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If you'd like to keep track of how much rainfall you're getting, you can buy a rain gauge, or you can make one out of inexpensive items from your local hardware store. These instructions discuss how to put one together and calculate exactly how much rain has fallen. It's not only easier on the wallet, but it's also a great science project and learning experience for kids.
Steps
 Assemble the gauge by putting the funnel securely onto a container that's large enough to collect the amount of rain you would normally get in your area without overflowing.
 Work out the surface area of the funnel you will be using.
 Measure the widest inside diameter of the funnel, divide that by 2.
 Square that number (multiply it by itself) then multiply by 3.142 to get the area of the top of the funnel.
 Divide the number you came up with by 10. This gives you the factor to multiply by when you are working out how much rain has fallen in millimetres.
 Using a measuring jug or any other container calibrated in millilitres (ml) measure the amount of rainwater you have collected then divide that number you arrived at in step 4. This gives you the amount of rain in millimetres that has fallen.
Tips
 If you use a taller thinner collecting vessel for the gauge itself then you could calibrate it so that it could be read directly without need for the calculation every time you take a reading.
 The collecting funnel could be connected to the collecting vessel via a tube so that the collecting vessel could be shielded from the sun to reduce evaporation.
Warnings
 Place in an open space where there are no overhanging plants, trees, hedges, buildings etc.
Things You'll Need
 A funnel of around 100mm diameter at its widest
 A container to collect the rainwater that the funnel will securely sit in.
 A measuring jug or tube calibrated in millilitres (ml)
 Rain!
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