It’s not a matter of “You like tomato and I like tomahto" from “Let’s Call the Whole Thing Off.” Dew point and humidity are not identical. They are both used to measure moisture content in the air, and they are both very useful and accurate. But when it comes to measuring human comfort as we more frequently encounter sticky conditions in the summer, dew point is more accurate.
The dew point temperature is the temperature to which you’d have to cool a parcel of air at constant pressure for it to become saturated with water vapor. Dew point can be measured with an instrument called a hygrometer.
Relative humidity is the percentage of water vapor in the air relative to its air temperature. Warmer air has more room for water vapor than arctic air. It’s an oversimplification, but let’s just say the molecules are more active with heat energy and that there is more space for water vapor molecules. Humidity can be measured with what is called a sling psychrometer.
When a parcel of air is saturated, its relative humidity has reached 100 percent. Its dew point has also been reached, and the water vapor in the air will condense into droplets. The droplets may appear as fog or a cloud, or even as dew on a surface. In the air the droplets may continue to coalesce into precipitation if a relatively warm, moist parcel is lifted into even colder air with lower dew points by physical processes, like convection.
Dew point is superior to relative humidity in gauging human comfort more accurately because it enables us to determine whether conditions “feel” sticky, as opposed to merely damp. You can have relative humidity of 90 percent with a temperature of 28 degrees, and the air may impart a feeling of dampness or, with a breeze it may feel raw, with a penetrating cold. It’s humid with those numbers, but it’s certainly not sticky.
Mugginess is more directly tied to dew point. When dew points are in the 40s and low 50s, warm temperatures in the 80s will feel much more comfortable than if dew points are in the 60s. There is enough spread between the air temperature and its dew point temperature for perspiration on your skin to evaporate into the dry air, carrying off body heat efficiently. That’s called evaporational cooling, and it’s nature’s air conditioning. If temperatures are in the 80s and the dew point has crept up to the mid-60s, perspiration doesn’t evaporate so readily, so less evaporational cooling occurs. Even at that muggier stage, a breeze can help this limited cooling process along very nicely. If your home has central air, you can save some real bucks by using a ceiling fan and setting your thermostat up to a higher temperature.
Once dew points go above 70 — which doesn’t happen around Western New York all that often — evaporational cooling slows down to a crawl, as the sweat stays put on your skin and clothes.
Using dew points is probably a more effective communication tool than just saying “warm and humid” for someone in my line of work. As mentioned, dew points in the 70s are fairly uncommon around here, unlike in Florida and Gulf states, where people are seemingly more acclimated to them. Such high dew points can be a cue to cut down on strenuous outdoor activities such as running and yanking weeds in the garden, even in the deep South. When your body can’t shed excess heat, of course, problems can go beyond discomfort into the realm of danger.
Even where dry air dominates in the summer, at some point the joke about Phoenix heat being a “dry heat” (it usually is, but not during the short summer monsoon season) wears thin if you’re stuck in an oven. When temperatures head up over 100, even with low dew points, people can run into trouble in short order. For one thing, you’ll be less aware of how much fluid you’re losing through perspiration because it evaporates so quickly. You may subconsciously do less to replenish your fluids until you feel great thirst, at which point your core body temperature may already be too high.
So, it can be humid at virtually any temperature, but the range for mugginess (or as I often say with my made-up word, “muggification”) is more specific.
By the way, if you’re not a fan of being muggified and you are trying to pick a more comfortable vacation spot, you can readily track what kinds of dew points are more commonly found with a link like this one, updated hourly.
In a pinch, one quick way to gauge whether the dew point is high is to take a glass of ice water or chilled beverage of your choice outside. If droplets quickly condense on your glass, voila! You may not know the precise dew point, but you’ll know it has been reached on the outside of your glass in short order, suggesting a sticky air mass. And you’ll know you have a possible excuse for not mowing the lawn until this whole dew point thing dies down. Feel free to toss around terms like “oppressive.” Get back to me on whether that works with your spouse.