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Disasters
Part 2: Monitoring and Predicting Weather
define the term ‘atmospheric pressure’ and
describe the movement of air
currents between areas of high and low pressure
Atmospheric Pressure
• The force per unit area that the air is exerting on
the body.
Atmospheric Pressure Units
• Air pressure is measures in hectopascals (hPa).
• The atmosphere pushes down on the Earth’s surface
with a pressure of about 100 000 pascals. With such
large numbers it is common in meteorology to use
kilopascals (kPa) or hectopascals (hPa).
• 1 kilopascal = 1000 pascals
1 hectopascal = 100 pascals.
Atmospheric Pressure
• The higher you are from
the surface of the Earth,
the less dense the air is so
the lower the air pressure
is.
• Sea level average =
1013 hPa
• During severe storms =
970 hPa
Atmospheric Pressure
• High Pressure
• Cold air
• air gets denser and
heavier
• Low Pressure
• Warm air
• air becomes less
dense and lighter
Movement of Air
Write down what
you think is
happening here.
Movement of Air
During the day, especially in summer, land areas heat up
more than adjoining waters. This warms the air over it and,
as the air expands, it rises. Air pressure becomes less over
the land. This makes room for cool air to blow from the
ocean to the land. The warmed air now moves out over the
ocean to take the place of the cool breeze. In doing so it is
cooled down and contracts. This creates a higher pressure
over the water and so the air descends.
Textbook: Spotlight
Page 304
Summarise section: “Too Much Pressure”
Video: https://www.youtube.com/watch?v=aiYyCurh_SU
Measuring Pressure Changes
• Barometers are used to measure air pressure
in our atmosphere.
• Invented in 1643 and used mercury. The
mercury would be pushed down by the air
pressure, moving it slightly.
• Other liquids were tried but moved by the
air more easily
Measuring Pressure Changes
• Modern (aneroid) barometers consist of a
closed metal capsule with flexible walls.
Only a small amount of air inside the
capsule.
• When air pressure increases outside the
capsule, the capsule squeezes a little
• When air pressure decreases outside the
capsule, the capsule bulges out a little.
identify that the distance between isobars on a
weather map indicates the relative change of
atmospheric pressure in an area
Air Pressure on Maps
• Lines are drawn on weather maps joining places which have
the same air pressure. These lines, called isobars, show
high and low pressure systems. Pressure is shown in
hectopascals (hPa).
Air Pressure on Maps
• For the Southern
Hemisphere, in a low
pressure system is
clockwise and moving
inwards, while that
around a high pressure
system is anticlockwise
and moving outwards.
Air Pressure on Maps
• If isobars are close
together the pressure
difference between two
points will be great and
therefore the winds will
be strong.
• If they are a fair distance
apart, wind speeds will
not be as great.
Air Pressure on Maps
• Worksheet: Typical Weather Map
Answers
1. The interval between successive isobars is 2 hPa.
2. X is 1008 hPa as it lies between the 1006 hPa and 1010 hPa isobars.
3. a) 1011 hPa
b)1008.5 hPa
4. Hobart has the lowest air pressure, being about 1008 hPa.
5. a) Air moving approximately in a northwesterly direction (a South-Easterly wind).
b) Air moving approximately in a southwesterly direction. (a North-Easterly wind).
6. Winds are stronger in Brisbane as isobars closer together.
Describe the relative pressures involved in the
formation of tropical cyclones and tornadoes
What is a Tropical Cyclone?
A cyclone begins to form when moist air is
heated by the Sun and rises from the surface
of warm, tropical seas. This air is funneled in
a natural updraft. On rising, the moist air
cools and condenses into rain.
What is a Tropical Cyclone?
The condensation of gaseous water vapour to
liquid water droplets releases large amounts of
heat energy, and this adds to the storm’s updraft.
Air continues to spiral upwards, with hot moist air
rushing in from all sides to replace it. These winds
spiral around an ‘eye’, an area of calm and light
rains.
What is a Tropical Cyclone?
The cyclone begins to revolve because
of the Earth’s rotation. In the Southern
Hemisphere the rotation of air in a low
pressure system is in a clockwise
direction towards the centre, where it
rises then flows outwards at higher
altitude. The energy released from this
process goes towards increasing the
speed of the spinning column of air.
How Do Tropical Cyclones Occur?
How Do Tropical Cyclones Occur?
How Do Tropical Cyclones Occur?
How Do Tropical Cyclones Occur?
Cyclone Category
Measurements of wind
speed and air pressure
made before a cyclone
reaches a location can
be used to predict the
size of any storm surge
and the likely damage
at that location.
Where Do Cyclones Occur?
• About 80-100 tropical
cyclones develop each year
from low air pressure areas
in tropical oceans. This is
usually between the
latitudes of 5° and 30° on
either side of the Equator.
• Cyclones never develop
directly on the Equator as
there is no Coriolis Effect
there to start the required
circular motion of warm
moist air.
Textbook
• Spotlight Senior Science
• Pg 305-307
• Summarise “Cyclones and Tornadoes”