CLOSE
iStock
iStock

What Is the Dew Point, and How Does It Relate to Humidity?

iStock
iStock

Humidity has been a part of weather forecasts for as long as we’ve gotten our news over the air. At the beginning of most weather forecasts, our friendly neighborhood weatherperson tells us the sky conditions at the moment, the current temperature, and the relative humidity. Over the past couple of decades, though, the relative humidity has started to fall by the wayside in favor of the dew point. The dew point is a much more useful measure of how much moisture is in the air, but how does it relate to relative humidity?

The amount of water vapor in the air can dictate what kind of weather we see and how comfortable we are once we step outside. Relative humidity is technically defined as the air’s vapor pressure divided by its equilibrium vapor pressure. Equilibrium vapor pressure means that “there is no net evaporation or condensation,” according to Alistair Fraser, professor emeritus of meteorology at Penn State. At the equilibrium, otherwise known as the saturation point, water molecules are entering and leaving the condensed state at the same rate. When the relative humidity is cited as 50 percent, that means that the air is halfway to its saturation point, and that net evaporation is occurring. Warm air requires more water vapor than cool air to reach its saturation point, which is why an 85°F afternoon can get much muggier than a day that only makes it to 50°F—the latter can still be humid, sure, but it’s not like walking into a sauna.

The dew point is the temperature to which the air needs to cool down to in order to become completely saturated, or reach 100 percent relative humidity. Once the air temperature cools below its dew point, water vapor in the atmosphere will condense. This causes the relative humidity to go up and down like a roller coaster during the day. The relative humidity will go up at night when the air temperature approaches the dew point, and the relative humidity will go down as the air temperature warms farther and farther away from the dew point during the day.

The dew point is a little more abstract than the relative humidity, but it’s an effective way of telling you how much moisture is present in the air because it means the same thing no matter how warm or cold it is outside. A 40°F dew point is comfortable whether the air temperature is 60°F or 100°F. This consistency allows us to index the dew point to comfort levels, giving us a quick understanding of how muggy or pleasant it is outside.

It’s downright dry outside when the dew point is at or below the freezing point. Dew point readings between the freezing mark and about 55°F are pretty comfortable. A dew point between 55°F and 60°F is noticeably humid. It’s muggy when the dew point is above 60°F, and it’s uncomfortable outside when it ticks above 65°F. Any dew point readings above 70°F are oppressive and even dangerous, the kind of stickiness you experience in the tropics or during a brutal summer heat wave. It’s rare for the dew point to reach 80°F, but it can happen in extremely moist areas like corn fields or certain tropical areas.

The dew point and relative humidity are closely related, but the former is much more useful than the latter. Relative humidity helps meteorologists predict conditions favorable for wildfires and fog. Other than that, it’s mostly a relic of the old days that show up in weather reports out of habit. If you want to know the true measure of how comfortable or muggy it is outside, take a look at the dew point.

nextArticle.image_alt|e
iStock
arrow
Weather Watch
Thanks to Desert Dust, Eastern Europe Is Covered in Orange Snow
iStock
iStock

Certain areas of Eastern Europe are starting to look a bit like Mars. Over the last few days, snowy places like Sochi, Russia have experienced an unusual snowfall that coated mountains in orange powder, according to the BBC.

The orange snow was the result of winds blowing sand from the Sahara east to places like Moldova, Romania, Bulgaria, Ukraine, and Russia. The sand mixes with precipitation to form orange-tinted snow. According to the BBC, the phenomenon occurs semi-regularly, turning snow orange about once every five years, but this year is especially sandy. As a result, skiers are navigating slopes that look like they're from a different world, as you can see in the video below from The Guardian.

The Sahara rarely gets snow, but when it does, the landscape can look somewhat similar, as you can see in this image of the Atlas mountains in Morocco.

Instagram is currently filled with photos and videos from Eastern Europe featuring the odd-looking snow. Check out a few samples below.

[h/t BBC]

nextArticle.image_alt|e
Jessica Kourkounis, Getty Images
arrow
Weather Watch
What Is Thundersnow?
Jessica Kourkounis, Getty Images
Jessica Kourkounis, Getty Images

The northeastern United States is dealing with its second major nor'easter in a week, with rain and heavy snow—and the associated power outages—cutting a path across the Mid-Atlantic and New England. But news of the adverse impacts of the snowstorm is being accompanied by an unusual buzzword: thundersnow. Thundersnow occurs during a thunderstorm that produces snow instead of rain. The mechanisms that produce rainy thunderstorms and snowy thunderstorms are largely the same, even if the air temperature is below freezing.

A band of snow can become strong enough to produce lightning through two processes known as convection and forcing. Convection occurs when an area of warm air quickly rises through cooler air above it. Convective snow is most common during lake effect snow events like those you’d find on Lake Ontario or Lake Erie, since the process requires extreme vertical temperature gradients that can result from bitterly cold air flowing over a warm body of water.

Forcing is slightly different. A strengthening low-pressure system involves fast, dynamic changes in the atmosphere, especially when one of these storm systems quickly gains strength. Such a fast-developing storm can cause large amounts of lift in the atmosphere, a process that forces air to swiftly rise like you’d see during convection. This creates intense bands of snow that can grow so strong that they produce thunder and lightning. This process is responsible for the thundersnow that occurs during blizzards and nor’easters, those powerful storms that regularly hit the eastern coast of the U.S. during the winter. Thundersnow can be pretty exciting—just ask The Weather Channel's Jim Cantore:

The name “thundersnow” can be a bit misleading. One of the most enjoyable things about a snowfall is how silent it is outside when there’s a thick blanket of snow on the ground. Snow absorbs sound waves so efficiently that you can usually only hear ambient noises immediately around where you’re standing. Snow muffles the sound of thunder for the same reason. Thunder that might be audible for many miles during a rainy thunderstorm might only be audible for a few thousand feet away from where the lightning struck. Unless the lightning strikes very close to where you are, you might only see a bright flash during thundersnow without ever hearing the thunder.

While thundersnow is a fascinating phenomenon to encounter, it does involve lightning, after all, and it’s just as dangerous as any other lightning bolt you’d see in a rainy thunderstorm. If you’re ever lucky enough to experience thundersnow, the event is best enjoyed indoors and out of harm’s way.

This piece originally ran in 2017.

SECTIONS

arrow
LIVE SMARTER
More from mental floss studios