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


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.

A Pile of Manure and a Heat Wave Combined to Cause Spain’s Worst Wildfire in 20 Years

Lumppini, iStock/Getty Images Plus
Lumppini, iStock/Getty Images Plus
A pile of manure in Spain ignited on June 27, sparking a wildfire that has torn through more than 10,000 acres of forest and other Catalonian landscapes, CNN reports. The manure combusted in part because of a scorching heat wave that has swept across Europe this week; temperatures in the affected region hit 104°F, and Germany, Poland, and the Czech Republic all reached record-breaking June temps. Catalan regional interior minister Miquel Buch reported that authorities believe the manure was improperly stored on a chicken farm in the village of La Torre de l’Espanyol, where it was exposed to the worst of the heat, according to NBC News. The wildfire—Spain’s most devastating in 20 years—is affecting the region just west of Tarragona, a port city that is best known for its Roman ruins, which is situated along the Balearic Sea about 60 miles southwest of Barcelona. [[{"fid":"315211","view_mode":"width-constrained-728","fields":{"format":"width-constrained-728","field_file_image_title_text[und][0][value]":false,"field_file_image_alt_text[und][0][value]":"Map of Catalonia, Spain","field_image_subhead[und][0][value]":"","field_image_subhead[und][0][format]":"unfiltered","field_caption[und][0][value]":"","field_caption[und][0][format]":"unfiltered","field_credits[und][0][value]":"PeterHermesFurian%2C%20iStock%2FGetty%20Images%20Plus","field_credits[und][0][format]":"unfiltered"},"type":"media","field_deltas":{"1":{"format":"width-constrained-728","field_file_image_title_text[und][0][value]":false,"field_file_image_alt_text[und][0][value]":"Map of Catalonia, Spain","field_image_subhead[und][0][value]":"","field_image_subhead[und][0][format]":"unfiltered","field_caption[und][0][value]":"","field_caption[und][0][format]":"unfiltered","field_credits[und][0][value]":"PeterHermesFurian%2C%20iStock%2FGetty%20Images%20Plus","field_credits[und][0][format]":"unfiltered"}},"link_text":false,"attributes":{"alt":"Map of Catalonia, Spain","height":728,"width":728,"class":"media-element file-width-constrained-728","data-delta":"1"}}]] The firefighting force includes about 350 firefighters, 12 fire engines, seven aircraft, two hydroplanes, and additional vehicles equipped with water tanks—all of which are techniques regularly used to fight massive blazes. Though high temperatures are expected to continue and the rough terrain itself is also a contributing factor in the rapid spread of the fire, the strong winds should soon abate; authorities have warned people to stay inside. Though more than 50 people have been evacuated from the area, no deaths have been reported thus far. While the spontaneous combustion of flammable materials like manure, hay, or compost is definitely possible, as evidenced by this situation, it’s not the most common way for wildfires to start. More common causes include lit cigarettes, unattended campfires, burning debris, and engine sparks. [h/t CNN]

Colorado Welcomed Summer With 2 Feet of Snow

Emmanuel Dunand/AFP/Getty Images
Emmanuel Dunand/AFP/Getty Images

While people in some parts of the country celebrated the first day of summer with barbecues and trips to the beach, residents of north and north-central Colorado had a snow day. Areas west of Denver—including Grand Lake, Aspen, and Steamboat Springs—experienced snow storms on summer solstice, with snowfall totaling nearly 2 feet at the highest elevations, CBS Denver reports.

Snow started falling in the Rocky Mountains the morning of Friday, June 21 and continued Sunday. Areas at ground level weren't cold enough to experience the unusual weather, but at elevations of 7000 feet and above, it looked like a winter wonderland. Steamboat Springs, a ski resort town in Colorado’s Yampa Valley, accumulated 20 inches of snow on the longest day of the year.

While Colorado mountain towns are used to seeing snow at odd times of year, the weekend's weather was still out of the ordinary. The average snowfall for Steamboat Springs in June is 0.1 inches. Prior to last Friday, it had been 91 years since a snowstorm hit the city in late June.

Snow has fallen in the Rockies later than average in six out of the past seven years. Though it feels like an extension of winter, the trend may actually be a product of the warming atmosphere. A warmer climate affects the jet stream, potentially pushing its course further south and leading to unusual weather patterns, such as unseasonable snowstorms in Colorado.

That means residents of some parts of the state will have to wait to have their summer hikes and picnics. The weather was serious enough to shut down one road in Rocky Mountain National Park.

[h/t CBS Denver]