High Pressure
&
Low Pressure
Weather
Air pressure - The weight of the
atmosphere covering a certain
area.
• Air pressure increases
closer to the surface of
the Earth.
• Other factors can affect
air pressure as well.
Cold air is more dense
(higher pressure) than
warm air
Dry air is more dense
(higher pressure) than
moist air
• H2O atomic weight = 10
• N2 atomic weight = 14
• O2 atomic weight = 16
• Water molecules take up
space in the air, but
don’t add as much mass
in the same amount of
volume.
What is denser?
• Warm air or cold air?
• Cold air
• Moist air or dry air?
• Dry air
• Which air masses are most
dense?
• cP- cold and dry
• Which air masses are least
dense?
• mT- warm and moist
• These air masses of different densities move around
• Remember: air moves from areas of high pressure to areas
of low pressure.
• Is this a land breeze or a sea breeze?
Observe
1. Fill a small beaker with water and
set it on a white piece of paper.
2. Get a dropper full of colored sugar
water.
3. Drop it slowly into the center of the
still water, record your
observations.
4. What happened? Why?
5. Clean out your small beaker and set
it to dry on the sink.
What happened?
• The colored sugar water is
more dense than the tap
water so it sinks.
• Dense air also sinks, now
the surface of the Earth has
a lot of densely packed air
molecules.
• This is called a high
pressure area.
As the air sinks down in a high pressure area, it
flows away from the high pressure spot and turns
right due to the Coriolis effect.
Clouds don’t form when air sinks, so
high pressure areas are associated with good,
clear weather.
High pressure
=
dense air sinking
&
good weather
•Air that is not very
dense (warm and/or
moist air) will rise, like
a hot air balloon.
•These air molecules
are not very tightly
packed.
•This is called a low
pressure area.
As these air molecules rise, they turn to the right
due to the Coriolis effect, causing a counter
clockwise wind.
• Hurricanes in the Northern Hemisphere
all show a counter clockwise swirl around
a low pressure center.
As the low density air molecules rise
(warm and moist),
they cool and condense, forming clouds.
Low pressure
=
air rising
&
stormy weather
High and Low Pressure Areas
• High pressure causes
air to sink
• Usually results in
several days of clear
sunny skies
http://www.youtub
e.com/watch?v=aiY
yCurh_SU
• Air rises in low
pressure areas and
condenses forming
water droplets
• Usually results in
rain and storms
Review
1.
2.
3.
4.
5.
6.
7.
8.
9.
What is denser, warm or cold air?
What is denser, moist air or dry air?
Which air will form a High Pressure system, denser or less dense air?
Which air will form a Low Pressure system, denser or less dense air?
The Coriolis Effect in the northern hemisphere causes wind to be deflected
to the right or the left?
Why do few clouds form in a High Pressure system?
Why are clouds expected to form in a Low Pressure system?
What kind of weather is expected in a high pressure system?
What kind of weather is expected in a low pressure system?