Building and reading a simple rain gauge

The calculation is simple: one square metre of area times one millimetre of height makes exactly one litre. So if 5 mm of rain falls, 5 litres of water have landed on every square metre. That applies to any area, however large – including the small opening of your cup.

For this to be right, your container needs vertical walls. Then the opening at the top is the same size as the base, and the water stands as high as it would on the ground outside. A glass that widens towards the top, or a bottle with a curved base, distorts the result.

If your container has a funnel whose opening is wider than the vessel below, it collects rain from a larger area. Then you must convert: rainfall equals the collected volume of water divided by the area of the funnel opening. An example: a round opening 10 cm across has an area of just over 78 cm². So one millimetre of rain brings just under 8 millilitres of water into it.

A smooth, cylindrical plastic bottle works well. Cut off the curved top. The base of many bottles is domed or has little feet, so fill it with some sand, gravel or plaster up to the point where the wall becomes straight. That level is your zero point.

Tape a ruler to the outside of the bottle from the zero point, or draw millimetre marks with a waterproof pen. You can put the cut-off top back in upside down as a funnel. Because it is just as wide as the bottle, you do not need to convert, and the funnel reduces evaporation.

So that the gauge does not fall over, bury it partly or fix it to a post. It must stand upright, and the opening should be level, otherwise it catches too much or too little depending on the wind direction.

The location determines the accuracy. Set the gauge up outdoors, well away from buildings, trees and fences. Obstacles shield rain or channel extra drops in – under a tree you measure too little, under the edge of a roof far too much.

Wind also plays a part. In a very windy, open spot, drops are blown over the opening and the gauge catches less than actually falls. A slightly sheltered spot in an open garden is therefore often better than the highest, windiest place.

The World Meteorological Organization describes in its guidelines how official rain gauges are built and sited, and stresses the distance from obstacles. Even so, a homemade gauge only gives approximate values and is not directly comparable with official measurements.

Read the gauge at the same time every day, for example in the morning. Bring your eyes level with the water surface, otherwise you read at an angle and get it wrong. The surface often curves up slightly at the edge; read it in the middle. Write down the value with the date and then empty the container.

Do not leave the water standing for days. In warm weather it evaporates and you measure too little. Mosquitoes can also lay eggs in standing water – another reason to empty it regularly.

To measure snow, let it melt in the container indoors and then read the water depth. You will be surprised: the meltwater stands much lower than the snow cover outside, because snow contains a lot of air.

For rainwater use you can roughly estimate how much water comes off the roof: rainfall in millimetres times roof area in square metres gives approximately the litres that can run off. In practice less reaches the water butt, because water evaporates, splashes or runs past the gutter. Collected rainwater is utility water – to drink it, it would first have to be treated and disinfected.

In the garden, the rain gauge helps with watering: you can see whether it really rained enough or only drizzled briefly. On trips or outings, a series of readings can show how wet paths and ground are after several rainy days.

What it cannot do: warn you. A homemade rain gauge is not a flood or severe weather forecast. In heavy rain, pay attention to official warnings, warning apps and the radio and follow the authorities' advice. As a project with children, on the other hand, the rain gauge is wonderful: it makes weather measurable and shows how science works.