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Rain Gauge

Wednesday, August 3, 2010

“How much rain fell last night? “, asked my wife, the gardener.

Whether you are a gardener and want to know if enough rain fell to water the garden or if you are just curious about how much rain you got, a rain gauge is a simple instrument to make.

Rain gauges are made of two parts – the entrance area – the area where the rain enters, and the collection tube – a straight-sided tube with a scale, where the rain ends up and where you measure it.
In the simplest type of rain gauge, the entrance area is the same diameter as the collection tube. In this type of rain gauge, one inch of rain falling through the entrance would measure one inch in height in the collection tube. Therefore, if a can is set outside in a rainstorm, 3 inches of water falling through the opening of the can will fill the can to a depth of three inches. Said another way, three inches of water in the can equates to 3 inches of rainfall.

While this type of rain gauge works perfectly well, a rain gauge with a higher degree of accuracy can be made where the entrance area is wider than the collection tube. In this case, the entrance area is usually a funnel that opens into the collection tube. Imagine 3 inches of rain falling through the entrance area- the funnel. Since this amount of rain must fit in a narrower collection tube, it will fill the tube to a greater depth than three inches. this will make our measurement more accurate.
Of course, to make this type of rain gauge, you must know how to convert the amount of water entering the entrance area – or the funnel – to the height in the collection tube.

Important to remember: We are measuring the area in square inches of water, not in linear inches. So when you do your calculations, remember to convert the diameter of your funnel and collection tube to area.
The formula for this is:

Or 3.14 x radius x radius

Remember that the radius is one half the diameter or opening.

Materials

• Straight-sided jar like an olive jar for your collection tube
• 2-liter soda bottle - cut the top off the soda bottle. The top will be your funnel and the bottom will house your rain gauge. (a one liter soda bottle will work just as well)
• plastic container lid
• graph paper
• clear tape
• pebbles

Directions:

 

 

 

 

 

 

 

Making the scale:

1. Calculate the area of the opening of the funnel.
2. Measure the diameter of the funnel.
3. Divide the diameter in half to get the radius.
4. Calculate the area by the formula
   area = 3.14 x radius x radius
5. Calculate the area of the opening of the collection tube

Plug your figures into this equation:

In my rain gauge, I have:
funnel opening of 4.25” for an area of 14.18 square inches
collection tube opening of 1.5” for an area of 1.77 square inches

So, following the equation above:

1.77 square inches/14.18 squate inches = 0.12 inches of rain at 1” height

Using the graph paper, make a scale, indicating the number of inches of rain for every inch of the scale. For instance, my scale will have 0.12 inches marked off at 1” height of the scale, 0.24 inches of rain at the two inch height and so on.

 

 

 

 

 

 

 

Assembling the rain gauge

1. The plastic container lid that should fit over the opening of the bottom of the soda bottle. Cut a hole in the lid so the funnel fits through the hole.
   The plastic container lid is to prevent rainwater from entering the soda bottle from the sides of the funnel.
   
   
   
   
   
   
   
2. Cover both sides of your scale in clear tape. This will protect it if it gets wet.
   
   
3. Tape your scale onto your collection jar. Make sure the bottom of your scale is at the bottom of your jar.
   
   
4. Place the collection jar inside the bottom part of the soda bottle. Place a few pebbles in the bottom of the soda bottle. This will prevent your rain gauge from blowing over in the wind. Make sure no pebbles get inside the collecting tube.
   
   
5. Place the plastic container lid over the top of the soda bottle bottom and place the funnel through the hole in the lid so rain water will run from the funnel into the collection jar.

Sit back and wait for the rain.

As you can see from the picture of my collection tube, my rain gauge will measure 0.66 inches of rain. So what happens if it rains more than that amount?
Any amount of rain above 0.66 inches will overflow the collection tube and flow into the soda bottle. To measure my total rainfall, I empty my collection tube and measure the amount of rain in the soda bottle using the collection tube. Adding the two measurements (or more if needed) I get the total rainfall.