Why do experiments usually test one variable at a time?

Because while testing a variable by itself you can see it's soul effect. But if added with other variables it's effect is different then as it started, or different from what it is meant to be, Making it impossible to see it's true effect.

So they will not get the results of each variable mixed up and to compare and contrast which variable will work better.

Advection fog occurs when warm air blows over a cold surface like snow and since tropical water is warm advection fog rarely forms over it