1. S4 – Weather Unit
Image copyright Joseph Kerski

2. 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.

3. 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

4. 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.

5. 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.

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

7. 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.

8. 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.

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

10. 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

11. 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.

12. 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

13. 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

14. 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?

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

16. 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..

18. 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.

22. Cold
Warm
Cold
Direction

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

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

28. Cold
Warm
Cold
Direction

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

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

34. Cold
Warm
Cold
Direction

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

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

40. Cold
Warm
Cold
Direction

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

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

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

44. Show Postman Pat does Depressions

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

47. 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.

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

49. 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

50. 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

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

52. 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.

53. 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?