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

Mr. T. Tonna

Mr. T. Tonna

Mr. T. Tonna

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