S4.14.2 Chapter 2
Weather and air pressure
The radio says a low pressure area is coming. Why does that mean cloud and rain, and what does a high bring?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
- Explain how high and low pressure bring different weather
- Build a rain gauge and a wind vane and keep records
Weather is what the air is doing now and today: warm or cold, wet or dry, calm or windy. Weather changes from hour to hour. To forecast it, people measure the air, and one thing they measure is its pressure.
You already know that air has weight. The air pressure at the ground is the weight of all the air above, pressing on each patch of ground. On each square centimetre it is about the weight of 1 kg. At sea level it averages about 1 013 hPa. A hPa, or hectopascal, is 100 pascals.
Air pressure is not the same everywhere, and it changes with the weather. A barometer measures it. If you keep a record of the pressure, you can see the weather coming. Here is a trace over three days.
Why? Think of air as a river with slopes. Air flows from places of high pressure towards places of low pressure. Where the ground is warm, or where winds meet, air is pushed upward. A place where air rises has a lower pressure at the ground. We call it a low.
In a low, air flows in along the ground and rises. As it rises it meets lower pressure, spreads out and cools. Cool air holds less water vapour. The vapour condenses into droplets, which make cloud. If the droplets grow big, they fall as rain.
A place where air sinks has a higher pressure at the ground. We call it a high. In a high, air sinks and then flows out along the ground. Sinking air is squeezed a little by the air above, so it warms, and cloud droplets dry up. The sky is clear, the winds are light, and the weather stays the same for days. We call that settled weather.
See it three ways
As a picture
In words
In a low, air rises, and cloud and rain form. In a high, air sinks, and the sky clears. The winds in a low are usually stronger than in a high.
In symbols
L → air rises → cloud and rain. H → air sinks → clear sky.
Weather forecasters draw the pressure on a map. They join places that have the same pressure with a line called an isobar. Each isobar has a number in hPa. Round a low the numbers fall towards the middle. Round a high they rise towards the middle.
The isobars also tell how windy it is. Where isobars are close together, the pressure changes quickly over a short distance, and the wind is strong. Where they are far apart, the wind is light.
Wind blows from high pressure to low pressure. But the Earth is spinning, and this turns the wind as it goes. In the northern hemisphere the wind turns to the right. So it spirals in anticlockwise round a low, and out clockwise round a high. In the southern hemisphere it is the other way round.
People name a wind by the direction it comes from. A west wind blows from the west, towards the east. A wind vane shows this. It is a pointer that turns freely. Its head swings round to face into the wind, and its tail points the way the wind is going.
Rain is measured with a rain gauge. It is a container with straight sides and a flat bottom. The depth of water in it, in millimetres, is the depth of rain that fell. Because the sides are straight, 1 mm in the gauge is 1 mm of rain on the ground everywhere. The picture strip “Rain adds up in the gauge”, further on, shows one.
Worked example
Over one week a gauge was emptied each day. The readings in mm were 0, 4, 12, 0, 0, 7 and 3. How much rain fell in the week?
- Add the seven readings: 0 + 4 + 12 + 0 + 0 + 7 + 3.
- 0 + 4 = 4, and 4 + 12 = 16.
- 16 + 7 = 23, and 23 + 3 = 26.
- So 26 mm of rain fell in the week.
Answer: 26 mm
There is a useful fact for roofs and farms. 1 mm of rain on 1 m² of ground is 1 litre of water. So the water from a roof is its area in m² times the rain in mm, in litres.
Worked example
25 mm of rain falls on a flat roof of 40 m². How many litres of water fall on the roof?
- 1 mm of rain on 1 m² is 1 litre.
- The roof has 40 m², so each mm of rain gives 40 litres.
- There are 25 mm of rain, so 25 × 40 = 1 000 litres.
Answer: 1 000 litres
Draw it first
Rain adds up in the gauge
1 Read the top of the water against the scale. After the first day it is just above the 5 mm tick, at 6 mm. That is the rain of one day.
2 After the second day, with the jar not emptied, it shows 14 mm. The second day gave 8 mm.
3 After the third day it shows 22 mm. The third day gave 8 mm again. Empty the gauge after each reading to get one day at a time.
Try it
Build a rain gauge and a wind vane, and keep a record
You need: a tin can or a jar with straight sides and a flat bottom, a ruler with mm marks, a drinking straw, a pencil with an eraser, and a pin, a piece of card, tape, a stick or a post to hold things in the open, paper and a pencil for a record.
Be safe: An adult should push the pin and cut the card. Push the point of the pin into the eraser at once, and keep the pin out of reach of small children. Do not use a can with a sharp rim: ask an adult to tape over the edge. Do not go out in a storm to read the gauge.
- Gauge: stand the can or jar in the open, away from trees and walls, on a flat spot. Wedge it with stones so it cannot tip over.
- Each day at the same time, stand the ruler upright on the bottom of the can. Read the depth of the water in mm. Many rulers have a gap before their 0 mark. If yours does, add that gap, or mark a strip of card from its very end. Write it down. Then empty the can.
- Vane: cut an arrow head and a tail fin from card. Tape the head to one end of the straw and the fin to the other. Ask an adult to push the pin through the middle of the straw into the eraser of the pencil.
- Fix the pencil upright in a post. Check that the straw turns freely. Mark north, east, south and west on the ground with stones. Find north with a compass. Without one, face where the Sun rises, which is roughly east, and north is on your left.
- Each day, read the way the arrow head points. Write the wind as the direction it comes from. Add a note about the sky: clear, cloud or rain.
What you will see: After a week you have a record of the rain, the wind and the sky. Compare it with what the pressure was doing, if a barometer is near. Days of rain often come with winds from one direction and falling pressure.
A wrong idea many people have
A west wind blows towards the west.
Why it feels right: Many words for direction mean where something is going: we go west, we head north.
The test that breaks it: Look at the vane. Its head points into the wind, to the west, and the wind arrows blow from there towards the east. Stand facing west on a day with a west wind. The wind hits your face.
What is true: A wind is named for where it comes from. A west wind comes from the west and blows towards the east.
A wrong idea many people have
Air does not weigh anything, so air pressure is not real.
Why it feels right: We cannot feel the weight of air on us, and air seems like nothing.
The test that breaks it: Air pressure at sea level is about 101 000 Pa. That is a push of about 10 N on each cm², the weight of about 1 kg. You do not feel it because the fluids in your body push back outwards just as hard. A tin can that has had its air pumped out is crushed flat.
What is true: Air has weight. Over each m² of ground there is about 10 000 kg of air, which is air pressure. We do not feel it because we are balanced from inside.
A wrong idea many people have
High pressure means stormy weather, because high sounds strong.
Why it feels right: The word high sounds like more of everything, so we expect more wind and rain.
The test that breaks it: Look at the two pictures of the air. In a high the air sinks, and sinking air clears cloud. In a low the air rises, and cloud and rain form. Strong wind needs close isobars, not a high number.
What is true: A high brings settled weather. A low brings cloud and rain. It is not the number that makes the storm. It is whether the air rises or sinks.
Key words
- weather
- what the air is doing now: warm or cold, wet or dry, calm or windy.
- air pressure
- the push of the air, from the weight of the air above. At sea level it is about 1 013 hPa.
- low
- a place of low air pressure, where air rises and cloud and rain form.
- high
- a place of high air pressure, where air sinks and the sky clears.
- isobar
- a line on a map that joins places with the same air pressure.
- wind vane
- a pointer that turns to face into the wind, to show where the wind comes from.
- rain gauge
- a straight sided container that measures the depth of rain in mm.
Back to your guess
The answer is A: A low brings cloud and rain, because air rises in it. A high brings clear, settled weather, because air sinks in it.
In a low, air flows in near the ground and rises. Rising air cools, its water vapour condenses, and cloud and rain form. In a high, air sinks and flows out. Sinking air warms a little, so cloud melts away and the sky clears.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Which weather does a low pressure area usually bring?
See the answer
Cloud and rain, because air is rising
In a low, air flows in and rises. The rising air cools, its water vapour condenses, and cloud and rain form.
- If you chose A: That is the weather of a high. In a low, air rises and makes cloud.
- If you chose C: Low pressure does not mean no weather. It means air is rising, and that makes cloud and rain.
- If you chose D: Settled weather comes with a high, where air sinks. A low is changeable.
What does the air do in a high pressure area?
See the answer
It sinks and then flows out along the ground.
In a high, air sinks, warms a little, and flows out along the ground. Cloud dries up, and the sky is clear.
- If you chose A: That is what air does in a low. In a high the air sinks. Go to S2.4.5.
- If you chose B: Air flows in along the ground towards a low. From a high, it flows away.
- If you chose C: The air in a high is moving. It sinks, and spreads out along the ground, with light winds.
The map is for the southern hemisphere. Which way does the wind turn round the low?
See the answer
Clockwise, because the turn is the other way in the south
The spin of the Earth turns the wind to the left in the southern hemisphere. So the wind spirals in clockwise round a low there.
- If you chose B: In the northern hemisphere the wind turns anticlockwise into a low. The southern hemisphere is the other way round.
- If you chose C: The Earth’s spin turns the wind. It spirals, in both hemispheres, rather than going straight in.
- If you chose D: The turn does not change with the time of day. It depends on the hemisphere.
The diagram shows a wind vane seen from above. What is the wind?
See the answer
A south east wind, blowing from the south east towards the north west
The head of the vane points into the wind, to the south east. So the wind comes from the south east and blows towards the north west.
- If you chose A: The tail fins point the way the wind goes. A wind is named for where it comes from, which is the side the head points to.
- If you chose B: The head points to the south east, not the north east. Look at which letters are on each side.
- If you chose D: The head points into the wind, to where it comes from. The wind blows away from the south east, not towards it.
This is a rain gauge with a straight sided jar. What is the depth of rain, in mm?
See the answer
17 mm
The top of the water is between the 15 mm and 20 mm ticks, two fifths of the way up. So the depth is about 17 mm.
- If you chose A: You read the number after the 10 as if it started again from 0. The scale goes on: 10, then 15, then 20.
- If you chose B: The water has not reached the 20 mm tick. Read where the top of the water is, not the next number.
- If you chose C: That is the tick just below. The water is a little above 15 mm, so read between the ticks.
Over a day the pressure falls steadily, from 1 016 hPa to 1 002 hPa. What weather is most likely to follow?
See the answer
Cloud and rain are more likely.
A falling pressure often means a low is coming. In a low, air rises, and cloud and rain are likely.
- If you chose A: That follows rising pressure. Falling pressure means air is starting to rise, and cloud may form.
- If you chose C: Falling pressure often comes with more wind, not less.
- If you chose D: Pressure is one of the best signs. A steady fall often comes before cloud and rain.
From an earlier chapter
Which layer of the atmosphere holds the weather?
See the answer
The troposphere
The troposphere is the lowest layer, up to about 12 km. It holds nearly all the water vapour and all the weather.
15 mm of rain falls on a flat roof of 60 m². How many litres of water fall on it?
See the answer
900 litres
1 mm of rain on 1 m² is 1 litre. The roof has 60 m², so 15 mm gives 15 × 60 = 900 litres.
A gauge was emptied each day. The readings in mm on five days were 3, 0, 12, 5 and 8. What was the total rain, in mm?
See the answer
28 mm
Add the readings: 3 + 0 + 12 + 5 + 8 = 28 mm.
Keep it
Copy this into your exercise book.
Low: air rises, cloud and rain. High: air sinks, clear sky. Close isobars, strong wind. A west wind comes from the west. 1 mm of rain on 1 m² is 1 litre.
Draw: A low and a high side by side, from the side, with arrows for the air and a cloud over the low.