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184
Experiential Science 10—Terrestrial Systems
GLOBAL
WEATHER
PATTERNS
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Weather
N
o matter where you live, the weather in your
community will be influenced by air movements occurring hundreds or even thousands
of kilometres away. That’s because much of the world’s
weather is the result of movements of large bodies of air
called air masses.
Air Masses
Air masses are immense bodies of air that share similar characteristics of temperature, atmospheric pressure,
and humidity. How large are these air masses? A single
air mass may cover hundreds of thousands or even millions of square kilometres. Air masses form over specific
areas of the world, called source areas. Source areas can
have different temperature profiles—equatorial, tropical, polar, or arctic. They can also have different moisture
characteristics. Maritime air masses are usually more
moist and continental air masses are drier. Canada is a
source area for several important air masses.
Continental polar and continental
arctic air masses
The continental polar and continental arctic air masses
are very similar. Meteorologists classify these air masses
according to their place of origin and their unique qualities. Continental arctic air masses develop north of the
Arctic Circle and continental polar air masses form over
the Canadian Subarctic and neighbouring Alaska. Both
areas receive low levels of radiant heat in the winter. As
well, much of the heat received is radiated away from the
surface by snow and ice, causing the temperatures to fall
even farther. Arctic air masses formed under these conditions are not only extremely cold, they are also very
dry and have a low water vapour content. Once these air
masses become large enough, they push south along the
Rocky Mountains, carrying cold air deep into the North
American continent, sometimes as far south as the Gulf
of Mexico. These arctic outbreaks of cold air are sometimes referred to as the Siberian Express or other colourful names such as the Blue Northern. Arctic outbreaks
rarely occur in the summer because warmer temperatures in the Arctic prevent the air masses from developing
such extremes of cold and of high pressure.
Maritime polar (mP) air masses
Maritime polar air masses form over the North Pacific
and North Atlantic oceans. However, since air tends to
move from west to east, the North Atlantic maritime
polar air mass has little effect on western Canada. The
North Pacific maritime polar air mass is the most important maritime polar air mass in the NWT. Maritime polar
air masses can form at any time of the year—not just in
winter—and they aren’t as cold as the arctic and continental polar air masses because the ocean moderates their
temperature. Maritime air masses are also more moist
than either the arctic or continental polar air masses and
they can produce heavy rain and snow, especially on
the western slopes of the coastal mountains. The North
Pacific maritime polar air mass is also responsible for the
frequent rain on the coast of British Columbia and heavy
snowfalls as it pushes over the Rocky Mountains.
Table 4.3 Air Masses
Air mass type
Temperature
Moisture
characteristics characteristics
continental arctic (cA)
extremely cold
very dry
continental polar (cP)
very cold
very dry
maritime polar (mP)
very cool
moderately moist
continental tropical (cT) very warm
dry
maritime tropical (mT)
very humid
very warm
DID YOU KNOW?
Yellowknife Sends Snow South
Continental polar and continental arctic air masses usually
don’t cause much precipitation. However, there is an
exception. Cold air from a continental polar air mass will
gain humidity and become less stable as it crosses the
Great Lakes. By the time it reaches the eastern shores
of these lakes, it can cause huge snowstorms in places
such as Chicago and southern Ontario. So an air mass that
developed near Yellowknife can cause snow thousands of
kilometres away.
Chapter 4 Weather185
Activity 4
field activity
lab activity
library activity
4 classroom activity
chapter project
research team activity
Following Air Masses
Purpose
To learn about air masses and their movement.
Background Information
Weather happens on a much larger scale than we can see from
our local weather monitoring station. Many stations across
the country and around the world compile their data to give a
better picture of what is going on across the globe. By studying
air masses and fronts, you can better understand why global
weather acts the way it does, and what this means for your
community and other parts of North America.
Air masses are named according to their properties:
• Any air mass that originates over land is called a
continental air mass.
• Any air mass originating over water is called a
maritime air mass.
• How far north the air mass originates indicates
whether it is called an arctic, polar, or tropical air
mass.
Summer Air Masses
CANADA
UNITED STATES
MEXICO
• There are 6 classifications of air masses. Their names
start with either continental or maritime, followed by
the geographic location where they formed.
• Air masses do not remain stationary; as a maritime air
mass moves across large areas of land, it will modify
to eventually become a continental air mass. The same
is true for a continental arctic air mass: as it moves
south, its properties change.
Materials and Equipment
• maps of North America showing air mass movements
over the past 5 days (download from internet)
• charts for recording predictions
Procedure
1. Look at the two maps below. Explain why the air masses
are in different positions in summer and winter.
2. Use Tables 4.3 and 4.4 to predict what the weather might
be like in 10 locations in Canada and the US on either side
of the front as it moves out of the Arctic and across North
America over a 5-day period. Use the charts provided to
record your predictions.
Conclusion
1. What do you think will happen as very cold, dry air comes
in contact with warm, moist air? Record your predictions.
Winter Air Masses
CANADA
UNITED STATES
MEXICO
186
Experiential Science 10—Terrestrial Systems
High- and Low-Pressure Systems
came up with the theory of “fronts” used the term “front”
because the place where air masses meet reminded him
of a front on a battlefield where opposing armies meet.
What do you think happens in the battle between air
masses?
In areas where air masses are moving, cold or warm
fronts develop. When a cold front overtakes a warm
front, an occluded front develops. Frontal zones where
Meteorologists often use the relative pressure associated
with air masses to characterize them as highs (highpressure systems) or lows (low-pressure systems). Lows
generally have mean surface pressures below 101.3 kilopascals (kPa)—the average sea-level pressure—but an
air mass may be considered a low as long as its pressure is
less than that of surrounding air masses. Near the ground,
air within a low flows towards the centre of
the air mass. At the centre of the low, air
Table 4.4 Fronts
currents meet and rise and then flow outCold fronts
ward at high altitude towards the edges of
Weather
Before front
the low. The Coriolis effect described in
warm
temperature
more detail below causes the air within the
decreasing
atmospheric
low to swirl in a counter-clockwise direcpressure
tion (also called a cyclonic flow). Cyclones
showers
precipitation
are intense low-pressure systems.
High-pressure systems are charactercirrus, cirrostratus
clouds
ized by surface air pressures greater than
Warm fronts
101.3 kPa and air moving at low altitudes
Weather
Before front
from the centre of the system towards its
cool
temperature
edges. Highs are often associated with clear
skies because dry, high-altitude air sinks in
decreasing steadily
atmospheric
the centre of a high to replace the air that
pressure
moves outward from the centre. The air in
showers, snow,
precipitation
a high-pressure system swirls in a clocksleet
wise direction as a result of the Coriolis
cirrus, cirrostratus,
clouds
effect, and it tends to follow low-pressure
altostratus,
systems.
nimbostratus
Although all highs and lows share basic
characteristics, they may also be categorized according to the processes that result
in their formation in different regions.
Based on these regional processes, meteorologists may distinguish, for instance,
tropical lows, thermal lows, extratropical
lows, or polar lows.
Contact with front After front
sudden cooling
cold
levelling off
increasing
heavy rain or snow, hail
showers then clearing
cumulus, cumulonimbus
cumulus
Contact with front After front
sudden warming
warmer, then levelling
off
levelling off
slight rise, then
decreasing
light drizzle
none
stratus, sometimes
cumulonimbus
clearing, scattered
stratus or scattered
cumulonimbus
Fronts
A front is the place where the leading edges
of air masses meet. Because air masses
have different characteristics—some are
cold and dry, others are warmer and more
moist—when they meet, they cannot mix
easily. As a result, the area where air masses
meet is often associated with dramatic
changes in weather. The scientist who first
Figure 4.5 A weather systems map showing highs, lows, and fronts.
Chapter 4 Weather187
Figure 4.6 Front symbols.
(From left to right): Cold front,
warm front, stationary front.
air masses aren’t moving
are referred to as stationary
fronts. These fronts are predictable and are associated
with specific changes in the
weather.
As shown in Figure 4.7,
a cold front is the leading
edge of a cold air mass.
When a cold front comes
through, there is usually
a quick but strong gust
of wind, and often there
is heavy rain or snow. A
cold front can bring severe
cold spells in the fall and
winter.
Upper Cumulonimbus (Cb)
air flow
Warm air
Col
d fr
ont
Heavy
Cold air
precipitation
(Above): Figure 4.7 A cold front is cold air displacing warm air.
(Below): Figure 4.8 A warm front is warm air displacing cool
air.
C irrus
Cirrostratus(Ci)
Altostratus (Cs)
(As)
Nimbostratus
(Ns)
Wa
rm
fron
t
Warm air
Moderate
precipitation
Cool air
In a warm front, shown in Figure 4.8, a warm air
mass is replacing a cold air mass. A warm front usually
moves more slowly than a cold front because cold air
that is being displaced is dense and hard to move. For
this reason, cloudy, wet weather can last for hours or days
before the warm air behind the front finally arrives. After
the front passes, the precipitation usually stops.
Wind
Wind is the horizontal movement of air that is set in
motion by the uneven heating of the Earth’s surface. As
you have seen, air flows from areas of high pressure to
low pressure. The force that sets the air moving is called
the pressure gradient force. The greater the pressure differences between parcels of air, the greater the force, and
thus, the stronger the winds.
Anemometers (wind gauges) are the most important
tool for measuring wind speed. In Canada, wind speed
is measured in kilometres per hour. Winds can also be
classified on the Beaufort scale. A calm day with no wind
at all is 0 on the Beaufort scale, while a full-blown hurricane is 12.
Wind vanes tell the direction of the wind. Winds are
always labelled with the direction they are coming from.
The weather forecast will report the winds as “winds
from the west,” for example, or “winds from the south.”
Windsocks, found at airstrips across the North, show
wind direction and approximate wind speed, providing
critical information for pilots flying into remote locations such as mineral exploration camps. Wind speed
is reported in knots and direction, referring to the four
cardinal directions: north, south, east, and west.
SCIENTIFIC TERMS
Fronts and Wind
Coriolis effect: the tendency of all particles in motion on the
surface of the Earth—including wind—to be deflected to the
right of the direction of travel in the northern hemisphere and
to the left in the southern hemisphere because of the Earth’s
rotation.
front: the transition zone between two air masses of different
density.
kilopascal (kPa): the internationally recognized unit for
measuring atmospheric pressure.
stationary front: a boundary between two different air masses,
neither of which is strong enough to replace the other, so the
front remains in place for some time.
wind: the lateral movement of air having fairly uniform
characteristics of temperature, atmospheric pressure, and water
vapour content.