# Homework Help: Very interesting estimation problem

1. Sep 19, 2006

### rocketboy

"Annie and Evelyn observe that, on average, it takes approximately 10 days for a small glass of water to completely evaporate when left outdoors. What would they estimate for the average annual precipitation on Earth?" (Answer in depth of precipitation)

Here's my though process but I have a feeling it is a bit off, if you could help me out it would be greatly appreciated! Thanks!

Ok, so first I assumed that a small glass holds 250mL of water and is 10cm tall, and that all water that evaporates in a year falls in that same year, so the quantity evaporated = quantity precipitated.

So the Volume of the glass = 250mL = 250cm^3

Thus 250cm^3 = Surface area of the glass x 10(cm)

Thus the surface area of the glass is 25cm^2

So it takes 10 days for 250mL of water to evaporate from an area of 25cm^2, and thus in 1 day, 25mL of water will evaporate from that surface area.

Then i found the surface area of the earth, and took 71% of that (the part of earch covered in water) and found the annual evaporation of water. Then I spread that out over the entire earth's surface area to find the average annual precipitation in depth.

2. Sep 19, 2006

### Cyrus

It seems to me whatever is in the cup would be the annual precepitation.

Every 10 days it will empty the cup, then rain it back down and fill up the cup. Over and over again in a cyclic pattern.

That's a very open ended question though.

Last edited: Sep 19, 2006
3. Sep 20, 2006

### Cyrus

on second glance, your work looks good to me.