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How Much Rain?

This week we were in Memphis, to help celebrate my grandmother's 104th birthday. My sister was in from California, and my brother was in town, and it was great fun visiting. I also went to lunch with some wonderful friends from the Memphis Pink Palace Museum. These are some of the folks that taught me how to teach science, and it is always a joy to visit with them.

For this experiment, we will measure some rain, and we will start by making a rain gauge.

For that, you will need:

- a container like a drinking glass with straight sides
- a ruler

Be sure that the container has sides that go straight up and down, with no slope. You want the bottom to be the same diameter as the top. I used a glass container for candles, and it worked very well. Place your container where rain can hit it. Do not put it near trees, buildings, or anything that would block some of the rain.

After a storm, use the ruler to measure how much rain is in the container. The depth of the water is how many inches (or centimeters) of rain fell on your area.

Now, imagine that your rain gauge shows one inch of rain. Just how much rain is that? One inch, right? Right, but what does that actually mean? It means that enough rain fell so that if all the water stayed, without running off or soaking in, the water would be one inch deep.

Lets think about how much water that really is. If your lawn is 75 feet deep and 100 feet wide, how much water fell on your lawn? 75 X 100 = 7500 square feet of lawn. Each square foot has 144 square inches (12 inches X 12 inches). 7500 square feet times 144 square inches tells us that you have 1,080,000 square inches in your lawn. If you got one inch of rain, then each square inch would have one inch of water, making it a cubic inch of water.

OK, so your lawn got 1,080,000 cubic inches of rain. There are 231 cubic inches in a gallon, so your lawn got 4675.3 gallons of rain! Each gallon of water weighs 8.34 pounds, which tells us that 38,992 pounds of rain fell on your lawn! That is 19.4 tons of water on your lawn.

You can try the same thing with a larger area. A square mile has 4,014,489,600 square inches. Divide that by 231 to get the number of gallons, which would be 17,378,742 gallons of water if you had one inch of rain. Multiply that by 8.34 and you find that 144,938,715 pounds of rain fell on that square mile.

OK, once you wrap your brain around that number, think about where all that water came from. The clouds! An average, fluffy white cloud can easily have more than 550 tons of water in it. Wait a minute! If clouds are that heavy, why don't they fall? The water is in very tiny droplets, small enough to be easily suspended by air currents. Each drop is very tiny, but when you add them all up, you get a tremendous amount of water. If you really want to stretch your mind, look at the size of a hurricane, and the amount of rain that it brings. Do a little math and you will get numbers that may be very hard to believe.

Have a wonder-filled week.