lesson 1: depressions and anticyclones
DESCRIPTION
Form 4 Option LessonTRANSCRIPT
Depressions and Anticyclones
Mr. T. Tonna
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
Key Words
DepressionAnticycloneWarm FrontCold FrontOccluded FrontWarm Sector
Mr. T. Tonna
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
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
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
Fronts
Warm Front Cold Front Occluded FrontThe 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
Warm Sector
• The warm sector is the area between the cold and warm front. It is usually associated with calm weather.
Mr. T. Tonna
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-WesterlyForce 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
Mr. T. Tonna
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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Mr. T. Tonna
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
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