The height of clouds in the sky affects sunlight, heat and much more.
On a road trip from Phoenix to Washington DC, you may notice the sky falling - or at least the clouds. We often admire the shapes and sizes of these cloud balls, but the height of these clouds can be equally dynamic, changing depending on the region and season. Humidity, temperature and topography play a role in how close we are to the clouds in every sense of the word.
“Above land, it mainly depends on how humid the air is,” says Roger Davies, a climate physicist at the University of Auckland in New Zealand. At low altitudes, fluffy, cotton-like cumulus and dark gray nimbostratus clouds adorn the sky, changing daily. Higher in the atmosphere, the clouds become thinner and finer, like cirrus clouds.
But beyond what they look like from our perspective on the ground, their altitude has a bigger impact on our day: it can determine the amount of sunlight and heat we experience. The thicker, low-lying clouds help keep solar radiation from reaching the surface, while the thinner, higher clouds trap the sun's energy like a greenhouse gas on Earth.
Scientists simulate the properties of clouds
In an effort to better understand how energy circulates on Earth, scientists simulate the properties of clouds in computer models. As the Earth warms, scientists are concerned about whether climate change is affecting the height of clouds. If so, this could affect the amount of sunlight and heat we perceive at Earth's surface. “Clouds are the master regulator of energy flow,” says Rachel Pinker, a meteorologist at the University of Maryland. “An important element is the height of the cloud base, because that is what the Earth sees.”
Pinker and her colleagues analyzed 15 years of cloud data from weather stations across the United States. They particularly examined the lower sun-blocking clouds (below 3,500 meters, or about 11,500 feet). The stations measured the height of the clouds from the surface daily by shooting a laser upward and measuring the time it took for the laser to bounce back.
Overall, it was found that relatively high clouds form in the western half of the country and in summer, while lower clouds form on the east coast and in winter. Clouds were more common on the East Coast and Pacific Northwest than on the Southwest and Central Plains.
Spring
On the humid east coast, clouds hang lower than on the ground. In the drier western parts of the United States, clouds hang higher in the air, making downpours visible from far away.
Summer
Warmer summer temperatures cause clouds to hang higher across the United States.
Autumn
As temperatures cool, the air can hold less moisture and contains fewer clouds.
winter
Cloud heights decrease the most in winter.
Clouds are made of liquid or solid ice water
Cloud formation has a simple physical basis. Generally, clouds are made of liquid or solid ice water. They form when water from the ground or elsewhere evaporates into the air as an invisible vapor. As this moisture moves to a colder temperature, the parcel of air becomes “saturated” with water (has 100 percent relative humidity). At this point, the water vapor condenses in liquid form onto tiny aerosol particles called cloud condensation nuclei.
“On the East Coast, humidity is higher and saturation occurs at lower levels,” Pinker said. “On the West Coast, on the other hand, the humidity is lower, so you have to cool it down a lot more. This means you have to go higher to reach the condensation.”
Accordingly, Pinker and her colleagues found that clouds were more common on the East Coast and Pacific Northwest than in the dry southwestern United States, where low humidity keeps clouds at bay. The East Coast, particularly the Mid-Atlantic and Great Lakes regions, had the most clouds during the winter. In the Pacific Northwest, most clouds formed in the spring.
While the East Coast is generally cloudier, some of the lowest clouds in the country form along the California coast. These coastal clouds also look different than the East Coast puffballs, which are heavy on stratus, stratocumulus clouds and fog. These clouds are associated with cooler atmospheric conditions at lower latitudes, according to climate scientist Rachel Clemesha.
How does warming of the atmosphere affect the height of clouds?
Normally, the temperature drops in higher regions of the atmosphere due to pressure changes, Clemesha said. In some places, such as B. on the California coast, however, warmer air lies above cooler air - a so-called temperature inversion. This is partly because the cooler air comes from the North Pacific, increasing the stark temperature contrast.
“These clouds are more like a blanket because they hit this temperature inversion and the air can't flow any further up. They're trapped there like a lid,” said Clemesha, a researcher at the Scripps Institution of Oceanography. “They don’t usually have a puffy top.”
Researchers aren't sure how warming the atmosphere will affect the height of clouds - or how clouds will affect the climate. On the one hand, a warmer atmosphere can “hold” more moisture, so there could be more low-lying clouds. These could reflect more energy into space and thus have a cooling effect, says Pinker. On the other hand, higher temperatures could push clouds higher where they could trap heat on Earth.
Cloud heights have decreased worldwide, a NASA satellite shows
"Which of the two possibilities is more dominant and what is the ultimate impact of warming?" Pinker said. “That’s the mystery.” But researchers are working to find an answer. A study has shown that cloud heights measured by a NASA satellite have decreased worldwide because fewer clouds have simply formed at higher latitudes. However, Davies, the lead author, said more long-term data is needed to determine whether this trend is due to climate change.
If this isn't due to climate change, Clemesha and her colleagues have shown that cloud heights are shifting due to other changing conditions - such as urbanization. In cities, building materials (such as concrete and asphalt) absorb more heat and increase the temperature in the city, especially at night. The team found that cloud heights in Los Angeles and San Diego increased with temperature and became less frequent at night.
If cloud heights change, it could impact fire conditions in certain parts of the country like California. For example, lower clouds could provide more moisture to water-scarce vegetation and reduce the risk of fires, Clemesha said. “We know the basic physics of why [clouds] are where they are and some of their properties,” says Davies. “What we don’t know about clouds is how they actually feedback on the climate system.
We are currently testing machine translations. This article was automatically translated from English into German.
This article was first published in English on September 11, 2023 at the “Washingtonpost.com” - as part of a cooperation, it is now also available in translation to readers of the IPPEN.MEDIA portals.