Air density is a key factor in how fast you can travel for a given power output. The higher the density of the air, the slower you will go, all else being equal. The major factors that influence air density are temperature, barometric pressure and altitude. Humidity also has an influence but relative to the other factors it is minor.
Air density increases as temperature, altitude and humidity reduce, and as barometric pressure rises. Air density can be determined with the calculator below. You'll need to know the:
local temperature (Celsius);
altitude above/below sea level (metres);
barometric pressure at sea level, which is what weather stations report (hPa); and
relative humidity (%)
Altitude data is typically available from mapping sites, local temperature might be worth checking yourself, and barometric pressure can be obtained from local weather station data. Barometric pressure does not change a lot over long distances, so the nearest weather station reading will be good - they all default to reporting the sea level equivalent barometric pressure. Since relative humidity does not make a lot of difference, if in doubt you can just leave it at the default value.
Just move the sliders to see the resulting air density in kg.m-3.
Note: The dial display is restricted to integer values and shows the air density result in units of g.m-3 or 1000 x kg.m-3.