S4 – Weather Unit Image copyright Joseph Kerski

Lesson 7 – Pressure Systems We Will: Know the causes of high and low pressure. Be able to explain the weather conditions associated with high and low pressure. Understand the impact of air pressure on people.

STARTER Complete a weather station circle for the following weather conditions: 4 oktas of cloud cover Rain Shower 7°C Temperature South Westerly Wind 15 knots wind speed

Pressure Air pressure is simply a measure of the amount of air above. When the air pressure is High, there is more air, and when the air pressure is Low, there is less air. As air rises, there is less air left above it and so its pressure falls. As air sinks, there is more air above it and so its pressure rises. So, if air rises up through the atmosphere the air pressure falls. This makes the temperature fall. If air sinks through the atmosphere the air pressure rises. This makes the temperature of the air increase.

Pressure The air is constantly moving. Generally speaking, warm air rises and cool air sinks. Draw the following 2 diagrams: Rising air cools down and condensation occurs. This makes cloud and rain. Falling air warms up and any moisture evaporates. This leads to clear skies.

High Pressure - Anticyclone 20 minutes including next slide. Students to draw their own version of diagram and label the weather conditions expected.

High Pressure (Anticyclones) Put the following statements in order: Cool air sinks as it is heavier than its surroundings. This leads to high surface pressure. Sinking air warms as it gets lower in the atmosphere. Clear skies in winter mean the ground cools down overnight. Weather conditions are cold and dry in the winter. Any moisture within the air evaporates. When sinking air hits the ground, it spreads out. Weather conditions are hot and dry in the summer. This means that there are no clouds. The spreading air is slowed down by friction at the ground. The friction causes a ‘traffic jam’ – and the pressure rises. Clear skies in summer allow the Sun to warm the ground.

High Pressure (Anticyclones) ANSWERS: Cool air sinks as it is heavier than its surroundings. Sinking air warms as it gets lower in the atmosphere. Any moisture within the air evaporates. This means that there are no clouds. Clear skies in winter mean the ground cools down overnight. Weather conditions are cold and dry in the winter. Clear skies in summer allow the Sun to warm the ground. Weather conditions are hot and dry in the summer. When sinking air hits the ground, it spreads out. The spreading air is slowed down by friction at the ground. The friction causes a ‘traffic jam’ – and the pressure rises. This leads to high surface pressure.

https://youtu.be/41K4ZIsFu90 Watch: https://youtu.be/41K4ZIsFu90 Positive effects on people Negative effects on people Summer Winter

What do you think? Your parents want to go camping this weekend. The forecast is for high pressure. Bullet point what you would pack. Explain why you have picked these things. 5 minutes

This is how a high pressure systems is shown on a weather (synoptic) chart. The lines are called ISOBARS and they connect areas of the same pressure. IMPORTANT FACT: The closer together isobars are on a synoptic chart, the windier it is.

Dry Clear Skies Cool nights Snow Hot Morning Frost Heavy Cloud Think about whether each of these conditions fit a summer anticyclone, a winter anticyclone, both or neither. Dry Clear Skies Cool nights Snow Hot Morning Frost Heavy Cloud Worth noting: the official definition of fog is visibility of less than 1,000metres. Fog is just tiny water droplets suspended in the air – low cloud. Steady Rain Very cold

Decide whether these statements about anticyclones are true or false Anticyclones form in areas where the air is descending There is little cloud with an anticyclone as evaporation increases as the air sinks. Isobars in an anticyclone are close together, indicating strong winds Anticyclones are areas of high pressure. In winter anticyclones, there are high temperatures. True, true, false, true, false

Low Pressure (Depressions) The UK often has ‘unsettled’ weather, where the conditions change frequently and sometimes rapidly. This is caused by low pressure systems, or depressions, that move over the UK. Sometimes they can last less than a day, but sometimes up to three days. The closer the pressure contours (isobars) are to each other, the faster the wind speeds will be. An area of low pressure is called a depression. 1. What is a depression and what is it associated with?

In what way does this look different from a high pressure system??

Low Pressure (Depressions) A warm front means that warm air is coming. At a warm front, warm air is pushing into and over cold air. This usually produces clouds and rain. A cold front means that cold air is coming. At a cold front, cold air pushes under the warm air, forcing it to rise. This produces heavy rain..

Divide your A3 sheet into four. For each round, draw and label what you see in the presentation, in one box. On the next round, swap your sheet with a partner, do the same as in point 2, but in a new box. It gets faster and harder with each round.

Cold Warm Cold Direction

Cumulonimbus Cirrus Warm Sector Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Cumulonimbus Cirrus Warm Sector 2 1 Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Cold Warm Cold Direction

Cumulonimbus Cirrus Warm Sector Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Cumulonimbus Cirrus Warm Sector 2 1 Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Cold Warm Cold Direction

Cumulonimbus Cirrus Warm Sector Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Cumulonimbus Cirrus Warm Sector 2 1 Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Cold Warm Cold Direction

Cumulonimbus Cirrus Warm Sector Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Cumulonimbus Cirrus Warm Sector 2 1 Cold Warm Cold Nimbostratus Heavy rain Continuous rainfall Showers Intermittent rain Direction

Redraw the diagram A3 size, but explain what is happening at points 1 and 2 to bring rainfall.

Show Postman Pat does Depressions https://www.radicalgeography.co.uk/weather-and-climate/

Area of lowest pressure Warm Front Occluded Front Warm sector Showers Behind the cold front. Cold Front

Fold your sheet so that you have a pop-up area to fill in showing what happens along a depression. Through the following explanation, you need to fill in the information.

Muddiest Point Write down one or two points on which you are least clear. We can then seek to remedy the muddiness.

1. 2 4. 5. 3. The depression moves from W to E due to the jet stream Heavy rain, towering clouds, strong winds Occasional showers Dark skies, persistent rain, strong wind Windy, warm and wet High cloud but mostly clear sky 1. 4. 2 5. 3. Link the numbers with the weather conditions The depression moves from W to E due to the jet stream

Explain the changes which will take place in the weather in Glasgow over the next 24 hours. (6) Describe how the weather will change in Glasgow. Synoptic Chart for 15th February 2007

SQA marking scheme – peer mark each others answers using the marking scheme

Study Reference Diagram Q3, Synoptic Chart,0800 hours10 March. Describe in detail the different weather conditions at Cape Wrath and Banbury at 0800 hours on 10th March. Make two points for each location.

Why are there clouds here? Why are there showers here? PLENARY Why are there clouds here? Why are there showers here? Frontal Convection or Relief rainfall. Why is it clearer here?