I recently received a message asking for a discussion of relative humidity (RH), wet-bulb (WB), and dry-bulb (DB). Time and time again, I hear techs say that condensation occurs when “hot meets cold,” which may be true in some cases. However, that is only a shorthand way to describe it and doesn't really address what is going on when we see condensation and undesirable growth.

RH = Relative humidity; the percentage of humidity relative to the amount of moisture can hold at that temperature.

I like to think of it as sugar in a cup of coffee. The hotter the coffee, the more sugar the coffee can hold.

When we say the air is “humid,” we can mean it contains a lot of absolute moisture in grains or pounds, or we can mean it is high “relative” humidity, which is the more common meaning. Air that's 95°F at 50% RH contains far more moisture per lb of air than 65° air does at 50% RH.

It's a common misconception that hot air is more humid. While it's true that hotter air can hold more moisture in the same way that hotter coffee can hold more sugar, it does not mean hotter means more humid. In fact, if you heat a mass of air and the amount of moisture doesn't change, the RH will go down as the air gets warmer. This is why the RH coming out of the top of a furnace is lower than the RH going in. No change has occurred in the actual amount of moisture present; the air is just hotter, therefore lower in humidity relative to how much it can hold.

DB = Dry-bulb; the temperature of air without taking account for evaporation/relative humidity

WB = Wet-bulb; the temperature of the air with the evaporative effect of a “wet bulb” taken into account. Quite literally, wet-bulb temperature is the temperature a thermometer bulb will be when covered in a wet fabric and whirled in the air or placed in an air stream.

If the RH is below 100%, the WB will always be lower than the DB. The differential between the DB and WB illustrates the RH. The higher the differential, the lower the RH. The lower the differential, the higher the RH. When DB and WB read the same, the RH is 100%, and the air is “saturated.” No more evaporation can occur.

When air hits 100% RH, the dry-bulb and wet-bulb temperatures are the same, and this point is known as “dewpoint.”

—Bryan