Rain Gauge

Understanding how Rain Gauge works

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

What the rainfall measure is telling us is the height of rain water collected in a vessel that has vertical walls and has uniform cross section from 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 rain water 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 leafs etc. can fall into it.

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

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

Then:

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

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

Download Microsoft Exel file for recording your rain fall measurement. Right click on d ownload and Save target as .. in appropriate directory. You will have to replace the value of 'dc' as per the diameter of the funnel used by you.

Trap:
If we do not convert the units properly, we get very absurd results.

Volume is measured in milliliter and the rain fall is given in millimeter.

1 milliliter (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 value of h in cm as (measured volume) * cm divided by the constant value.
Multiplying this by 10 gets us value in millimeter or mm

An example
We used a funnel whose diameter is 4.5 inch = 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

The error factor:
Suppose in the above example we made a mistake in measuring volume by 1 ml. In our case where diameter of the funnel is 11.43 cm, 1 ml rain water collection would give h = 0.1 mm.

In other words if we make an error of +/- 1 ml in noting the volume of collected rain water our percentage error in reporting rain fall would be +/- 0.4 percent.

On 1st of Juy 2007 we measured volume to be 367 ml. This gives us 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 of a 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 rain fall we can directly divide the volume by 100 and we will not be too off the correct reading.)

Tips:
Rain showers can be quite windy at times and you'll want to fasten your rain gauge somewhere so that it doesn't thrown off by 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.

For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .01 inches or .02 mm may be recorded as a trace.

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 gauge must be placed clear of obstacles so that raindrops do not splash into, or out of, the collecting funnel.

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

Direct Measure of rain fall level: If we can somehow prevent evaporation of rain water then we can stick a scale to the vessel and directly measure the height 'h' giving the rain fall. For this, one could use a transparent glass vessel that has a uniform cross section from bottom to top. One can the stick a plastic scale on it or xerox a scale on transparent plastic sheet (used for making overhead transparency). But as you can see that you can estimate the height of the collected wher to say about half a millimeter. And, therefore, measuring water volume is worth all the trouble.

Make your own rain gauge: Click on the DIY links in the index column. Here we show you how to make some simple rain gauges that can give you reasonably accurate rain fall readings. If you have some other ideas do let us know.

Please note:
Some of the observations taken may not be as good as those taken by professional observatories but remember the aim here is to learn to do scientific experiments and take observations, as accurately as possible.

Send us your design (s) - we would be happy to share it with others