Temperature Inversions Can Have Amazing or Dangerous Effects
Millions of people have gasped at the Grand Canyon, one of the seven wonders of the natural world, but in January 2015, they gasped for an unusual reason: the entire canyon was filled with a sea of fog. Battleship Rock, a rock formation on the Canyon’s South Rim, “truly looked like a ship today, cutting through the waves,” the Interior Department said.
This beautiful and rare phenomenon was caused by an atmospheric condition that most everyone has experienced at some point: temperature inversion. The cold air in the canyon was trapped by warmer air above it; the cold air had enough moisture to condense, creating the fog that made cameras click even more furiously than they normally do in the great national park.
While the Grand Canyon event was particularly eye-catching, temperature inversions themselves are quite common. At times, they can have a significant impact on the weather, on air quality and even on people’s health and safety.
So what is a temperature inversion, when do they occur, and why are they important for safety?
Hot Air Rising
Typically, air temperature follows a predictable pattern. The sun heats the ground, which then warms the air at the surface. The warm air rises, expands and cools. Consequently, temperature is lower at higher elevations. For example, if you drive up a mountain pass, for every 1,000 feet you gain, the temperature drops by roughly 3.3°F (-16°C).
Temperature inversion is a case of the air getting topsy-turvy, with a warm layer of air trapping cooler air below it, as it did in the Grand Canyon example.
Temperature inversions come in two basic varieties. The most common form is surface inversions, which happen near the surface of the earth. The second form, aloft inversions, occur above ground.
Surface inversions can happen for a variety of reasons. For one, nightfall can lead to temperature inversions. When the sun goes down, the ground loses heat rapidly — this is called radiative heat loss. During the winter, nights are longer, so the ground is cooled for an extended period of time, causing the air near the surface to be cooler. Also, if there is little wind, the warm air on top might not get mixed with the cooler air below it. Additionally, hills or mountains surrounding an area can cause the cool air to remain trapped beneath the warmer air. While episodes like the Grand Canyon are rare, temperature inversions can occur in a variety of conditions.
Slip Sliding Away
Have you ever wondered why roads get icy and people slip on relatively “warm” winter days? Temperature inversion is a common culprit.
All air contains water vapor, which remains an invisible gas in warmer temperatures. If the roadway temperature and dew point temperature of the surrounding air drops below freezing, frost can form on the road. This “black ice,” named because the ice on the surface is not clearly visible to drivers, is responsible for up to 25% of all traffic accidents in some areas of the U.S.
Often, temperature inversions caused on a calm, cool night dissipate when the sun warms the ground the next morning. But, under certain conditions, such as a high-pressure area or a cloudless night, the temperature inversion can persist for several days.
Temperature inversions can also cause black ice through freezing rain. As snow moves through the warm upper layer, it can melt. When it reaches the cooler level, it becomes “super-cooled,” meaning it is cooled below freezing without becoming solid. The raindrops become ice when they strike cars … or the road.
They can create hazardous conditions for travelers and pedestrians alike, so it’s important to keep an eye out before stepping out onto unknown wet surfaces.
More Risks Than Just Slipping
Temperature inversions can also contribute to smog and poor air quality. Because the warmer layer of air acts as a lid, pollutants produced by vehicles, fireplaces and other sources close to the ground remain trapped near the surface of the earth, raising the air quality index (AQI) level.
Temperature inversions can have other interesting effects, like magnifying sound. If an explosion or sound occurs near the ground, the soundwave travels up to the warmer upper level and bounces back to the ground, traveling much farther than normal. Consequently, an airplane taking off or a train rumbling across the landscape sounds much louder than normal.
Like other natural phenomena, temperature inversions can turn everything upside down, and result in surprises.
As part of our commitment to safety, we encourage you to do a double take and check for potentially slick conditions caused by temperature inversions before you step out the door into seemingly “warm” winter weather.