Air pressure decreases with increasing altitude and is affected by temperature, humidity, and elevation. It is measured using a barometer. Differences in air pressure cause winds to move from high to low pressure areas. Global wind patterns like trade winds and jet streams curve due to the Coriolis effect. Local winds are influenced by geographic features and temperature variations between land and bodies of water. Changes in weather occur when different air masses characterized by their temperature and moisture content meet at fronts like cold fronts and warm fronts, which can lead to thunderstorm formation.

1. Air PressureCan you feel it?

2. Understanding Air Pressure
• Air pressure sometimes called
barometric pressure is simply the
pressure exerted by the weight of air
above.
• A barometer is a device used for
measuring air pressure (bar = pressure,
metron = measuring instrument).

3. Air Pressure
• Air pressure is the measure of the force with
which air molecules push on a surface.
• Air Pressure is GREATEST at the surface of
Earth because there is more of the
atmosphere above you to push down on you.

4. • As you move UP through the
atmosphere, air pressure decreases.

5. Air Pressure is dependent on
DENSITY
• More dense air will have a higher air
pressure- there are more air molecules in a
given space to push down on you
• Less dense air will have a lower air pressure-
there are fewer air molecules to push down on
you.
More Dense= more particles
to push down on you
Less Dense= fewer particles
to push down on you

6. Air Pressure is affected by 3 factors
• 1- Elevation, or altitude
• 2- Temperature
• 3- Water content

7. Impact of Elevation on Air Pressure
• As you move up through the atmosphere, air
pressure decreases.
– There are fewer air molecules above you to push
down on you, so the force of the air will be less.

8. Impact of Water Content, or
humidity, on Air Pressure
• Moist air is less dense than dry air, and
therefore has a lower air pressure.
– A water molecule has less mass than other
molecules that make up the air. If you
replace some of the air molecules with water
molecules, the water lowers the density (and
lowers the air pressure)
O N
O N
H2O
O H2O
N Weighs less since
H2O is lighter
than Nitrogen and
Oxygen

9. Impact of Temperature on Air Pressure
• Warm air is less dense than cold air.
Therefore, warm air has a lower air
pressure and cold air has a higher air
pressure.
– The molecules in warm air are moving fast
and are spread farther apart. Therefore there
are fewer air molecules in a given area to push
down on you.
Warm
Air
Cold Air

10. Barometer
• The piece of equipment used to measure
air pressure is a Barometer
Aneroid Barometer Mercury Barometer

11. Mercury Barometer

12. Air Pressure Affects the Weather
• Air pressure in a weather system reflects the
amount of water in the air, which affects the
weather.
Low air pressure
usually results in
Bad weather:
stormy, cloudy,
overcast.
High air pressure
usually results in
Good weather:
clear skies, no

13. Gradient Wind
• A rotating motion of air relative to earth’s
surface.
• If this gradient wind circulates in counterclock
direction, then it is termed as cyclone in
northern hemisphere while it is called
anticyclone in southern hemisphere.
• Similarly, the clockwise motion of air is called
cyclone in southern hemisphere while it is
called anticyclone in northern hemisphere.

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How does air pressure affect weather?
• Cyclonic flow
– Counterclockwise
around a low in Northern
Hemisphere
– Clockwise around a low
in Southern Hemisphere
• Anticyclonic flow
– Clockwise around a low
in Northern Hemisphere
– Counterclockwise
around a low in Southern
Hemisphere

16. Air Pressure on a Weather Map
• Areas of High and Low pressure on shown
on a weather map with an H or an L.

17. Wind
Global Wind and Local Wind

18. Wind
• Wind is a moving air caused by differences
in air pressure.
• What Causes Wind?
• Air moves from areas of high pressure to
areas of low pressure. The greater the
pressure difference, the faster the air
moves, and the stronger the wind blows.

19. • You can see how
air moves if you
blow up a balloon
and then let it go.
The air inside the
balloon is at a
higher pressure
than the air around
the balloon. If you
open the end of
the balloon, air will
rush out.

20. What Causes Differences in Air
Pressure?
• Most differences in air pressure are
caused by differences in air temperature.
• Temperature differences happen because
some parts of Earth get more energy from
the sun than others.
• For example, the sun shines more directly
on the equator than on the poles. As a
result, the air is warmer near the equator.

22. Convection cells
• The warm air near the equator is not as
dense as the cool air near the poles.
Because it is less dense, the air at the
equator rises, forming areas of low
pressure. The cold air near the poles
sinks, forming areas of high pressure. The
air moves in large circular patterns called
convection cells.

24. What Are the Major Global Wind
Systems?
• Global winds are large-scale wind
systems.
• There are three pairs of major global wind
systems, or wind belts: trade winds,
westerlies, and polar easterlies.

25. Major Global Wind System
• Trade winds are wind belts that blow from
30° latitude almost to the equator.
• They curve to the west as they blow
toward the equator.

26. Major Global Wind System
• Westerlies are wind belts that are found
between 30° and 60° latitude.
• The westerlies blow toward the poles from
west to east.
• Most of the United States is located in the
belt of westerly winds. These winds can
carry moist air over the United States,
producing rain and snow.

27. Major Global Wind System
• Polar easterlies are wind belts that
extend from the poles to 60° latitude.
• They form as cold, sinking air moves away
from the poles.
• In the Northern Hemisphere, polar
easterlies can carry cold arctic air over the
United States. This can produce snow and
freezing weather.

28. Major Global Wind System

29. Why Do Global Winds Curve?
• Remember that pressure differences can
cause air to move and form winds.
• If Earth did not rotate, these winds would
blow in straight lines. However, because
Earth does rotate, the winds follow curved
paths.
• This deflection, or curving, of moving
objects from a straight path because of
Earth’s rotation is called the Coriolis
effect.

32. Resultant winds
● Winds blow from
areas of high pressure
to areas of low
pressure.
● The Coriolis effect
deflects these winds
and produces the
angled paths of Earth’s
dominant wind systems.

33. • As Earth rotates,
places near the
equator travel faster
than places closer to
the poles. This
difference in speed
causes the Coriolis
effect.
• Wind moving from
the poles to the
equator is deflected
to the west. Wind
moving from the
equator to the poles
is deflected east.

34. • The polar easterlies, prevailing westerlies,
and trade winds are all winds that we feel
on the ground. However, wind systems
can also form at high altitude.
• Jet streams are narrow belts of very high-
speed winds in the upper troposphere and
lower stratosphere. They blow from west
to east all the way around the Earth.

35. Jet Streams
• Jet streams can reach speeds of 400
km/h. Pilots flying east over the United
States or the Atlantic Ocean try to catch a
jet stream. This wind pushes airplanes
along, helping them fly faster and use less
fuel. Pilots flying west try to avoid the jet
streams.

36. Jet Streams
• The global wind systems are always found
in about the same place every day.
• Unlike these global wind systems, jet
streams can be in different places on
different days. Because jet streams can
affect the movements of storms,
meteorologists try to track the jet streams.
They can sometimes predict the path of a
storm if they know where the jet streams
are.

38. Local Winds
• What are they?
• Are winds that generally move over short
distances and can blow from any direction.
• Local winds are caused by differences in
temperature. Many of these temperature
differences are caused by geographic
features, such as mountains and bodies of
water.

39. Local winds: sea-land breeze, mountain-valley winds
Winds caused by geography.

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• Differential solar
heating is due to
different heat
capacities of land
and water.
• Sea breeze
– From ocean to land
• Land breeze
– From land to ocean
Sea and Land Breezes

41. Air masses
• Have you ever been caught outside when
it suddenly started to rain? What causes
such an abrupt change in the weather?
• Changes in weather are caused by the
movement of bodies of air called air
masses.
• What are air masses?
• Is a very large volume of air that has a
certain temperature and moisture content.

42. Air masses
• Classified by the water content and
temperature of the air.
• These features depend on where the air
mass forms. The area over which an air
mass forms is called a source region.
• One source region is the Gulf of Mexico.
Air masses that form over this source
region are wet and warm.

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Storms and Air Masses
• Storms – disturbances with strong winds and
precipitation
• Air masses – large volumes of air with distinct
properties

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Education, Inc.
Fronts
• Fronts – places where
two air masses meet.
– Warm front
– Cold front
• Storms typically
develop at fronts.

45. What are Cold Fronts?
• A cold front forms when a cold air mass
moves under a warm air mass. The cold
air pushes the warm air mass up. The cold
air mass replaces the warm air mass.
• Cold fronts can move quickly and bring
heavy precipitation. When a cold front has
passed, the weather is usually cooler.
• This is because a cold, dry air mass
moves in behind the cold front.

47. What are Warm Fronts?
• A warm front forms when a warm air mass
moves in over a cold air mass that is
leaving an area.
• The warm air replaces the cold air as the
cold air moves away.
• Warm fronts can bring light rain. They are
followed by clear, warm weather.

49. Thunderstorm
• A thunderstorm is an intense storm with
strong winds, heavy rain, lightning, and
thunder. Many thunderstorms happen
along cold fronts.
• Thunderstorms are associated with
cumulonimbus or “thunderhead”
clouds.

50. • Two conditions are necessary for a
thunderstorm to form: warm, moist air near
Earth’s surface and an unstable area of
the atmosphere.

51. Thunderstorm Formation
• The atmosphere is unstable when a body
of cold air is found above a body of warm
air.
• The warm air rises and cools as it mixes
with the cool air.
• When the warm air reaches its dew point,
the water vapor condenses and forms
cumulus clouds.
• If the warm air keeps rising, the clouds
may become dark cumulonimbus clouds.

52. LIGHTNING
• As a cloud grows bigger, parts of it begin to
develop electrical charges.
• The upper parts of the cloud tend to become
positively charged. The lower parts tend to
become negatively charged.
• If this charge builds up enough, it is discharged
within the cloud as lightning.
• Sheet lightning rises from the base to the top;
forked lightning extends the quickest way down
from the cloud to the ground.

54. THUNDER
• You have probably seen large lightning
bolts that travel between the clouds and
the ground.
• When lightning moves through the air, the
air gets very hot.
• The hot air expands rapidly. As it expands,
it makes the air vibrate.
• The vibrations release energy in the form
of sound waves. The result is thunder.

55. Hurricane
• A hurricane is a violent tropical maritime
storm, known as a tropical cyclone in the
Indian Ocean and a typhoon in the Pacific.

56. Hurricane
• Most hurricanes form between 5°N and
20°N latitude or between 5°S and 20°S
latitude.
• They form over the warm, tropical oceans
found at these latitudes. At higher
latitudes, the water is too cold for
hurricanes to form.

57. HOW HURRICANES FORM?
• A hurricane begins as a group of
thunderstorms traveling over tropical
ocean waters.
• Winds traveling in two different directions
meet and cause the storm to spin.
• Because of the Coriolis effect, hurricanes
rotate counterclockwise in the Northern
Hemisphere and clockwise in the
Southern Hemisphere.

58. HOW HURRICANES FORM?
• The sun’s energy causes ocean water to
evaporate. As the water vapor rises in the
air, it cools and condenses. A group of
thunderstorms form and produce a large,
spinning storm. A hurricane forms as the
storm gets stronger.

59. Major Global Wind System

60. HOW HURRICANES FORM?
• The hurricane will continue to grow as
long as it is over warm ocean water. When
the hurricane moves over colder waters or
over land, the storm loses energy. This is
why hurricanes are not common in the
middle of continents. The storms lose their
energy quickly when they move over land.

63. Tornado?
• A tornado can form when a rapidly
spinning column of air, called a funnel
cloud, touches the ground. The air in the
center of a tornado has low pressure.
When the area of low pressure touches
the ground, material from the ground can
be sucked up into the tornado.

65. What is climate?
• Describes the average weather conditions
in a region over a long period of time.
• Determined by temperature and
precipitation.

66. What are the reasons for the changing
seasons on Earth?
• The tilted axis of Earth (tilted at 23.5° )
• Factors affecting climate on Earth:
– Earth’s rotation and revolution
– Latitude
– Elevation
– Amount of precipitation due to topography
– Wind Patterns

67. Seasons of the Earth

68. Solstice and Equinox
• An equinox occurs when the Sun is
directly overhead at the equator.
• A solstice occurs when the Sun is directly
overhead at a tropic.

69. Solstices and Equinoxes
• June solstice – summer in the northern hemisphere, they have
longer days and shorter nights; winter at southern hemisphere, they
have longer nights and shorter days.
• September equinox – have almost equal hours of day and night
(12 hrs day; 12 hrs night); northern hemisphere is at autumn
season, southern hemisphere is at spring season.
• December solstice - summer in the southern hemisphere, they
have longer days and shorter nights; winter at northern hemisphere,
they have longer nights and shorter days.
• March Equinox - have almost equal hours of day and night (12 hrs
day; 12 hrs night); southern hemisphere is at autumn season,
northern hemisphere is at spring season.

70. Seasons of the Earth

71. The five main latitude lines
• - The difference between the latitude lines relate to the
changes in climate.
• Five latitude lines:
-The reason why there
are places where it’s
always cold or hot year
round
• 1. Arctic circle
• 2. Tropic of cancer
• 3. Equator
• 4. Tropic of capricorn
• 5. Antarctic circle

72. The five main latitude lines
Temperate
region
Tropic
region
Temperate
region
North Polar region
South Polar region
Equator line (tropics)
– receives large
amount of sunlight all
year round.
Middle latitude lines
(Temperate)– sun
never shine directly in
these areas so places
located here are cooler
than in the tropics.
Polar regions – areas
here receives least
amount of sunlight so
places located here
are very cold, in fact
for half a year these
areas does not receive
sunlight at all.

73. The five main latitude regions of Earth’s
surface.
• North Frigid zone; Arctic circle; North Polar Region; High
Latitudes - (4.12% of Earth’s surface)
• North Temperate Zone; Middle Latitudes – between the
arctic circle and the tropic of cancer (25.99% of Earth’s
surface).
• Tropic region; Low Latitudes; Torrid zone; Tropical Circles
– between the tropical circles (39.78% of Earth;s surface)
• South Temperate Zone; Middle latitudes; South
Temperate Region – between the tropic of capricorn and
the antarctic circle (25.99% of Earth’s surface)
• South Frigid zone; Antarctic circle; South Polar Region
(4.12% of Earth’s surface)

75. END!