# Which paramteres in the Weber number belong to the object and which to the fluid?

The Weber number (We) "can be thought of as a measure of the relative importance of the fluid's inertia compared to its surface tension" (Wikipedia):

$$\mathrm{We} = \frac{\rho v^2 l}{\sigma}$$

So assuming a droplet is moving through air, the droplet is the object and the air is the fluid. Clearly, $l$ and $v$ are the characteristic length and velocity of the object (the droplet), but which density $\rho$ and surface tension $\sigma$ of the two should be used?