Dew Point and Relative Humidity

For this post, I will discuss a little on dew point and relative humidity.

 Water is present as a liquid and as a gas in the atmosphere. When water is a gas, it’s called water vapor. The naked eye can’t see water vapor in the air, because water molecules are very small. When water is a liquid, you can see it in the form of clouds. It can also fall from the sky as precipitation.

The amount of water vapor in the air is called humidity.

There are two kinds of humidity, Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. Relative humidity also measures water vapor but relative to the temperature of the air. For the purpose of this post I will only briefly discuss relative humidity due to its relationship to dew point.

Relative Humidity:

Relative humidity is a percentage of the amount of water vapor in the air of the total amount that could be held at its current temperature. The warmer the air the more water vapor it can hold.

Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal. If the amount of water vapor in the air stays the same, but the temperature goes down, the relative humidity will increase. This is because the colder air cannot hold as much water vapor. If the temperature gets cold enough, the air gets to the point that it is holding the most water vapor it can hold. The relative humidity for this temperature would be 100 percent. This is also known as the dew point temperature.

Dew Point:

All Y’all hear the term dew point all the time on your local weather forecast during the summer. But what the heck is dew point really? And why should you care?

Dew point is the temperature to which a parcel of air must be cooled in order to reach saturation by the water vapor already present in the air. A parcel is like a helium balloon as it rises up into the air.  The higher the dew point, the more moisture there is in the air.

To put all of this in simpler terms, is that the dew point is the temperature at which the moisture in the air becomes liquid water. It is a complicated calculation to determine the dew point, but fortunately y’all don’t need to do the math. Here’s a chart that will show you the approximate dew point.

Dew point temperature is a marvelous indicator of how comfortable you'll feel outside. The higher the dew point rises, the greater the amount of moisture in the air, the higher the dew point, the harder it is for the sweat on your skin to evaporate, and the hotter and muggier the air will feel.

Going back to relative humidity, many times it can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, but a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more "humid" on the 80-degree day with 50% relative humidity than on the 30-degree day with a 100% relative humidity. This is because of the higher dew point.

When the air temperature gets down to the dew point, some of the water vapor turns back to liquid water (condensation). That fog you see on the mirror after you take a shower; is the result of dew, which is just liquid water that has condensed out of the air.

Human body temperature is dependent on the air as it absorbs and removes moisture from our skin to cool us down.  If the relative humidity is high, the amount of water evaporating from our skin is limited so we feel warm and smothered.

A dew point temperature less than 50 degrees is considered dry, 50-68 degrees is comfortable, 69-76 degrees is uncomfortable, while greater than 77 degrees is considered extremely uncomfortable.

The basic meaning of other heat related terms.

HEAT WAVE:

A period of abnormally and uncomfortably hot weather. To have an official heatwave the air temperature must at least 90 for at least three days, A heatwave can last from several days to several weeks.

HEAT EXHAUSTION:

The effect of excessive heat, particularly when combined with high humidity, on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

HEAT STROKE:

Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105 degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.