Humidity

Humidity is the amount of water or moisture (a vapor)  in the air. When the Humidity is HIGH, there is a lot of water vapor in the air. If the Humidity is LOW, there is little water vapor in the air.

The air is like a sponge; there are places inside of it that can hold water. A sponge, however, can only hold a specific amount of water. A glass can hold water until it is filled to the top. If one adds more water, the glass will overflow. The sponge too, will overflow if more water is added to it than it can hold. This is called Capacity.

Capacity is the amount of material (in this case water) that something can hold. As air warms up, heat energy is transferred to the air which speeds up and moved the molecules of the air. The warm air moves around more crashing into other molecules and moving them around too. The cooler pieces of air move around less and can be closer together.

In a specific area, cool air has more pieces per unit volume and is heavy. Warm air has less pieces per unit volume, so it is lighter. Since warm air has less pieces, there is more room or space between the pieces. This means more moisture can fit between the pieces. Warm air, therefore, can hold more water vapor than cool air can.

When you reach the point where the air, glass or sponge can no longer hold water, you are at that mediums Saturation Point. When you get to the Saturation Point, it can not hold any more; IT IS FILLED UP. As you lower the temperature, the saturation point becomes less and the amount of molecules per unit volume increases. Less space between molecules says less “stuff” can fit between.

The amount of water vapor in the air compared to it’s carrying capacity is called Relative Humidity. The actual amount of water vapor in the air is called Specific Humidity. Specific Humidity is expressed as the mass of water vapor (in grams) in one kilogram of air t a given moment in time. It does not depend on temperature or pressure. Adding more water vapor or taking some water vapor away, however, can change the specific humidity.

The Relative Humidity of air changes as water moves into or leaves the air. It can also change as the temperature changes. Cool air means less room between molecules or pieces of air and therefore less Relative Humidity. Air that is 50% filled with water vapor has a Relative Humidity of 50%.