Making a simple rain gauge:
All we need is
- a tin can or a plastic bottle with a flat base,
- a funnel with the same diameter as the can,
- some paper clips or tape,
- some slips of card
- and a ruler marked with mm.
If you don’t have a funnel you can make a simple one by cutting off the top of a plastic bottle. It is important that your makeshift funnel and the can have the same diameter at the top, so you know over what surface area you are measuring your rainfall.
I think it is also important that the container has a flat base, however, many designs use plastic bottles with rounded bases. If you think your child will not mind whether the measurements are precise or not, do the simpler designs with a plastic bottle and don’t worry too much about the shape of the base.
Invert the funnel into the can. Clip or tape it on.
Place the rain gauge somewhere in the open, away from overhanging buildings or vegetation, and anchor it somehow e.g. by fixing it onto a post or by sinking it in a bucket of sand, or using a larger container with rocks in.
Choose a time of day when it will be easy to make your measurements, i.e. you will be awake and you are fairly sure you will be at home most days.
Make a prediction – how much rain do you think you will get in your gauge each day next week? If you like, you can also check a weather report each day and see what they predict. (Try to use the same website or news channel so that you are consistent.)
Making your measurements
Try to do this every day at the same time.
If you find leaves or other ‘foreign’ material in your rain gauge, make a note as this might have affected the measurement for that day.
Dip your card into the container so that it reaches the bottom and the water soaks into the card.
Take the card out and mark a line where the water stopped, then use your ruler to measure the height of the water level in mm.
Tip all the water out to reset your gauge for the next day.
If you miss a day, leave the water in and just make a measurement the next day. Then you can divide by two to give an average for the two days.
If you want, you can keep the pieces of card, write the date and time at the top and draw a line to show how deep the water was. Then you can arrange them as a ready made bar graph.
Or you can use graph paper to make a rainfall chart.
To think about:
(1) Why did we use a funnel? What difference might it make to the measurements if we didn’t have a funnel? (If you really want, you can make two rain gauges, one with a funnel and one without, and compare the measurements from each of them.)
(2) Why is it important to put the rain gauge away from buildings or vegetation? What difference would this make to your measurements?
(3) How much rain was there in your gauge each day? Did this match your predictions? Did it match the weather report?
(4) What do you think would be the measurement on a day when it had been raining for most of the day? What about a day when it rained for a little in the morning and then was sunny for the rest of the day?
(5) Did you have any practical problems with the rain gauge, and with taking measurements? How might you be able to fix these?
Many simple rain gauges for children just say to cut up a plastic bottle and invert the top part into the bottom. This is good because you are able to see the water in your container, but the measurements then become very imprecise. I like this version because the sand in the bottom both helps to anchor the gauge and also gives a flat base for measuring a precise depth. However I am not sure about measuring the precipitation every day by tipping it out. It seems to me that it would become quite inaccurate because you would have to refill it with just the right amount of sand and water each time.
I like the diagrams on the first page of these instructables but you really don’t need the rest of the pages. If you make sure the container has a flat bottom you don’t need to do any conversions between volume and height. (This is, however, the way commercial rain gauges work.)