Feeling slow in the winter? Blame the cold air because it's denser.

Temperature affects air density, which has a linear relationship to air drag, so yes, you are slower when it's colder outside.

You raise an interesting question and you’re observations are correct. The drag force on a cyclist is provided by the following equation: D = ½ p V2 Ap Cd where D is the drag force (N); p is the air density (kg/m3); Ap is the frontal area of the cyclist (m2) and Cd is the drag coefficient (dimensionless).

As you mentioned, air density is affected by temperature, pressure, and also by humidity. Temperature has a much more pronounced effect on air density than humidity: cold air contains more molecules per cubic meter. If the air pressure is constant at 1000 kPa, then the air density at 25 degrees Celcius (77 degrees Fahrenheit) will be around 1.169 kg/m3 while at -5 degrees Celcius (23 degrees Fahrenheit) the air density will be 1.3011 kg/m3 — about 10-percent higher, and the drag would also be increased by 10 percent.

Air drag is also affected by you wearing more stuff, like a jacket, and your muscles can't get as warm either, so now you have a whole bunch of excuses..., eh, reasons for being slower in the colder air.