The science idea
Weather is what the air around you is doing today: whether it is raining, how hard the wind blows and where it comes from, and how hot or cloudy it is. Weather stations measure these things every day, and when the records of many years are put together they show the climate of a place. You can start your own record with two simple instruments, a rain gauge and a wind vane, made from things that would otherwise be thrown away.
Rainfall is measured as a depth. If all the rain that fell on a flat piece of ground stayed where it landed, with no drainage or evaporation, how deep would the water be? Meteorologists measure that depth in millimetres. A rainfall of 1 millimetre means 1 litre of water on every square metre of ground. The rain gauge works because a straight-sided container, whose opening is the same as the area of its floor, collects rain of exactly the same depth as fell outside: if the funnel opening is exactly as wide as the cylinder, 10 mm of water in the bottle means 10 mm of rain. If the funnel is wider than the cylinder, the water is deeper than the rainfall by the ratio of the two areas, which makes small amounts of rain easier to read.
A wind vane shows where the wind comes from. The vane balances on a pin, so it can swing freely. It has a small head and a large tail. When the wind blows, it pushes on both, but the larger tail receives the larger push, so it is blown downwind. The head ends up facing into the wind. Winds are always named by the direction they come from, so a west wind blows from the west towards the east, and the vane points to the west.
Wind is air moving from a region of high pressure to a region of low pressure. Sunlight heats some places more than others, for example land more than the sea in the daytime. Warm air rises, which lowers the pressure near the ground, and cooler air moves in to replace it. In India this idea explains the monsoon: in summer the land heats up much more than the Indian Ocean, and moist south-westerly winds blow in from the sea, bringing the rains.
Taking readings at the same time every day, in the same place, is what turns the instruments into a project. After two weeks you will be able to say how much rain fell in total, how many days it rained, which way the wind blew most often, and whether rain came with a wind from a particular direction. You can then compare your results with the local forecast and with the figures from the weather department.
Materials and tools
Cost: Free
A plastic bottle, pebbles, a straw, a pin, a pencil, card and a cup are all household or waste items. A magnetic compass or a phone compass is needed once, to find north.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Plastic bottle with straight, round sides (1 to 2 litres), such as a mineral-water or cold-drink bottle | 1 | Choose one with a smooth straight body, without ridges, and a flat or almost flat base. |
| Pebbles or small stones | A handful | From the garden or the road side. |
| Waterproof tape, or clear tape and a permanent marker | 1 strip, 25 cm long | Cut a strip of tape and mark it with a pen, then cover it with a second layer of clear tape. |
| Plastic or paper cup, a pencil with a rubber at the end, plasticine or damp sand | 1 each | Any cup, an unsharpened end of a used pencil, and sand from the garden. |
| Drinking straw (not bendy) | 1, about 20 cm | From a juice carton. Cut off any bendy part. |
| Straight pin (a sewing pin) | 1 | From the sewing box. Cover the point with the pencil rubber after use. |
| Thin card for the arrow head and tail | 1 piece, about 12 cm × 8 cm | The side of a cereal box. |
| Magnetic compass (or the compass on a phone) | 1 | Borrow one. You only need it once, to find north. |
| Notebook, ruler and pen | 1 each | From school. |
Tools
- Scissors
- A craft knife for starting the bottle cut (adult)
- Tape or a stapler
- A ruler
- A permanent marker pen
- A measuring cylinder or a small cup and syringe for calibration (optional)
Safety first
Read this before you start
- An adult should start the cut in the plastic bottle with a knife or sharp scissors. The cut edges can be sharp, so cover the rim with tape.
- A pin is sharp. Push the pin into the pencil rubber so the tip is hidden, and never leave it pointing up at eye level. Keep it out of the reach of younger children.
- Do not go out to read or fix the instruments during a thunderstorm or lightning. Wait until it has passed.
- Do not put the weather station near electric wires, on a roof edge or on a high wall. A balcony rail is not a safe place for a student to lean over.
- Empty the rain gauge after each reading and turn it upside down. Standing water breeds mosquitoes within a week.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Choose the bottle and mark the cut
Wash a clean bottle. Draw a line around it at the point where the sloping shoulder meets the straight body. An adult makes a small cut on the line with a knife and then you cut right round with scissors.
Tip: Use a ruler held against the bottle to check that the line is level all the way round.
Make the funnel
Turn the top part upside down and push it into the body so that the neck points down and the rim of the funnel sits at the top of the body. You now have a funnel and a straight cylinder. Fold a strip of tape over the sharp cut edges.
Common mistake: Using the lower, wider part as a funnel. The funnel rim must be exactly as wide as the cylinder, so the cut must be made on the straight part.
Weigh down the base
Take out the funnel and put the pebbles into the body. Pour in water up to the level where the bottom of the bottle becomes a flat floor with straight sides. Mark that level with a pen. This is the 0 mark.
Tip: A bottle with a rough or petal-shaped bottom has a curved floor. The pebbles and the water level off the floor, and the 0 mark starts where the sides become straight.
Mark the scale
Stick a strip of tape up the side of the bottle, with the bottom of the strip at the 0 mark. With a ruler, mark a line every 5 mm and write the numbers 10, 20, 30 and so on at every 10 mm. Cover it with clear tape to protect it from water.
Common mistake: Starting the ruler at the bottom of the bottle instead of the 0 mark. The readings will then be too high.
Check the scale (optional)
Measure the inside width D of the bottle in millimetres. The area is about 0.785 × D × D mm², and 1 mm of water needs that many cubic millimetres. For D = 70 mm, one millimetre is about 3.85 mL. Pour that into the bottle and check that the level rises by 1 mm.
Tip: 1 mL is 1000 mm³, so volume in mL = 0.785 × D × D ÷ 1000 for each millimetre of depth. Write the value on the strip.
Make the arrow head and tail
Cut a small triangle from the card about 3 cm tall and 3 cm long for the head. Cut a tail about 6 cm tall and 5 cm long. Tape the head to one end of the straw and the tail to the other end, with the card standing upright (vertical).
Common mistake: Making the head and tail the same size. The tail must be bigger than the head, or the vane will not swing round to face the wind.
Find the balance point
Lay the straw across your finger and slide it until it balances level. Mark that point. Push the pin through the straw there and then into the top of the pencil’s rubber (eraser).
Tip: If one end drops, tape a small piece of paper clip or clay to the other end until it balances.
Check the spin
Blow gently on the tail. The straw should swing freely. If it sticks, widen the hole in the straw by moving the pin round, and slip a small piece of cut straw or a bead between the straw and the pencil as a washer.
Common mistake: Pushing the pin too tightly in the straw. The straw should be loose enough to turn.
Make the base and mark the directions
Fix the pencil upright in a cup full of damp sand or plasticine. Cut a square of card and mark N, E, S and W on its edges (north at the top). Place the card under the vane and use the compass to turn it so that the N points to true north.
Tip: Hold the compass away from metal objects, because iron and steel pull the needle off.
Place the instruments
Put the rain gauge and the wind vane in an open place, such as a garden, a school ground or a balcony floor, away from walls, trees and roofs. Put the rain gauge on a firm stand about 30 cm above the ground to avoid splashes and tie it in a bucket or a stake so that it does not fall.
Common mistake: Putting the gauge under a tree or an overhang. Rain will be blocked or will drip into it.
Record every day
At the same time each day (for example 8 a.m.), read the water level in the gauge to the nearest mm, note the direction the vane head points to, describe the wind strength and the sky, and write them in the table. Then empty the gauge and turn it upside down if no rain is expected.
Tip: Always read at eye level with the water surface level. Read the bottom of the curve.
Analyse and present
After 14 days, add up the rain and count the rainy days. Draw a bar chart of daily rain and a tally of wind directions. Look for a pattern: did it rain when the wind came from a particular side? Compare with the weather forecast.
Tip: A “rainy day” is usually a day with at least 2.5 mm. State the rule you use in your report.
How it should look when it is finished
A clear plastic bottle with an upside-down funnel in its top, standing in the open with a strip of millimetre marks up its side, and next to it a pencil in a cup of sand with a straw balanced on a pin. The straw has a small pointed card at one end and a tall tail at the other, and it swings to point into the breeze, over a card marked N, E, S and W.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
The funnel gathers the rain that falls on its opening and sends it into the cylinder. Because the opening and the cylinder have the same width, the depth of the water collected is the same as the depth of the rain that fell: 1 mm of water in the bottle means 1 mm of rain, which is 1 litre on every square metre of ground.
The wind vane is a pivoted pointer. The wind pushes sideways on both the head and the tail. The tail has the larger area, so it gets the greater push and swings downwind, and the head is turned to face the wind. As the wind changes, the vane swings round and points to the new direction. The card marked N, E, S and W lets you read it.
Wind itself is moving air that flows from higher pressure to lower pressure. The differences in pressure come from the heating of land and sea by the Sun, so winds change with the season and the time of day.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The gauge has no water even though it rained | It was under a tree or a roof, or the funnel was blocked, or someone emptied it. | Move it to an open place, check the funnel is clear, and keep a note of when you empty it. |
| The reading is much lower than the weather report | Wind blew rain past the gauge or the cut was not level. | Place it in a less windy spot and cut the bottle level. Small differences between places are normal. |
| Water rises past the 0 mark in dry weather | Dew, or a leak at the funnel joint. | Check the tape and the funnel fit. Do not count dew as rain. |
| The vane does not move | The pin is too tight, or the straw is rubbing the pencil. | Loosen the pin, add a bead or a straw washer, and balance the straw. |
| The vane points the wrong way (the tail points into the wind) | The head is larger than the tail, or the vane is not balanced. | Make the tail bigger than the head and rebalance it. |
| The vane swings without settling | The wind is gusty and the vane is very light. | This is normal in gusts. Note the direction it points to most of the time. |
Test and record
Take readings at the same time each day. Rain in mm is the depth of water in the gauge, read at eye level. The wind direction is the direction the vane head points to (where the wind comes from). Strength: calm, light, moderate or strong.
| Date | Time | Rain (mm) | Wind from (N, NE, E ...) | Wind strength | Sky (clear, cloudy ...) |
|---|---|---|---|---|---|
| Day 1 | |||||
| Day 2 | |||||
| Day 3 | |||||
| Day 4 | |||||
| Day 5 | |||||
| Day 6 | |||||
| Day 7 |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- In what unit is rainfall measured and what does it mean?
In millimetres. 1 mm of rainfall means that the water would be 1 mm deep over level ground, which is 1 litre per square metre.
- Why must the funnel rim be as wide as the cylinder?
So that the area that collects rain equals the area of the floor on which it collects. Then the depth of water equals the depth of rain.
- What happens if the funnel is wider than the cylinder?
The water is deeper than the rain by the ratio of the two areas, which makes light rain easier to read. You then divide the reading by that ratio.
- Why does the wind vane point into the wind?
The wind pushes harder on the large tail than on the small head, so the tail is blown downwind and the head turns to face the wind.
- If the vane head points to the north-west, from where is the wind blowing?
From the north-west. Winds are named by the direction they come from.
- What causes wind?
Differences in air pressure caused by uneven heating of the Earth’s surface. Air moves from high pressure to low pressure.
- Why should you empty the gauge after reading?
So that you measure each day’s rain separately, and so that standing water does not breed mosquitoes.
- Why is a record over several weeks more useful than one day’s readings?
One reading is only a snapshot. A longer record shows patterns, such as how often it rains and which wind brings rain, and lets you compare with the forecast.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a rain gauge and a wind vane and to record the rainfall and wind direction at my home for two weeks.
- Apparatus
- A plastic bottle, pebbles, tape, ruler, a straw, a pin, a pencil, a card, a cup of sand, and a compass.
- Procedure
- Describe how you made the funnel and the scale, how you balanced and mounted the vane, where you placed them, and what time you took readings each day. Include the diagrams.
- Observation
- The daily table of rainfall, wind direction, wind strength and sky, with a bar chart of rainfall and a tally of wind directions.
- Result
- Total rainfall in 14 days was ... mm over ... rainy days. The wind blew most often from ... State any link between wind direction and rain.
- Conclusion
- A funnel and a straight cylinder measure rainfall as a depth, and a vane with a larger tail shows the direction the wind blows from. The record shows the pattern of the weather at my home over two weeks.
Take it further
- Make a cup anemometer: four paper cups on a cross of straws, turning on a pencil. Count the turns in 30 seconds on different days.
- Add a thermometer in the shade and a simple barometer from a balloon and jar, and record temperature and pressure.
- Measure the rainfall with a funnel that is twice as wide as the cylinder, and show that the depth is four times as much.
- Compare your readings with those published for your city and explain any differences.
- Study the dew: leave a cold bottle in the open in the morning and record when water drops appear.
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