Download Pressure Systems.

Pressure systems.
• We are going to learn about two
different types of pressure systems:
• Low pressure systems: A depression.
• High pressure systems: An anticyclone.
But first……..
• We have to know a few things
Things we need to know.
• IsoIsobars
barsshow air pressure
on weather charts.
The level of pressure will
be shown by numbers.
Generally under 1000mb
is low pressure, and over
is high pressure
Isobars also tell us what the
wind will be like. If the isobars are
close together it will be very windy.
If they are far apart it will be calm.
Low pressure usually brings bad weather.
High pressure usually brings
calm nice weather.
Fronts
• Fronts are VERY important too.
• Important: Fronts of any sort ALWAYS
bring rain!
• Fronts
• Weather
Fronts
• Warm front: Warm air comes behind it.
Generally brings light steady rain.
Cold sector
Warm sector
Direction of front
Fronts
• Cold front: Cold air comes behind it.
This front will cause very heavy rain
and thunderstorms on occasion.
Warm sector
Cold sector
Direction of front
Fronts
• Occluded front: This will bring the
worst weather. Very wet, windy and
usually storms.
•
Frontal weather
Direction of front
High pressure and low pressure.
• The amount of air pressure is how much the air
in the atmosphere is pushing down on the earth.
High pressure and low pressure.
• If the pressure is high that means that the air is
low and pushing down on the earth’s surface.
Air is low
=
Lots of pressure = high pressure
High pressure and low pressure.
• If the pressure is high that means that the air is
low and pushing down on the earth’s surface.
Air is low
=
Lots of pressure = high pressure
High pressure and low pressure.
• If the pressure is low it means
airhigh
is higher and
Air the
is up
therefore not putting as much pressure on the earth.
=
Low pressure
High pressure and low pressure.
• QUESTION: If the air is close to the
ground (high pressure) what do you
think we will not get?
Answer:
 No Clouds. The air is close to the
Copy this: ground and therefore too warm for
clouds
to form.
In areas of high
pressure
you do not get



many clouds in the sky.
High pressure and low pressure.
• QUESTION: If you don’t get clouds in
areas of high pressure what happens
in areas
of
low
pressure?
Copy this:

In areas of
low pressure the air is high in
Answer:
the atmosphere and it is colder. This
 The air is higher up and therefore
means that it cools, condenses and forms
it cools, condenses and forms
clouds. (The 3 C’s)
clouds.
High pressure – what does it mean?
• From what we know already about isobars, air
pressure and fronts what can we tell about this
pressure system. What will the weather be like?
• Talk with each other come up with as many ideas as
you can.
High Pressure = an anticyclone
Isobars are far apart.
This means that the winds
will be calm.
Air pressure is getting
higher as it moves into the middle,
also over 1000mb = high pressure
Air pressure is high so
no cloud cover
One last thing you can
tell is wind direction.
In an anticyclone wind
blows clockwise.
(In the northern hemisphere)
Low pressure systems = Depression
Again you can tell wind
direction. It travels anti
Isobars close together,
clockwise in a depression
this means strong winds.
Low pressure, this
means very cloudy
Pressure decreases
towards the centre.
This shows a depression
Weather during a depression
• The weather during
a depression
depends a lot on the
fronts that come
with it.

Each stage of a
depression brings
very different
weather.
Cold sector
=
cold air
Cold sector
=
cold air
Warm sector
=
warm air
Depressions – how are they
formed?
• Depressions form where warm air
meets cold air
• The boundary between the two air
masses is called a front
• Along a front there will usually be thick
cloud and heavy rain
A Depression
Warm Front
Cold Front
Passage of a depression
• Q: Describe
the weather
in Wick over
the next 12
hours.
Depressions bring very
changeable weather.
The weather in Wick would
be very different to
Stornoway
Now, using the
information
from the
synoptic chart,
explain why this
weather will
affect Wick.
Cold front
brings cold air
so
temperatures
will decrease
Rapid
condensation
at cold front
means very
heavy rain
As cold front
approaches
isobars tighten
and wind strength
increases.
As warm front
passes then the
warm sector,
temperatures in
Wick will
increase due to
warm air
Will start off with
relatively clear skies.
Isobars spaced out so
wind speed not too
strong
As warm front approaches,
warm air is cooled so there will
be clouds then steady rain
Look at the
synoptic chart
Describe and
explain
the weather
you would
expect
Edinburgh to
be experiencing
on the
11th of
November
2012.
High Pressure
Weather 12 noon on 11th of November 2012
High pressure in winter
No clouds due to
high pressure
Clear skies in winter
mean that
temperatures will be
cold.
Dry conditions due to
high pressure and lack of
weather fronts. Air is
falling so not being
cooled
Isobars are far apart so
wind speed will be light