What is the difference between dew point and humidity?

Dear Tom,

Can you help me understand the difference between dew point and humidity?

Thanks,
Dawn Lenza
Homer Glen

Dear Dawn,
We sure can. While relative humidity and dew point are both concerned with the amount of water vapor in the air, there are differences. Dew point is the temperature at which the air becomes saturated (100 percent relative humidity). It is dependent on only the amount of moisture in the air. Relative humidity is the percent of saturation at a given temperature; it depends on both moisture content and temperature. As air is heated, its ability to hold water vapor doubles with about every 11 degree increase in temperature. If air is at 100 percent relative humidity at 60 degrees but is heated to 93 degrees, its relative humidity decreases to about 33 percent, even though its dew point remains at 60 degrees.