After nearly three straight weeks of no rainfall during the month of May, the weather pattern has finally broken — and humid air has returned to the Tennessee Valley.
When people say “it’s humid outside”, they are referencing the fact that extra moisture is present in the air. The phrase “relative humidity” is more familiar compared to the term “dewpoint”, but humidity can fluctuate depending on the temperature.
In fact, higher relative humidity values occur in the morning when the temperature is low, and the relative humidity falls during the day when the temperature is high — but the amount of moisture stays the same.
This is why the “dewpoint” is helpful in communicating the amount of moisture in the air: Unless wind is transferring warmer or drier air into a region, the dewpoint is going to stay relatively constant until there is a change in the wind.
What is the “dewpoint”?
The air we breathe consists of a mixture of gases including oxygen, nitrogen, carbon dioxide and water vapor (as well as mix of other trace gases).
Water vapor is simply water in gaseous form, and greater amounts of water vapor can be present in warmer temperatures compared to cooler temperatures.
When air is cooled to a certain point, the water vapor within the air is also cooled to the point that it can no longer remain in a gaseous state — it then condenses into liquid water, which we call “dew”.
So the dewpoint is the temperature at which air needs to cool in order for water vapor to condense into liquid water. The dewpoint is the limit at which air can cool, so on dewy mornings, the coolest air temperatures are close to — if not equal to — the dewpoint.
The dewpoint also impacts stormy weather: If the air has higher amounts of water vapor within it, this water vapor will provide the “fuel” necessary for thunderstorms to develop. In a nutshell, the higher the dewpoint, the more humid the atmosphere — and the more “fuel” available to develop thunderstorms.
What does the dewpoint have to do with comfort level?
In a nutshell: the higher the dewpoint, the more muggy or “miserable” it feels to be outside. But why?
The higher the dewpoint, the more water vapor that is present in the atmosphere. This means that any liquid water on the earth’s surface (or perhaps the surface of your skin) will have a hard time evaporating and changing from liquid to gas — it’s as if the atmosphere is saying “no thanks, we’re full”. In true scientific terms, the atmosphere is close to saturation when the air temperature is close to (if not equal to) the dewpoint.
Sweat contains a mixture of salt as well as liquid water — and if the dewpoint is high enough, the water within the sweat won’t evaporate off of your skin. It’s that evaporative process that cools your body: Heat is drawn from your body to the atmosphere when sweat evaporates, causing your body to cool.
So if the dewpoint is too high, the body’s cooling mechanism breaks down — sweat builds up, but its got no where to go. As a result, you feel damp and clammy and…well, miserable.
The dewpoint and you: Your daily forecast
You may not necessarily notice the dewpoint in our on-air presentations, but we do reference it from time to time in some of our tower camera banners.
For the next week or so, dewpoints will be rather high (in the mid to upper 60s) as tropical air continues to flow into the Southeast. This muggy air will continue to provide the possibility of rain showers and thunderstorms for the next several days. For the latest forecast information, check the WHNT News 19 Forecast Discussion.