Humidity vs Dew Point

7 Sep

The difference between Relative Humidity and Dew point.

Dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface.

Humidity is the amount of water vapor present in the air. Water vapor is the gaseous state of water and is invisible to the human eye. Humidity indicates the likelihood of precipitation, dew, or fog. Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin.

Point of interest: At any moment, the atmosphere contains an astounding 37.5 million billion gallons of water, in the invisible vapor phase. This is enough water to cover the entire surface of the Earth (land and ocean) with one inch of rain.

Water vapor is also the most important greenhouse gas in the atmosphere. Heat radiated from Earth’s surface is absorbed by water vapor molecules in the lower atmosphere. The water vapor molecules, in turn, radiate heat in all directions.
Sometime we complain about how muggy our air is without realizing just how important the humidity really is.

This summer has had many days of hot and sticky weather. You may have checked on the humidity, only to find it was at a meager 50%. How could the humidity be so low, when it feels so high? The answer: dew point!

Dew point is the temperature at which water vapor in the air condenses into liquid water, such as in the form of dew, fog, or possibly rain. The dew point is always lower or equal to the air temperature, hence why dew or fog often occurs during the early morning hours, when the air temperatures are typically lowest and the dew point highest.

On the other hand, the relative humidity is the ratio between the amount of water vapor in the air at a given temperature to the maximum amount of water vapor possible in the air at that temperature. The relative humidity is not an exact ratio between the current air temperature and dew point!

For example, a 95°F day may only have a relative humidity of 45%, but it still feels incredibly hot and humid. Instead of checking the humidity on such days, one should focus on the dew point and the resulting heat index to determine how sticky it will feel outside. The heat index utilizes temperature and dew point (or relative humidity) to determine how hot the body perceives the environment. Because the body cools down through the evaporation of sweat, moist air will not allow evaporation as readily as dry air. So, in turn, the body will feel hotter on days with higher dew point values. In general, a dew point of 60 – 63°F begins to “feel” more humid and a dew point of 70°F or higher becomes rather oppressive on a summer day.

Most people have a tough time understanding the difference between the “meaning” of RH (relative humidity) and dew point. Meteorologists have the challenge of explaining these concepts to the general public and generally do a poor job of it.

We have a good grasp on how the weather makes us feel. One approach to explaining dew point would be to say, dew points above 65 F make it feel sticky and humid outside while dew points less than 65 F are comfortable with respect to the stickiness of the air. The higher the dew point is, the more moisture that is in the air. The higher the dew point is above 65 F, the stickier it will feel outside (feels like you have to breathe in a bunch of moisture with each breath). 75 F or above dew point, the air really feels sticky and humid.

RH can be more difficult to explain. We pretty much understand that a RH of 100% means it is foggy, very wet, or saturated outside. One misconception people have is that the RH is 100% only when it is raining. Example 1: The RH is often 100% in the early morning hours when temperature has dropped to dew point. Example 2: When rain first begins, it takes time for the air to saturate. RH is often much less than 100% when it is raining (it takes time and lots of evaporation to saturate air that previously has a RH of 50% for example).

RH can be explained as the “closeness the air is the saturation”. When the RH is less than 40%, it feels dry outside, and when the RH is greater than 80% it feels moist outside (dew point will determine if it is uncomfortably moist or just regularly moist). Between 40 and 80% RH is comfortable if the temperature is also comfortable.

The worst combination for human comfort is a high dew point (65 F or above) combined with a high RH. If the dew point is above 65, it will generally always feel uncomfortably humid outside. Obviously, the temperature could climb to over 100 and result in a low RH, but the quantity of moisture in the air is still high and will be noticed.

The optimum combination for human comfort is a dew point of about 60 F and a RH of between 50 and 70% (this would put the temperature at about 75 F). The air feels dry outside when BOTH the dew point is below 60 F AND the RH is less than 40%.

Now the dilemma, how do we differentiate the “meaning” between a high dew point and a high RH when they both indicate the air is humid??? Dew point is related to the quantity of moisture in the air while relative humidity is related to how close the air is to saturation. How we understand this difference in meaning can be a challenge. The challenge can be overcome by describing how the weather feels and relate that information to the current dew point and relative humidity.

I hope this makes it a bit easier to understand. If you have any questions, please feel free to ask.

Mr. Ed


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