, the descending air
(expands) (is compressed) and its temperature (increases) (decreases).
Most clouds form by the cooling of air. Air, if sufficiently cooled, will become saturated with
water vapour. Continued cooling will result in condensation, cloud formation, and possible
precipitation. The vertical motion in a (High, Low) often leads to cloud formation.
Warming causes clouds to evaporate. Cloudy air is saturated with water vapour.
With sufficient warming, it will become unsaturated and existing cloud particles (water
droplets or ice crystals) will evaporate. The vertical motions in a (High, Low) produce
warming, promote cloud dissipation, and lead to clear skies.
Descending air in a High leads to (fair)(stormy) weather and ascending air in a Low tends to
make weather (fair)(stormy).
The broad horizontal expanses of Highs and Lows cover large geographical areas such that
their circulations transport colder air from higher latitudes and warmer air from lower latitudes.
Consequently, in a High, air to the east of the system's centre is generally (colder)
(warmer) than air to the west.
In a Low, air to the east of the system's centre is generally (colder) (warmer) than air to the
west.
In the table below, describe the typical characteristics of Highs and Lows.
Highs
Lows
Pressure Change Towards
Center(increase, decrease)
Surface Winds Around Center
(clockwise, counter-clockwise)
Surface Winds Around Center
(inward, outward)
Vertical Motion (up, down)
Change in Temperature of
Vertically Moving Air (increases,
decreases)
State of the Sky Around Center
(clear, cloudy)
General Weather at Center (fair,
stormy)
Adapted for Ms. Crews’s Science 10 class from
http://www.msc.ec.gc.ca/education/teachers_guides/documents/08_pressure_hi_low_activity.pdf, Accessed April
7, 2009