In reply to SueK, who commented that she could not fathom 20″ of rain– what’s forecast for NOLA.

20″ is indeed a lot of rain. We could get up to 10″. Here’s something I once had my students do– a very long time ago– when teaching both metric system, and some environmental biology.

Take some paved outdoor space of reasonable dimension– driveway, small parking lot. Measure length & width in feet or yards. Set aside.

1 inch of rain is a volume of water– 1 cubic inch.

1 inch = 2.54 cm. 2.54 * 2.54 * 2.54 = 16.39 cubic cm. Round down (so as not to over inflate the eventual stat) and once inch of rain is 16.3 cubic centimeters of rain.

By definition, 1 cubic centimeter of water = 1 ml (and has a mass of 1 gram but that’s irrelevant here) [and I can’t remember but there’s a pressure component to this too]

1″ rain = 16.3ml.

Convert your measurement of the driveway to cm. Let’s say 10′ x 20′ … 304.8cm & 609.6cm, respectively.

Now, drop an inch of water on that surface.

304.8 * 609.6 * 16.3 = 3,028,639.1 cubic centimeters or ml of water.

How many liters is 3 MILLION milliliters? 3028.6391 call it 3028.

Divide by 2 b/c everyone knows how much is in a 2-liter bottle.

One inch of rain falling on a paved surface measuring 10′ x 20′ is more than 1500 2-liter bottles of water. It can’t go down, so it runs off.