Pressure and Rainfall

Investigating the Link Between Between Pressure and Rainfall

Teachers Notes

Here is some data collected by a weather station on the outskirts of Edinburgh, at the start of 2019.

Date

Atmospheric Pressure (hPa)

Rainfall (mm)

10/12/2018

1025

0.0

11/12/2018

1020

0.0

12/12/2018

1019

0.0

13/12/2018

1022

0.0

14/12/2018

1017

0.0

15/12/2018

988

1.0

16/12/2018

1005

5.1

17/12/2018

1005

0.3

18/12/2018

996

1.5

19/12/2018

995

0.3

20/12/2018

995

0.5

21/12/2018

1000

0.5

22/12/2018

1014

0.0

23/12/2018

1027

0.0

24/12/2018

1032

0.3

25/12/2018

1026

0.3

26/12/2018

1023

0.0

27/12/2018

1023

0.0

28/12/2018

1022

0.0

29/12/2018

1030

2.3

30/12/2018

1030

0.3

31/12/2018

1026

0.0

01/01/2019

1044

0.0

02/01/2019

1043

0.0

03/01/2019

1041

0.0

04/01/2019

1039

0.0

05/01/2019

1034

0.0

06/01/2019

1031

1.0

07/01/2019

1024

0.0

08/01/2019

1033

0.0

09/01/2019

1031

0.0

Using this data, draw a graph of rainfall against pressure.

graph paper

Now use this information to complete the following sentences:

The most it rained in one day was _______________mm.

It didn’t rain at all on ____________ days.

The highest pressure recorded was ______________hPa (a hPa is the same as a millibar).

The lowest pressure recorded was _______________hPa.

Does it always rain when the pressure is low? Use figures to justify your answer.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Does it ever rain when the pressure is high? Use figures to justify your answer.

____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Many weather apps assume that if the pressure is low, it will rain. Does your graph justify this assumption?

_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

 

 

Extension:

Here are the weather maps for 4 of the days when it rained: the first 3 show when the pressure was low and the 4th shows when the pressure was high and it rained.

1)

2)

3)

4)

Passage of a Depression – Animation

Depressions, Anticyclones and Fronts

Passage of a Depression – interactive animation

Worksheet to accompany the animation.

For 11+

Depressions from our Weather and Climate teacher’s guide

Pop-up depression

Cold and Warm fronts activities for differentiation and revision

Finding weather features on a simple synoptic chart

Red sky at Night, Shepherd’s Delight worksheet and Teacher’s Notes – a resource looking at how our prevailing wind direction means this saying is largely true.

Depressions Taboo

For 14+

Weather systems PowerPoints and cross section practice

Using WOW data to investigate a depression passing across the UK with  worksheets for students including isoline drawing practice.

Anticyclones, depressions and fronts with student worksheets 

Depressions and anticyclones with a synoptic chart exercise

A case study of orographic rainfall in Scotland.

What is the weather? Work out what the weather is like at several UK locations based on some simplified weather maps.

Interpreting weather charts basic information on synoptic charts, with Isotherm map exercise and Synoptic chart exercise.

Isotherm and Isobar drawing exercise based on a depression on our contour resources page.

For 16+

17. Depressions

Weather and Climate: a Teachers’ Guide

Pathway: Extending Weather

 Pressure and WindWater in the AtmosphereAnticyclones – Depressions

Lesson overview: In this lesson we look at the causes of Low pressure systems and the weather they bring to the UK.

Depressions or low pressure weather systems, bring most of the ‘normal’ and ‘extreme’ weather that we experience in the UK.  Driven by rising air, they mix colder and warmer air masses, forming fronts where these meet and bringing a predictable progression of weather as they pass over the UK.  Their approach is typically first signalled by high cirrus cloud, which eventually develops into rain-bearing stratus.  Once this passes there may be higher temperatures and less precipitation for a while, before towering cumulonimbus clouds arrive bringing heavy rainfall followed by lower temperatures and scattered showers.

Learning objectives:

  • To understand what low pressure is.

  • To know what weather a depression system brings to the UK.

  • To be able to draw and explain weather fronts.

  • To understand how weather changes in the UK as a depression passes over

Key Teaching Resources

Depressions PowerPoint
Depressions PowerPoint (easier)
Depressions Worksheet
Depressions Worksheet (easier)
Depressions Homework
Depressions animation worksheet
Pop-up Depression

Teacher CPD/ Extended Reading

Depressions_More for Teachers

Alternative or Extension Resources

Further resources for teaching about depressions, anticyclones and fronts.

Alternatively … you could get today’s weather chart and ask students to write a forecast for a place in the UK … or capture the weather forecast, from YouTube or similar, without the sound and ask students to write (and present) the commentary to go with it.

Weather and Climate: a Teachers’ Guide

Depressions – Identifying Features

Identify the weather features on the synoptic (weather) chart below:

Cold Front

Warm Front

Highest pressure

Depression

Anticyclone

Isobar

Occluded Front

Strong Wind

Light Wind

Lowest Pressure

 

Advanced

Northerly Wind

Northwesterly Wind

Southwesterly Wind

 

(hint the clouds in the images below are cirrus, cumulonimbus, cumulus and nimbostratus)

cirrus
cumulus
Cumulonimbus
synoptic chart extract

Pop-up Depression

Print, cut-out and fold along the lines to make a 3D model of a depression. 

Depression
Depression with folding instructions

Passage of a Depression – Account

Passage of a depression

Use the following help sheet to write a fictional story about the passing of a depression. Imagine you are in the field taking observations and measurements. You should use all the statements included below.

 Coming of the warm front At the warm front Within the warm sector At the cold front Within the cold sector
 Barometer – shows pressure is falling Barometer – continues to show the pressure is falling. No significant changes on the barometer. Suddenly the pressure on the barometer begins to rise. Pressure continues to rise.
 Thermometer – shows the temperature is steady Thermometer – shows the temperature is rising. Feeling a little sticky in this heavy coat! Still quite mild. Time for a hat, and to button up that coat, the temperature is falling, it’s becoming chilly. It remains cold.
 Cirrus clouds high in the sky, they begin to descend and thicken becoming altostratus. Becomes darker, the clouds are low and cover the sky like blankets – nimbostratus. Starting to see breaks in the cloud, a little drizzle now – nothing heavy. Angry looking clouds, towering and grey, begin to form – cumulonimbus. The clouds have thinned out now – some cumulus.
 No need for the umbrella yet! Time to get out the umbrella, continuous heavy rain. A steady breeze, nothing too uncomfortable. Heavy rain! Is that thunder I hear? A few showers.
 Slight increase in wind speed. The wind is much stronger now – struggling to hold onto my umbrella.  Struggling to stay on my feet, as I’m battered by the strong winds. Time to go home I think!

Drawing Pressure and Temperature Contours (Intermediate)

Drawing Temperature and Pressure Contours

Lines of equal temperature – temperature contours, are called isotherms.

In this exercise, the task is to draw the 8.5°C, 6.5°C and 2.5°C isotherms. The 10.5°C and 4.5°C isotherms have already been drawn to help.

Start with the 8.5°C isotherm. This is the line which has everything warmer than 8.5°C on one side of it, and everything colder than 8.5°C on the other side. You could use a coloured pencil to colour in the dots for everywhere that is warmer than 8.5°C. Your isotherm is then the line which separates your coloured dots from the uncoloured ones.

Next, try the 2.5°C isotherm. For this, you could use a different colour to shade the dots for everywhere that is colder than 2.5°C.

Lastly, draw the 6.5°C isotherm. This will be very close to the 4.5°C isotherm in places, so be careful! To help with this, you could use a 3rd colour to shade the places where the temperature is 7 or 8°C, and a 4th colour to shade the places where the temperature is 5 or 6°C. Your line will divide the two.

Remember, contours can never touch or cross each other (it can’t be 8.5°C and 6.5°C at the same place!). The line should start and finish at an edge of the page.

Drawing of isotherms

Lines of equal pressure – pressure contours, are called isobars.

In this exercise, the task is to draw the 1016, 1012, 1008 and 1004mb isobars. However, to save space on the map, the pressure values have been recorded in shorthand – so 9 or 09 is short for 1009, 11 is short for 1011 etc.

On this map, we have some extra clues – these weather observations indicate the wind speed by the tail on the symbol. The wind is blowing from the tail of the arrow to the centre (the bars on the tail of the arrow tell you the wind speed) so –o is a wind blowing from the west to the east, or o—is a wind blowing from the east to the west. The wind tends to blow along the pressure contours, so your contours need to be roughly parallel to the tails on the closest weather symbols.

Start with the 1016mb contour. Where on the map are there places where the pressure is higher than that? If it helps, colour in those symbols where the value is greater than 16. Your pressure contour needs to divide those symbols from the others.

Now use a different colour to shade the symbols where the pressure value is 13, 14 or 15. Can you now draw the 1012mb isobar?

Again, use a different colour to shade the symbols where the pressure value is 9, 10 or 11. Can you now draw the 1008mbar contour?

Lastly, is there anywhere where the pressure is under 1004mb? If so, draw the 1004mb contour to separate off that observation from the others.

Drawing of isobars

These two maps both correspond to the same weather situation. Can you work out what is going on?

Clues:

– Where is the pressure lowest?

– How is the wind direction changing across the map?

– In general, where is the temperature lower and where is it higher?

– Where is the temperature changing most rapidly? Remember that a front is where cold and warm air meets. You can use the wind information on the second map to see where the cold air is pushing into the warm air (a cold front) or whether the warm air is pushing into cold air (a warm front).

Drawing Pressure and Temperature Contours (Harder)

Drawing Isobars

Isobars are lines of constant pressure. Drawing the isobars reveals features (eg highs, lows, ridges and troughs) which help us understand the weather.

When trying to draw isobars, remember the following

– You are trying to draw the isobars of pressure for the values below the
graph

– The symbols on the map give the observed pressure and wind speed and
direction. Remember that the wind is blowing from the tail of the arrow
to the centre. The bars on the tail of the arrow tell you the wind speed:

– The wind blows almost parallel to the isobars (they are usually blow
slightly more towards the centre of the low pressure area). If you stand
with your back to the wind in the northern hemisphere the pressure is
lower on your left than on your right.

– Isobars tend to be parallel to each other, don’t wiggle and never cross.

– The closer the isobars are to each other, the stronger the wind. You can
use the bars on the tail of the weather station symbol to give you the
Beaufort force of the wind. Look at the scale at the top of the map (this is
called the ‘geostrophic scale’). The distance from the left hand edge of
the scale to the force at the symbol gives you the spacing between 2mbar
isobars at that point.

Beaufort scale

Draw the 1004, 1008, 1012 and 1016 isobars (noting that they have been recorded in shorthand, such that 1004 becomes 04 etc.) remembering that the wind tends to blow parallel to the isobars. Can they see what sort of a weather system it is, and where the fronts might be?

Drawing of isobars

Draw the 2.5, 4.5, 6.5, 8.5 and 10.5 isotherms (lines of equal temperature), remembering that sometimes, if there isn’t any data in a particular place, you have to make an educated guess about what the contour might look like. Is it more obvious now what the structure of the weather system is?

Drawing of isotherms

Shade the areas of rain, heavy rain and drizzle in the map below. Is it more obvious now what the structure of the weather system is?Weather map

What is the Weather

For each location marked on each map, what do you expect the weather to be doing? Think about whether it is windy, what the wind direction is, whether it is raining and whether it is warmer or colder.

 

weather map

Galway __________________________________________________________________

Aberdeen __________________________________________________________________

London __________________________________________________________________

weather map

Glasgow __________________________________________________________________

Norwich __________________________________________________________________

Exeter __________________________________________________________________

weather map

Tiree __________________________________________________________________

Carlisle __________________________________________________________________

Norwich __________________________________________________________________

weather map

Tiree __________________________________________________________________

Leeds __________________________________________________________________

Brighton __________________________________________________________________