Today’s caption further explains images highlighted in our new feature: A Celebration of Clouds.
It may not be obvious to the naked eye, but events on Earth’s surface—such as a burning fire or a floating iceberg—can affect the development and shape of clouds in the sky. The connection stems from how these events influence the rise and fall of air masses.
In general, clouds form where the air is ascending. Air cools as it rises, and because
cold air holds less water, it quickly saturates and reaches the point of condensation as it cools in the atmosphere.
The most common way to get moist air masses to rise is to heat the ground with sunlight. But the energy to lift air can come from other sources too, such as from the heat of a volcano or a fire. On August 5, 2014, the Moderate Resolution Imaging Spectroradiometer on NASA’s Aqua satellite captured an image of a pyrocumulonimbus cloud south of Yellowknife, Canada. Sometimes called “fire clouds,” pyrocumulonimbus are the white, cauliflower-shaped structures capping the darker column of smoke. With the exception of the energy source—fire versus sunlight—these clouds are otherwise similar to cumulus that form on warm afternoons.
While cloud growth favors ascending air, the converse also applies. Clouds generally fail to form where the air is descending. On June 1, 2016, the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured this image of a void in the low stratus clouds over iceberg A-56 as it drifted in the South Atlantic Ocean.
The exact reason for the hole is somewhat of a mystery. It could have happened by chance, although imagery from the days before and after this date suggest something else was at work. Steve Palm, a research meteorologist at NASA’s Goddard Space Flight Center, thinks the iceberg could have disrupted the air flow or modified the atmosphere in such a way to cause the clouds nearby to dissipate. If an obstacle is large enough, it can divert the flow of low-level air around it. At the same time, air above and downstream of the obstacle converges and sinks.
“The sinking motion warms and dries out the air, causing a hole in the clouds,” Palm said. “It is a common phenomenon often caused by islands.“