What does it mean for the relative humidity of an air mass to be 70 percent?

Answer 1

Let's do a quick review of some facts about water vapor in air, and then we'll tackle this question. An air mass will have a given temperature and a given pressure. For air of a given temperature and pressure, only a certain amount of water vapor can be "suspended" in that air before it begins to condense and precipitate out (as rain, snow or something else). When the amount of water vapor in the air is at it maximum (for whatever temperature and pressure we cite), that air has 100% of the water vapor in it that it can hold. Any more water vapor and water will condense and precipitate out, as we stated. That's all we need to know to take on this question.

When we consider the amount of water vapor in a given air mass, we use a "standard" or and "index" to relate that amount of water vapor to something "fixed" to make our measurement. The reference in this case will be the maximum amount of water vapor that an air mass can hold at that temperature and pressure (whatever they are). In the case of this question, if the relative humidity of an air mass is 70%, that air mass (whatever its temperature and pressure) is holding 70% of the water vapor that it can possibly hold. Note that term we use is relative humidity. The "amount" of humidity in an air mass that has a relative humidity of 70% is 7/10ths (70%) of the amount of water vapor that it can possibly hold altogether. We've compared the amount of water vapor in the air to the maximum amount of vapor that it can hold.

While it is true that an air mass may have this or that temperature and pressure, in any given air mass of whatever temperature and pressure, there is some maximum amount of water vapor that it can hold, and when we look at the amount of water vapor in that air mass, we compare it to that maximum amount that it can hold. That's relatively humidity, and when relative humidity reaches 100%, that air is saturated with water vapor and we can expect it to begin to precipitate out in the form of rain or another form of precipitation (depending on temperature and conditions aloft).

It might help to note that warmer air and air at higher pressure can hold relatively more water vapor that cooler, less "pressurized" (less dense) air. But whatever the temperature and pressure of an air mass, there is some maximum amount of water vapor that it can hold, and we look at how much is in it, and compare that to the maximum amount, and call the comparison the relative humidity.