Heavy, Heavy Hangs Over Your Head – Weighing a Thunderstorm

1. Heavy, Heavy Hangs Over Your Head – Weighing a Thunderstorm

2. You can look at the “weight” of a thunderstorm in various ways. We’ll explore two of them: • The weight of water falling as rain over a parcel of land of a given size. • The weight of water suspended in a thunderstorm cloud.

3. Suppose we have a parcel of land that is one square mile in area… To make life easier, we’ll suppose our parcel is essentially flat and non-porous.

4. Let’s further suppose that enough rain has fallen on our parcel to cover it uniformly to a depth of 1 inch. So, how much does all that water weigh? (For this example we’ll use English units of measurement rather than metric.)

5. |--------------- 5,280 feet ---------------------------------| 27,878,400 sq. ft. in 1 sq. mile |----------------------- 5,280 feet ----------------| |--------

6. 1 square foot = 12 x 12 inches = 144 square inches

7. 1 sq. mile So… 1 square mile = 5,280 feet X 5,280 feet = 27,878,400 square feet 1 square foot = 12 inches X 12 inches = 144 square inches 27,878,400 square feet X 144 square inches = 4,014,489,600 square inches in 1 square mile (that’s 4 billion + square inches!)

8. |----- 1 inch ---| So, if enough rain were to fall such that all 4,014,489,600 square inches of land surface were uniformly covered to a depth of 1 inch, that would be 4,014,489,600 cubic inches of water. How much do you suppose all that water would weigh???

9. Let’s look at it in a slightly different way… A cubic foot is 12 X 12 X 12 = 1,728 cubic inches, so the water standing on our parcel represents…. 4,014,489,600 cu. in. / sq. mile divided by 1,728 cu. in. / cu. ft. = 2,323,200 cubic feet of water per square mile

10. 1 cubic foot of water weighs 62.31 pounds approximately – depending on temperature and any contaminants, so…

11. 2,323,200 cubic feet of water times 62.31 pounds per cubic foot equals a whopping 144,758,592 pounds!!! That’s equivalent to about 1,800 fully loaded 18-wheelers – enough to stretch for just over 22 miles along I-81! Clearly, there is A LOT of water in a thunderstorm!!!

12. As an aside, 2,323,200 cubic feet of water is equivalent to 17,377,536 gallons – there are 7.48 gallons in a cubic foot – so the amount of water covering a square mile to a depth of 1 inch would be enough to fill 68.65 Olympic sized swimming pools! (An Olympic sized pool holds about 253,125 gallons.)

13. Okay, then, let’s look at this another way – from the storm cloud’s point of view!

14. This time, we’ll use metric measurements. Let’s assume that our storm cloud is pretty average—in fact, fairly small as storm clouds go. Obviously, a real cloud is irregularly shaped, but we’re going to think of our cloud as an oblong box. Our cloud will cover an area of 1.6 km2 (This is a square mile, in case you were wondering—but I won’t use that term again!) So how tall is our cloud? Some storm clouds can reach incredible heights, but let’s assume that our cloud is only 10,000 meters tall.

15. |---- 1600 m ----| So we can think of our cloud as a box with a volume of 1,600 meters X 1,600 meters X 10,000 meters. That works out to a total of 25,600,000,000 cubic meters! Scientists, of course, would prefer to state that using “scientific notation”. Thus it would become 2.56x1010 m3 Either way, we’re looking at over 25 billion cubic meters of volume. |---- 1600 m ----| |------------------- 10,000 m -------------------|

16. So how much water can that big a cloud hold? The answer depends upon the density of the water vapor that makes up the cloud. An “average” storm cloud may have a density of about 10 grams per cubic meter (10 g/m3) so….

17. 2.56x1010 m3 times 10 g/m3 equals 2.56x1011 grams of water in a thunderhead. There are 1,000 grams in a kilogram, so there are 256,000,000 kilograms of water in our storm cloud. A kilogram weighs about 2.2 pounds, so the weight of our storm cloud in pounds is about 563,200,000 pounds!

18. By comparison, a fully loaded 747 weighs about 875,000 pounds. Thus, it would take 644 fully loaded 747s to equal the weight of a small thunderstorm! And all that water just floats around up there….!

19. The End