Contrast humidity and relative humidity?

I'm assuming you're asking for the difference between specific humidity and relative humidty. If so...

Relative humidity is exactly that: it is relative to the temperature. Warmer air can hold more moisture than cooler air, so if warm air is 40% humid, and cool air is 40% humid, then the warmer air is actually more humid than the cooler air, even though the RELATIVE humidity is the same.

Specific humidity is expressed in GPP, or Grains [of water] Per Pound [of air].

Example:

If the air is 60 degrees, and the relative humidity is 40%, then a pound of that air contains 31 grains of water.

If the air is 80 degrees, and the relative humidity is 40%, then a pound of that air contains 61 grains of water.

As a result, even though the relative humidty is the same in both examples (40%), the air that is 80 degrees is almost twice as humid as the air at 60 degrees.

Elevation (feet above sea level) plays a factor.

The formulas for computing relative/specific humidity (among other things, such as vapor pressure, heat index, dew point) are in the science called psychrometrics, and note the spelling: psychro (with an "r"), not psycho.