Make a Rain Gauge

In this instructable, I discuss a simple rain gauge that is essentially made from trash.

This is a wonderful project for any group of people. We, possibly, cannot claim that this would be a substitute for a professional instrument but as you will see that it is quite accurate. And at places where there are no weather instruments this will give some reasonable indication of rainfall. 

Having said this instrument has lots of educational value. Children learn to be disciplined observers, take readings and learn about the rainfall at their own location. 

Step 1: Understanding How Rain Gauge Works

A rain gauge is an instrument to measure rainfall, which is measured in millimetres. Occasionally the rainfall is also reported in centimetres or in inches.

What the rainfall measure is telling us is the height of rainwater collected in a vessel that has vertical walls and a uniform cross-section from the bottom to the top.

Therefore, in principle, any vessel that has a uniform cross-section can be used as a rain gauge. All the vessels will collect rainwater to the same height 'h' as shown below. For example, you can keep an aquarium in open with a scale pasted to it as a rain gauge. However, from an open rain gauge collected water will evaporate quickly when the rain stops affecting the final reading. Also, a lot of dirt, like leaves etc. can fall into it.

The rainfall can be measured in different ways. As said above one can simply measure the height 'h' of collected water in a vessel of a uniform cross-section. One can measure the volume of collected water and convert this value into rainfall. More sophisticated rain gauges have electronic devices to automatically record rainfall.

Step 2: Volume Measure Method

If we collect rainwater by placing a funnel over some vessel then volume V of rainwater collected in that vessel would be the volume of a cylinder whose radius is r, height is h,

Then   V= pi r² h  ........ (1)

or       h = V / pi r² ....... (2)

(see the next image)

we can easily measure the diameter of the funnel mouth and get the radius by dividing it by 2. This value is not going to change, the value of pi is also constant (you may take it as 3.142). The denominator in the equation is, therefore, a constant. Let us call it dividing constant 'dc'

Therefore, all we have to do is to collect rainwater using a funnel, measure the volume of water collected in it and then divide by the constant above to get the rainfall measure.

Step 3: Trap

We must remember to convert the units properly, or else we will get very absurd results.

Volume is measured in millilitres and rainfall is given in millimetres.

1 millilitre (or 1 ml) = 1 cm³ = 1* cm * cm * cm,

this is to be divided by (r cm)² = r * r * cm* cm

we then have the value of h in cm as (measured volume) * cm divided by the constant value.

Multiplying this by 10 gets us a value in millimetres or mm

An example

We used a funnel whose diameter is 4.5 inches = 4.5*2.54 cm = 11.43 cm

or radius r = 5.715 cm, giving the value of dividing constant as 102.608 cm

Now if the volume measured is 245 ml

then: h= (243/102.608)cm = 2.368 cm = 23.68 mm or rounding it to 23.7 mm

Step 4: Error Factor

Suppose in the example given in the last step we made a mistake in measuring volume by 1 ml. In our case where the diameter of the funnel is 11.43 cm, 1 ml rainwater collection would give h = 0.1 mm.

In other words, if we make an error of +/- 1 ml in noting the volume of collected rainwater our percentage error in reporting rainfall would be +/- 0.4 per cent.

On the 1st of July 2007, we measured the volume to be 367 ml. This gives us the value of 'h' to be equal to 25.7672 or rounding it to 35.8 mm. Now suppose we read the volume to be 366 or 368 then we would get 'h' to be equal to 35.6697 (~ 35.7) or 35.8646 (~35.9).

(If you are a bit mathematically minded you would notice that in the example above the dividing constant is very nearly equal to 100, which tells us that for light rainfall we can directly divide the volume by 100 and we will not be too off the correct reading.)

Step 5: Placement of the Rain Gauge

One must keep in mind that the rain gauges, like most meteorological instruments, should be placed far enough away from structures and trees to ensure that any effects caused are minimized. Rain gauges must be placed clear of obstacles so that raindrops do not splash into, or out of, the collecting funnel.

On the ground, one should have at least 10-meter clearance from all sides. This could be a kind of luxury for schools in metropolitan cities. One may then place the rain gauge on top of a building then it might be sufficient to see that it is about 50 cm above the ground or rather terrace level and about 2 m away from nearby railing walls.

Rain showers can be quite windy at times and you'll want to fasten your rain gauge somewhere so that it doesn't fall off by the wind. Locate a good place for your gauge. There should be nothing overhead, like trees, electric wires, or the edge of a roof. These obstructions can direct rainwater into or away from your gauge, creating a false reading. The edge of a fence, away from the building, is often a good place for your gauge.

In the next two steps we will see two models of the rain gauges.

Step 6: 100 CD/DVD Cake Box Rain Gauge

Material needed:

The basic material needed is a funnel and a measuring cylinder. A good quality plastic funnel can be purchased from a grocery shop. We would require a measuring cylinder that can read at least 1 millilitre of volume. This can be purchased from a laboratory supplier or the science teacher of a nearby school should be able to tell you where to buy one. For the rest of the material, you may follow the instructions below and/or improvise on it.

For our rain gauge, we used 100 CD/DVD cake boxes with their spindle removed. On the top of the cake box cover we cut a hole and inserted the neck of the funnel through it and glued it water resistant glue such as M-seal. A vessel to collect the rainwater is placed on the base of the cake box. The cover can be locked to its black base.

This unit turned out to be quite light and could easily fly away with moderately strong wind. We found a 40 x 40 cm and 18 mm thick waterproof wooden board in our carpentry workshop. So we screwed the base of the CD cake box to the board. Underneath this board, we also fixed four caps of mineral or soft drink water bottles. One of the bottles could be cut to fit inside the cake box to serve as a vessel for collecting rainwater.

We simplified the above design by removing the wooden board and putting lead weights and using a pet bottle as a vessel for collecting the rainwater. We also found a tall stool to place this rain gauge.

Step 7: Pet Bottle Design

This is an interesting design for collecting rainwater. In this design, we have used one small bottle (1 litre) and a thermal cover bag for it. This is kind of cover is available in the stores where material for school-going children is sold. In this design, we have used a discarded bag. This bag (with the bottle inside it) can slide into a 2-litre pet bottle with its bottom part cut. We then drilled holes in the cap of the small bottle to fit in the funnel. We sealed the funnel to the cap using M-seal glue.

We found a long (waterproof) wooden plank (one can use a rod or similar material). The plank was about 8 mm thick, 4 inches (10 cm) wide and about 5 feet (1.5 m) long (tall). We used the other 2-litre pet bottle and cut off its bottom part and fixed it upside down to this plank at one end. To do this we made two matching holes in the plank and the bottle and tied the bottle using nylon rope. Then the first bottle is slid into this bottle.

Our rain gauge is now ready. We then placed it at some convenient place. The rainwater collecting bottle easily comes out of the bigger bottle. We can now uncap the funnel and either collect water in a baker and measure the volume of the water collected or measure it directly.

Step 8: A Suggestion:

If you do not have a good measuring cylinder at hand you can use two identical small bottles. A funnel is attached to one cap. And the other cap is used on the second bottle.

Students can use one bottle for collecting the rainwater and other transporting. 

This idea can be used for collective classroom exercises. Students can be asked to set up their own rain gauges at home and bring the collected rainwater to the school for measuring volume.