#Quadratically.

[NASA page #1](http://www.grc.nasa.gov/WWW/K-12/FoilSim/Manual/fsim0005.htm) says that, as an approximation,
$$\text{Lift}\propto v^2$$
where $v$ is the airspeed.

[NASA page #2](https://www.grc.nasa.gov/www/k-12/problems/Dalette/LiftvsAirspeed_ans.htm) says that, for a better calculation,
$$\text{Lift}=av^2+bv+c$$
where $a$, $b$, and $c$ are constants. This is odd because it implies that lift is not merely proportional to $v^2$, because there is the $bv$ term in there, as well as the $c$ term.

But there is a better equation, given [here](http://www.av8n.com/how/htm/4forces.html) (and [here](http://www.langleyflyingschool.com/Pages/CPGS+4+Aerodynamics+and+Theory+of+Flight+Part+1.html)), among other places:
$$\text{Lift}=\frac{1}{2}\rho v^2 \times \text{lift coefficient} \times \text{area}$$
where $\rho$ is air density.

This is similar to the drag formula.