Pressure systems. We are going to learn about two different types of pressure systems: Low pressure systems: A depression. High pressure systems: An anti- cyclone.

But first…….. We have to know a few things

Things we need to know. Iso bars Isobars show 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. Cold sector Warm 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 the air is higher and therefore not putting as much pressure on the earth. Air is up high = 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 ground and therefore too warm for clouds to form. Copy this: 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? Answer: The air is higher up and therefore it cools, condenses and forms clouds. Copy this: In areas of low pressure the air is high in the atmosphere and it is colder. This means that it cools, condenses and forms clouds. (The 3 C’s)

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 Isobars close together, this means strong winds. Low pressure, this means very cloudy Pressure decreases towards the centre. This shows a depression Again you can tell wind direction. It travels anti clockwise in 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 Cold Front Warm 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.

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 As warm front passes then the warm sector, temperatures in Wick will increase due to warm air As cold front approaches isobars tighten and wind strength increases. Rapid condensation at cold front means very heavy rain Cold front brings cold air so temperatures will decrease

High Pressure Weather 12 noon on 11 th of November 2012 Look at the synoptic chart Describe and explain the weather you would expect Edinburgh to be experiencing on the 11 th of November 2012.

High pressure in winter No clouds due to high pressure Isobars are far apart so wind speed will be light Dry conditions due to high pressure and lack of weather fronts. Air is falling so not being cooled Clear skies in winter mean that temperatures will be cold.