Form 4 Option Lesson

1. Depressions and Anticyclones
Mr. T. Tonna

2. Index
• Key Words
• What are Depressions & Anticyclones?
• What are weather Charts?
• Isobars
• Fronts
• Warm Sector
• Different Weather Conditions
• Forecasting
• Class Task
• Summary
Mr. T. Tonna

3. Key Words
Depression
Anticyclone
Warm Front
Cold Front
Occluded Front
Warm Sector
Mr. T. Tonna

4. What are Depressions
• ‘Normal’ air pressure is 1016mb.
• Depressions are low pressure systems that form in the mid-
latitude region (30 -60 N/S) when warm air meets cold air.
• When this occurs the warm air is forced above the colder air. It
then cools and clouds form which produce rain.
• Winds in depressions move in
an anticlockwise direction in the
northern hemisphere and
clockwise in the southern
hemisphere.
• Characterised by fronts.
Mr. T. Tonna

5. Anticyclones
• Anticyclones are the opposite of depressions.
• They are high pressure systems
• Less common than depressions.
• They usually bring nice weather.
• Air descends, warms and acquires moisture.
• Clear skies and little chance of rain.
• If ground is cold low fog can occur.
Mr. T. Tonna

6. Isobars
• Isobars are lines of equal atmospheric pressure
drawn on a meteorological map. Each line passes
through a pressure of a given value, provided
certain rules are followed. The rules for drawing
isobars are-
– Isobar lines may never cross or touch.
– Isobar lines may only pass through pressures of 1000
+ or - 4. In other words, allowable lines are
992, 996, 1000, 1004, 1008, and so on.
– The atmospheric pressure is given in millibars (mb
– Pressure lines are usually corrected or sea level so any
differences in pressure due to altitude are ignored.
Mr. T. Tonna

7. Fronts
Warm Front Cold Front Occluded Front
The Beginning of
the depression
The End of the
Depression
When the warm
and cold fronts
join.
A Warm Front is labelled
with a Red Semi-Circle.
A Cold Front is labelled
with a Blue Triangle.
Occluded fronts are labelled
with a mix of the two fronts.
Mr. T. Tonna

8. Warm Sector
• The warm sector is the area between the cold
and warm front. It is usually associated with
calm weather.
Mr. T. Tonna

9. Weather Conditions in Depressions Mr. T. Tonna
Precipitation/
Clouds
Temperature Wind (Speed/
Strength)
Visibility
Before
Depression
No Rain, Cirrus /
Cirrostratus
Clouds
Approx. 6
Degrees
Calm, North
Easterly Wind
Good
Approaching
Warm Front
Light Drizzle,
Altostratus
Clouds
Approx. 6
Degrees
Strong, Southerly
Winds
Good to Fair
Arrival of Warm
Front
Continuous rain,
Nimbostratus
Clouds
Slight Fall in
Temperature
South-Westerly
Force 3-4
Worsens
Warm Sector Little Rain, Patchy
Stratus Clouds
Rises to around
12 Degrees
Westerly, Force 3 Limited
Passing Cold
Front
Heavy Showers,
Cumulonimbus
Clouds.
Falls Further North-Westerly,
Force 8
Limited
Post-
Depression
Showers,
Cumulous Clouds
clearing
Low 3-4 Degrees. North-Westerly
Force 6
Good

10. Mr. T. Tonna

11. Forecasting Weather Patterns
• The Following Weather Charts will show areas
experiencing a Mid-Latitude Depression.
• Our Task is to analyse the chart and determine
which weather characteristics a marked area is
experiencing.
• As well as this we shall discuss the future weather
that area will be experiencing according to the
mentioned weather characteristics of
depressions.
Mr. T. Tonna

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16. Class Task
– Read Pages 204-206 (A New Wider World-
Waugh, D; Bushell, T)
– Answer the following:
• A Low pressure system is called a ________?
• True or False: Anticyclones have pressures below
1016mb?
• Are Depressions are associate with nice weather?
• Warm air moves _____ cooler air during a depression.
• London is currently in the warm sector of a depression
what kind of weather would you expect there to be?
Mr. T. Tonna

17. Summary
• Depressions and Anticyclones are caused by
pressure change.
• Depressions are more common than Anticyclones
in the Mid-Latitude regions.
• Depressions bring complex whether phenomena
with alternating bursts of showers and drizzles.
• Using weather maps we are able to locate
depressions and predict the weather of a certain
area/locality.
Mr. T. Tonna