Imagine rolling two balls at each other for a head-on collision. After they hit, which way they roll depends on a simple law of physics: F=MV2, or "force equals mass times velocity (squared)." This means that a small ball rolling fast can potentially overpower a large ball rolling slowly.
The same concept goes for the air curtain. The force of the air blowing from the curtain has to be strong enough to overpower and turn back the force of the air coming from the outside. If your giant, open warehouse door is constantly getting pummeled by strong winds, you'll need a hefty air curtain -- one that blows a lot of air with great velocity.
The other thing to take into account is the direction of that velocity. You might think that an air curtain would most effectively fight horizontal winds with its own horizontal winds. But that's not really the case. When air is blown directly out the door for a head-on collision with other air (like those two balls we mentioned above), much of the power of the air is lost over distance. Instead, air curtains direct their flow of air mostly downward toward the floor of the doorway. The system must blow hard enough to ensure the air is still traveling at the needed velocity as it reaches the bottom of the doorway, and that the velocity is also stronger than the velocity of the outside, inrushing air. As such, the curtain acts like a wall that outside air comes toward and then bounces off of.
To give you an example: If wind were blowing horizontally toward the door at 20 feet per second (13.6 mph/21.2 kph), an air curtain angled outward at 15 degrees would have to blow 133 feet per second (91 mph/146.4 kph) to counteract the force of the outside air.
All of this means that you'll need to play with your air curtain to get its force just right -- at just the right speed and angle to most efficiently counteract the force of the inrushing air. You can tweak your air curtain three ways: Either crank up the volume of air blown, the speed of the air blown, or direct this velocity outward a bit, usually at an angle of between 10 and 20 degrees [source: PoweredAire.com].