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PART 1
Air Masses
Fronts
Winds
What is Weather?
Weather is the state of the
atmosphere: hot or cold, wet or
dry, calm or stormy, clear or cloudy.
• Weather is driven by air pressure differences between
one place and another (in turn, air pressure is defined
by temperature and moisture).
• Most weather phenomena occur in the troposphere:
 On Earth, the common weather phenomena include
wind, clouds, rain, snow, fog and dust storms.
 Less common events include natural disasters such
as tornadoes, hurricanes, typhoons and ice storms.
• Weather is one of the fundamental processes that shape
the Earth through weathering and erosion.
Heating of the Atmosphere
The Sun shines more directly near the equator bringing these
areas more warmth; the polar regions are at such an angle to
the Sun that they get little or no sunlight during the winter,
causing colder temperatures.
Unequal heating of the atmosphere creates a restless
movement of air (and water!) to distribute heat
energy from the Sun across the planet.
Air Masses and Fronts
Air mass is defined as a large body of air that has
similar temperature and moisture.
A boundary that separates two different air masses
is called a weather (atmospheric) front.
Air Masses
Cover many thousands of square kilometers.
• Originate in source
regions: flat, uniform
composition areas with
light winds.
• Examples: snow covered
Arctic plains, tropicsubtropic oceans,
forests, mountains, large
bodies of water.
• Classified by their origin:
 Land (continental)
 Water (maritime)
 Tropical (within 25° of
equator)
 Polar (poleward of 60°
north and south)
Air masses affecting the U.S. weather
during the course of a typical year.
Weather Fronts
Cold front:
Warm front:
fast moving cold (dense)
air mass runs into a slow
moving warm air mass
and slides under.
fast moving warm (less dense)
air mass collides with a slow
moving cold air mass and
moves over.
Weather: showers
and thunderstorms.
Weather: fog, rain, snow.
Weather Fronts
Stationary front:
Occluded front:
cold air mass and warm
air mass meet, but
neither has enough
force to move the other.
warm air mass is caught
between two cooler air
masses and the denser cold
air masses move underneath
and push it upward.
Weather: rain, thunder.
What types of weather fronts can
you find on this map?
Air Pressure Differences
H
L
• Air pressure differences can
occur due to:
 the Sun angle at any
particular spot
 surface temperature
differences (higher
altitudes are cooler than
lower altitudes)
• As air warms, it expands and
becomes less dense creating
lower air pressure (L).
• Cool air sinks and becomes denser
creating higher air pressure (H).
Differences in air pressure cause wind:
flow of air on a large scale.
Wind
• Winds blow (that is air moves) from areas of high
pressure to areas of low pressure.
• Wind speed is often a way of classifying storms.
Meteorologists distinguish two types of winds:
Local Winds and Global Winds.
How to Measure?
• Air pressure is measured
using a barometer (from
Greek baros ‘weight’).
Winds are characterized by
their direction and speed.
• Wind speed
is measured by
• Winds are named
by the direction from
which they come.
anemometer
(from Greek
anemos ‘wind’).
Wind Rose
• A wind rose shows how
wind speed and direction
are typically distributed at
a particular location over
a specified period of time
(year, season, month):
 typically uses 16 cardinal
directions
 can quickly indicate the
dominant wind directions
and the direction of
strongest wind speeds.
• Compiling a wind rose is
one of the preliminary
steps taken in constructing
airport runways, as aircraft
typically perform their best
take-offs and landings
pointing into the wind.
Local Winds
These are the winds we feel on the ground:
• cover short distances
• blow from any direction
• created and influenced by local conditions, local
temperature variations, and local topography.
 Sea Breeze
 Mountain
Breeze
 Land Breeze
 Valley Breeze
Local winds are
often seasonal.
Local Winds
Sea and land breezes are formed by varying
temperature differences between the land and water.
• During the day the land heats
up faster than the water:
 the air above the land
warms up and rises, forming
a low pressure area;
 the wind will blow from the
sea to the land, called
a sea breeze.
• At night, land cools off faster
than the sea:
 the air above the sea
surface warms up and rises;
 and wind will blow from the
land to the sea, called
a land breeze.
H
L
H
L
Local Winds
Mountain and valley breezes are examples of
local winds caused by the topography of an area.
• During the day the
mountain slopes heat up:
 the warm less dense air
flows up the mountain;
 this is called a valley
breeze.
• At night, the mountain
will cool off faster than
the valley:
 the cool mountain air
descends;
 this is called a
mountain breeze.
Local Winds of North America
Chinook: A warm,
westerly wind found
in western North
America – Canada
and the USA, when
air from the Pacific
blows over the Rocky
Mountains and other
upland areas.
Norther usually occurs
in winter but occasionally
occurs at other times of
year during periods of big
change in temperature.
Nor'easter blizzards
is a macro-scale storm
along the East Coast of
the United States and
Atlantic Canada that
forms repeatedly
during winter months.
Norte is a strong cold
northeasterly wind which
blows in Mexico along the
shores of the Gulf of Mexico.
It results from an outbreak
of cold air from the north.
Find more local winds by continent at www.thecompetitionworld.com
Our Local
Winds