Transcript Air Masses and Wind Systems Powerpoint

Global winds and Air masses
Air Density
Hot air is less dense than cool air (due
to the activity of the molecules)
 As heat is added to an air mass, the air
molecules absorb energy and their
activity or movement increases
 This results in fewer air molecules per
unit of volume in the atmosphere

Warm Air

Less dense, warm air tends to rise (this is
why hot air balloons rise)
 Additionally, hot or warm air is buoyant
 Because of this increased molecular activity,
the air is able to support greater amounts of
other atmospheric components such as water
vapor
 Areas of warm, rising air have low
atmospheric pressure
Cold Air

The number of air molecules per unit volume
of atmosphere in cold air is greater
 This causes heavier or more dense air which,
due to gravity, has a tendency to sink to the
earth’s surface
 This creates high pressure areas
 Cold air is less buoyant than warm air, as the
air molecules are less active, so cold air
masses are less able to hold water vapor and
other atmospheric components
What causes wind systems?

The necessity to equalize pressure on the
earth’s surface
 Rising air in low pressure areas (warm
air)must be replaced by air flowing in from
high pressure areas (cold air)
 Winds always blow from areas of high
pressure into areas of low pressure
 The greater the difference in pressure, the
greater the velocity
 This difference is called the pressure gradient
(see weather and air pressure video)
The theoretical wind system

In theory, the global wind system would
involve two giant cells of air movement, one
in each hemisphere
 The cells would be triggered by hot air rising
at the equator and cold air flowing down from
the polar regions to replace it
 A low pressure system would exist at the
equator (as air rises) and a high pressure
system would exist at the polar regions (as
cold air falls and presses down on the earth’s
surface)
BUT…
The real way things happen are affected
by many forces
 The most important of these forces is
the Coriolis effect

Global Winds

The two main factors in wind direction are the rotation
of the earth (it turns from west to east) and the
location of high and low pressure air masses.


In the northern hemisphere winds curve to the right.
In the southern hemisphere winds curve to the left.
Global Winds

This results in continual winds as the air flows from
high presure to low pressure.

We have the trade winds which flow between 0 and
30 degrees.
The Westerlies which flow between 30 and 60
degrees.
The Easterlies which flow between 60 and 90
degrees.
Near the equator there are few winds and these are
known as the Doldrums.

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Coriolis Effect
As the earth rotates, we see the winds
curving to the right in the Northern
Hemisphere and the left in the Southern
Hemisphere. This is the Coriolis Effect.
(see Coriolis effect movie)

Coriolis Effect
Surface Winds

Remember: Air flows from HIGH to LOW pressure!
So in a low pressure system the winds flow counter
clockwise= into the centre where the pressure is the
lowest.

In a high pressure system the winds flow
clockwise=away from the centre where the pressure
is the highest.

This is reversed in the southern hemisphere.
The Jet Stream

Jet Stream: a fast moving (300 km/hr) air current that
occurs in the atmosphere where the troposphere
meet the stratosphere.

Migrates with the seasons.

Weather systems and temperatures are greatly
influenced by the jet stream.


Moves from west to east.
Their path is meandering in nature and they mark the
division between cold polar air and warm tropical air
Cyclones and Anticyclones

The waves or meanders in the jet
streams are important in both the
development of cyclones (low pressure
systems) and anti cyclones (high
pressure systems)
(see video of ship caught in cyclone
The Jet Stream
(video: “how does the jet
stream work?”
Chinook winds: the snow eaters
-Chinook winds are the result of colliding
pressure fronts
-Air pressure is lower at high elevation, and
greater at low elevations.
-When wind comes down off of the Rocky
Mountains, the low pressure air collides with
the high pressure air near the base of the
mountains.
-The moisture in the air condenses, warming
the winds by releasing latent heat as the wind
moves downhill to the east
(see Chinook winds movie)
What is a land breeze?
Land breezes are the result of land heating during the day much
faster than the ocean next to it
 The warm air over the land will rise throughout the day, causing
low pressure at the surface
 Over the water, high surface pressure will form because of the
colder air
 To compensate, the air will sink over the ocean.
 The wind will blow from the higher pressure over the water to
lower pressure over the land causing the sea breeze
 The sea breeze strength will vary depending on the temperature
difference between the land and the ocean.
(see video land breeze)

What is a sea breeze?

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At night,the air over the ocean is now warmer
than the air over the land
The land loses heat quickly after the sun
goes down and the air above it cools too
The oceanis able to hold onto this heat after
the sun sets and not lose it as easily
This causes the low surface pressure to shift
to over the ocean during the night and the
high surface pressure to move over the land
This causes a small temperature gradient
between the ocean surface and the nearby
land at night and the wind will blow from the
land to the ocean creating the land breeze
(see video on sea breeze and land breeze)
Sea breeze
Land Breeze
What is an Air Mass?

An air mass is a large body of air that is
homogenous in terms of temperature and
moisture.

A front is the leading edge of an air mass.

We have fronts because air masses
cannot mix together.
Classifying Air Masses

Air masses are classified according to their
place of origin.

If they occur in the high latitudes they will be
colder and therefore called arctic or polar.

If they occur in the low latitudes they will be
warmer and therefore called tropical.
Classifying Air Masses

If an air mass begins over water and
therefore contains a large amount of potential
precipitation they will be called maritime.

If an air mass begins over large and therefore
contains little to no water in it they will be
called continental.
Air Masses over North America
1) Continental Polar
• Originates in the vicinity of the North Pole and
flows as far south as Florida.
• Air in this air mass is very cold, stable and dry.
• Cold air is more dense and stays in contact
with the ground.
• Clear, sunny skies and high pressure are
common in this air mass.
Air Masses Over North America
2) Maritime Polar
• Originates over the oceans and polar regions.
• Often originates over the Gulf of Alaska.
• It is cool, unstable and moist.
• Brings a lot of the precipitation to BC in the
winter.
• Often the cold air is kept away by the
mountain ranges.
• When the two meet is when we mainly see our
snow.
(see “Windy weather wrecks havoc on Van
Air Masses Over North America
3) Maritime Tropical
• Originate over water in lower latitudes
(Mexico-ish)
• More stable than maritime polar air masses.
• Sunny skies with some precipitation in the
winter months.
• When they are much warmer they can cause
stormy conditions.
Air Masses Over North America
4) Continental Tropical
• Originates over land in the lower
latitudes (Texas).
• Brings with it hot and dry conditions.
Air Masses Over North America
QuickTime™ and a
decompressor
are needed to see this picture.
What is a weather front?

A weather front (or “front”) is a boundary
separating two masses of air of different
densities
Types of Fronts

Warm Front: leading edge of a warm air
mass.
Types of Fronts

Cold Front: leading edge of a cooler
front
Types of Fronts

Stationary Front: warm and cold air
boundary that remains in one place
Types of Fronts

Occluded Front: cold front overruns a
warm front mixing the air from the two
Practice
1) If a wind is called a westerlie it is flowing from
the__________________.
2) There is little wind near the equator and this
is known as the ____________.
3) If an air mass is composed of cold dry air it is
_______________________.
4) If an air mass is composed of warm dry air it
is______________________.
5) The coriolis effect causes winds in North
America to flow ___________.
6) All winds flow from _____________ to
_________pressure.
7) True or False. Areas located North of the jet
stream will be colder than areas south of it.