Velodrome atmospheric conditions
The equation for total aerodynamic drag force on a bicycle is well-understood and, you're right, air density has a role in it: the higher the density, the greater the drag. As you guessed, density decreases with increasing temperature so warm air is less dense than cold air. It's also probably no surprise that density decreases with pressure. However, evidently counter-intuitively, humid air is less dense than dry air. The simplest explanation is that water molecules contain two hydrogen atoms and one oxygen atom so they have a molecular mass of 18 g/mol, while dry air has a molecular mass of around 29 g/mol. You can check the wikipedia page on air density for more details.