Air density, viscosity and Reynolds number
Air density comes from the temperature, the pressure and the humidity, treating air as a mix of dry air and water vapour. Humid air is slightly lighter than dry air. When you enter an altitude, the pressure comes from the standard atmosphere.
The dynamic viscosity follows Sutherland's law and depends only on temperature. The Reynolds number Re = V · c / ν compares inertia with viscosity and decides how the boundary layer behaves, which is why an airfoil polar is only valid near the Reynolds number it was computed for.
Mach number
Below about Mach 0.3 the air can be treated as incompressible, which is what XFOIL and most 2D CFD runs assume. The Prandtl-Glauert factor shows how much the pressures grow at your speed, roughly 2 % at 250 km/h.
What are the standard conditions?
15 °C and 1013.25 hPa at sea level with dry air, which gives 1.225 kg/m³. Cold air is denser, so a wing makes about 9.5 % more downforce at −10 °C than at 15 °C.