DBCherry very accurately said:
If your plane has a 9 mph stall speed and you're flying INTO a 10 mph headwind you can hold the plane pefectly still in the air. However, if you're flying downwind in that same 10 mph wind, you BETTER be traveling at a minimum of 19 mph or you're going to fall out of the sky! (19 mph downwind will require the same throttle as 10 mph into the wind.)
By the way, this causes an awful lot of crashes. Picture a landing approach; people are use to seeing their plane traveling at say, 20 mph on the downwind leg on a calmer day, on a windier day that same 20 mph may be stall speed. (Especially when you make your turn onto the base leg.)
Yeah, this bit of visual trickery kills a lot of airplanes, that's for sure. A lot of guys will be flying downwind with way too little airspeed, then bank in to a turn and fall out of the sky.
This is why I try to teach people to "feel" the plane. Meaning pay attention to where your sticks are. If you know that a certain thottle stick possition is required for level flight, then regardless of what your eyes are telling you about your plane's speed, you better have the throttle in that location if you are trying to maintain level flight.
Same kind of thing goes for elevator and other controls. You can get information about what is really going on with your plane though the sticks, so pay attention to your fingers.
(yes, there is some variation due to the air density on a given day or whatever, but it should be close)