Dew Point and Relative Humidity

A Basic Scientific Clarification Between Dew Point
and Relative Humidity.

There has often been much confusion and misinformation in the everyday, average person’s weather world regarding the relationship and difference between what is known as the Dew Point Temperature and the Relative Humidity. This is evident as such as television weather-casters rarely cite the dew point temp. but most always report the relative humidity. In actuality the dew point is a much more accurate measure of the actual water vapor content in the air. Water vapor content is sometime also referred to as humidity (but not relative humidity).

First it’s essential to understand the concept that warmer air (often considered and measured as a volume parcel) can hold or suspend more water vapor-humidity than that which is cooler. One may analogize an air parcel’s temperature and its humidity holding capacity with warm air being a large room and cool air a smaller room. Room size would decrease in volume as the air temperature decreases when water vapor-humidity holding capacity is considered.

Second, the higher the dew point temperature the more water vapor molecules, and thus, humidity is present in a particular parcel of air.

Consider ambient air in a given area having a temperature of 80 degrees Fahrenheit with a dew point temperature of 65 degrees. This basically means that the air’s temperature would have to decrease to 65 degrees in order for visible condensation to occur. The water vapor is cooled by the surrounding air’s decreasing temperature until it condenses to a visible form, in this case at 65 degrees. When this occurs the air is said to be Saturated and the relative humidity reaches 100 percent.

Ambient air that is saturated at a specific temperature can no longer suspend
and hold its humidity-water vapor so it condenses out to a visible form. Again, the relative humidity reaches 100 percent.

Relative Humidity is an index and a percentage ratio which compares the air’s temperature and the amount of water vapor-humidity present in the air compared its overall holding capacity at that temperature. Again, suppose the air temperature is 80 degrees (with a dew point of 65 degrees) and the relative humidity is 61 percent. This means the air at 80 degrees is suspending-holding 61 percent of the total water vapor it’s capable of holding with the existing amount of water vapor in the air, represented by the 65 degree dew point. Interestingly, if the 80-degree air became drier with the attending dew point lower to 58 degrees, the relative humidity would also decrease to 47 percent.

From this explanation a correlation between ambient air temperature, dew point temperature-humidity and the resultant relative humidity can be seen.