>I'm standing outside. I would have to guess at the height of the clouds, but is there a basic calculation to tell me how far away, by land, that cloud is? Is it simply a2 + b2 = c2? Not counting for the curvature of the earth? I think kind of yes, but since I'm only guessing at the height of the cloud (a2), and I don't know the distance on land from where I'm standing to directly underneath the cloud (b2) and I sure don't know the distance I am fro the cloud directly (c2) then I must have to use angles. This all came about because a little girl asked me how far away a cloud was. What was it over?

Pythagoras doesn't work, since you need two sides to calculate the third (in addition to making sure that you have a right angle). You have to do trigonometry, as explained in some of the other replies.