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Chapter 2 Section 3
Winds
What are winds?
 A wind is the horizontal movement of air
from an area of high pressure to an area
of lower pressure
 Winds are caused by differences in air
pressure
Convection Currents
 Form when an area of Earth’s surface is
heated by the Sun’s rays
 Air over the heated surface expands and
becomes less dense. As the air becomes
less dense, its air pressure decreases
Convection Currents
 Cool dense air with a higher pressure flows
underneath the warm, less dense air= warm
air rising
How do we measure wind speed?
 Winds are described by their direction and
speed
 Anemometers are used to measure wind
speed
Wind Direction
 The name of a wind tells you where the wind
is coming from
 For example: A west wind comes from the
west and blows toward the east
 Wind direction is determined with a wind
vane
 The wind swings the wind vane so that one
end points into the wind
Wind Direction Represented on a Map
 Wind direction is shown by a line leading
into a circle
 The example below shows that the wind
blew in from the east
Wind Speed Represented on a Map
 The wind speed is
shown by the “feathers”
on the wind direction line
 One “feather” =10 knots
 Half of a “feather”=5
knots
 A circle within a circle=
calm, no wind
 A flag=50 knots
 (1 knot=1.15mph)
 (feathers are also
sometimes called
“barbs”)
Weather Symbol Examples
 Example #1: What is the wind speed and
direction?
 Wind coming from the east at a speed of 85 knots
Pressure and Wind
 Isobars are lines joining
places on a weather map
that have the same air
pressure
 Meteorologists use isobars
on weather maps to observe
atmospheric pressure
changes over an area and to
make predictions about wind
speed
 Remember-Wind is a result
of air pressure differences
and wind blows from high to
low pressure
 The closer the isobars are=
the faster the wind speed
 The further apart the isobars
are= the slower the wind
speed
Wind Chill Factor
 Wind blowing over your skin removes body heat
 The stronger the wind, the more friction is
created from the air molecules hitting your body
 This creates heat energy which is absorbed by
the water near the surface of your skin, causing
it to evaporate
 This evaporation removes heat from your body
and makes you feel colder
Local Winds
 Local winds are winds that blow over short
distances
 Local winds are caused by the unequal heating
of Earth’s surface within a small area
 Unequal heating often occurs along the shore of
a large body of water
 It takes more energy to warm up a body of water
than it does to warm up an equal area of land
(water has a high specific heat)
 During the day the land warms up faster than the
water (unequal heating)
 At night, the land cools faster than the water
(unequal heating)
Local Winds
 Land Breeze –Local
wind that blows from
the land to a body of
water during the night
 Sea Breeze -Local
wind that blows from
an ocean or lake to the
land during the day
Global Winds
 Global winds are winds that blow steadily
from specific directions over long
distances
 Global winds are created by the unequal
heating of Earth’s surface
 Warm air rises at the Equator and cold air
sinks at the poles
Jet Streams
 About 10 km above Earth’s surface are bands of
high-speed winds called jet streams
 Meteorologists use the location of some of the
jet streams as an aid in weather forecasting
 Jet streams generally blow west to east
 Jet streams form at the boundaries of adjacent
air masses with significant differences in
temperature (polar region vs. Equator)
 Jet streams have a meandering shape
Jet Stream Example
Chapter 3 Section 1 Weather
Air Masses
 A huge body of air that has similar temperature
(warm or cold), humidity (dry or wet), and air
pressure (high or low) at any given height is called
an air mass
 Four major types of air masses influence the
weather in North America:
–
–
–
–
Maritime tropical
Continental tropical
Maritime polar
Continental polar
Types of Air Masses




Continental: land (dry)
Mar: sea (wet)
Tropical: warm/hot
Polar: cold
 The area that the air mass forms over will determine its
characteristics
 For example:
– If the source region is an ocean, the air mass will have a
lot of moisture
– If the source region is land, the air mass will be drier
Putting it together
 Maritime Polar…
– Moist and cold
 Continental Polar…
– Dry and Cold
 Maritime Tropical…
– Moist and warm
 Continental Tropical…
– Dry and warm
Air Masses and Their Origin
Air Masses
Origin
Maritime Tropical
Gulf of Mexico and Atlantic
and Pacific Ocean
Continental Tropical
Southwest and Northern
Mexico
Maritime Polar
North Pacific and North
Atlantic
Continental Polar
Central and Northern
Canada and Alaska
North American Air Masses
1
6
2
4
5
3
How Air Masses Move
 Prevailing Westerlies- push air masses
west to east
 Jet stream
Fronts
 The boundary where 2 unlike air masses meet is
called a front
 4 types of fronts
–
–
–
–
Cold
Warm
Stationary
Occluded
 Storms and changeable weather often develop
along fronts
Symbols for Fronts
Front Symbols and Direction
 Front symbols point in the direction of
movement
 Example: In the picture below, the warm,
cold, and occluded fronts are moving north
and the stationary front is not moving in any
direction
Front Symbols and Direction
 Draw a cold front moving east:
 Draw a warm front moving south:
 Draw an occluded front moving northeast:
Cold Fronts
 A cold front is when a fast-moving cold air
mass takes over a warm air mass
 Possible weather: Clouds with possible
thunderstorms with heavy rains or snow;
cooler weather on the way
Warm Fronts
 A warm front is when a warm air mass
overtakes a slow-moving cold air mass
 Possible weather: Humid, light rain or snow
for several days; warmer weather on the
way
Stationary Fronts
 A stationary front is when cold and warm air
masses meet, but neither can move the
other
 Possible weather: The air masses remain
stalled over an area and may bring many
days of clouds and precipitation
Occluded Front
 An occluded front is when a warm air mass
is caught between two cooler air masses
 Possible weather: Cloudy and rain or snow
Front Symbols on a Weather Map
Low Pressure and Weather
 If you look at a weather map, you will see areas marked with a letter L
and the L stands for “low pressure”
 Low pressure systems lead to cloudy conditions that often bring
precipitation
 Here’s how it works:
– Areas of low pressure are caused by massive amounts of air rising from the ground
into the atmosphere
 As the air molecules rise, they take their mass with them
– Air pressure is a force
– Force = mass x acceleration
– Less mass of air molecules left at ground level means low air pressure
– As this air rises it takes whatever water vapor it is holding along with it
– As altitude increases in the troposphere, the temperature decreases
– Eventually the air mass reaches a temperature at or below the dew point and the
water vapor condenses
 Relative humidity rises because colder air cannot hold as much water vapor
– This condensation forms clouds
 If there is enough water vapor in the air, it might also bring precipitation
High Pressure and Weather
 If you look at a weather map, you will see areas marked with a letter H
and the H stands for “high pressure”
 High pressure systems lead to clear and calm weather
 Here’s how it works:
– Areas of high pressure are caused by massive amounts of air sinking from
the upper troposphere down towards the ground
 As the air molecules sink, they take their mass with them
– Air pressure is a force
– Force = mass x acceleration
– More mass of air molecules coming to ground level means high air pressure
– As this air sinks it takes whatever water vapor it is holding along with it
– As altitude decreases in the troposphere, the temperature increases
– The higher the temperature is, the farther from the dew point you get
 Warmer air is less dense so it has more space to hold water vapor so relative
humidity drops
– This means that condensation does not occur so no clouds form
 Because there are no clouds, there will be no precipitation
Weather Map Example
Weather Map Example