Our daughter Nynke (7 years old) suddenly asked while cycling, and it was raining hard: “Daddy, why does the rain always fall at an angle?”.
I replied that this is because of the wind, but she said “but there is no wind now, papa”. Then I frowned, ‘well’. Then I looked again for another explanation. I said this is because there is wind in the higher air layers. Although, I believe that rain clouds do not always hang very high. Again I was unsure of my own answer. Does rain always fall at an angle, or is this an optical illusion? Still, I continued to search. In most drawings, the rain is always drawn diagonally (a cloud and slashes). On many clear photos you can also see the rain fall diagonally. Is our daughter right or wrong, and why?
Answer
Dear Jo,
Indeed, rain almost always falls at an angle, simply because there is almost always wind.
Rain that falls at an angle of less than 10 degrees hardly ever occurs. Raindrops are so light that the slightest horizontal force (wind) can move them from their vertical path (under the influence of gravity). Just try to drop a small piece of paper straight down: indoors this works, outside almost never.
Moreover, rain often accompanies a sudden change in weather conditions under the influence of wind: that is why raindrops are drawn obliquely.
So your daughter is right that the rain always falls at an angle, but probably not that there was no wind when you were cycling. You don’t feel a very light wind when you are cycling. Or maybe the wind was at your back, and you were cycling about as fast as the wind speed…
Answered by
Dr. ir. Jan Ronge
Bio-engineer. Electrocatalysis and renewable hydrogen production
Old Market 13 3000 Leuven
https://www.kuleuven.be/
.