the maximum viewing distance on a clear day is related to how high you are above the surface of the earth. this relationship can be approximated by the formula h=0.075d^2, where d is the max distance, in kilometers, and h is your height, in meters, above the ground.

Question

how high do you need to be in order to see a distance of 25km?

Answer 1

just plug in your numbers
h = 0.075 * 25^2