MIKE MOSS SAYS: Julia, I'm assuming you're asking about relative humidity (RH), the one (given in percent) that we typically show on air, although we prefer to track moisture in the air by way of the dew point more so than RH. Relative humidity can be measured directly by some types of instrumentation, while others measure something called wet bulb temperature or dew point, and those readings can be used along with air temperature measurements to compute the RH. For some links and an equation for calculating RH, see the earlier question and answer at the address below:
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