December 2008Departments

The Weather Never Sleeps

The Secret Lives of Clouds


And knowing them when you see them

Learning to fly requires understanding scores of things, including a fair amount of weather theory and how to interpret observations and forecasts. The weather theory includes learning the names of various kinds of clouds.

Fair Weather Cumulus: Fair-weather cumulus clouds can thicken from scattered to broken, and you might not be able to descend to land without flying through them.
Towering Cumulus
Cumulonimbus
Shelf Cloud
Pyrocumulus
Mammatus

One of the key differences is between generally flat stratus clouds and piled-up, lumpy cumulus clouds. How this theory relates to actual flying isn't always clear. The photos here show a few kinds of cumulus clouds, close-ups of features of some cumulus clouds, and rain falling from a couple of cumulus clouds. All of these illustrate aspects of weather that you might not learn about in weather theory classes.

Fair-weather cumulus clouds are, as their name says, a sign of generally good weather. This is because temperatures above the cloud tops are too warm for towering clouds to grow-the atmosphere is not unstable enough. If you fly above the tops of such clouds, you'll have a generally smooth ride. Below the clouds, however, you'll be flying into and out of the rising air currents that are feeding humidity into the clouds.

If the view from the cockpit is like the photo, you need to be alert to the possibility that clouds below could thicken from scattered (covering three-eighths to four-eighths of the sky) to broken (covering five-eighths to seven-eighths of the sky); you might not be able to descend to land without flying through clouds, which would be unsafe and illegal for a VFR pilot. Clouds in the distance will look closer together than those under you, but if you have any doubts, contact a flight service station for an en route weather update.

All pilots, including those on instrument flight plans, should avoid towering cumulus clouds because they threaten turbulence. Such clouds form when the humidity in rising air condenses into cloud drops or ice crystals. Clouds growing high into the sky suggest fast-rising air and turbulence. The clouds on the right in the photo, with the cauliflower appearance, are made mostly of tiny water droplets, which could be supercooled-colder than 32 degrees F but still liquid. Updrafts are pushing the water droplets up so quickly they haven't yet turned to ice. Clouds with this cauliflower appearance hold the dangers of turbulence and airframe icing. Updrafts have slowed in the smooth clouds on the left and water droplets have frozen into ice crystals.

Cumulonimbus (thunderstorm) clouds should always be avoided by several miles. They are packages of dangerous weather: severe turbulence, airframe icing, hail, rain heavy enough to drown engines, dangerous ground-level wind, lightning, and even tornadoes. All cumulonimbus clouds don't contain all of these dangers, but you don't want to find out which hazards lurk in any particular cloud.

The shelf cloud in the photo the next page formed when cool air flowing out from a cumulonimbus cloud pushed up warm air flowing into the cloud. Winds flowing down and away from thunderstorms can bring rapid, dangerous wind shifts to airports miles away from the storm. A thunderstorm doesn't have to be as close to an airport as the one in the photo for its winds to be dangerous. When rain begins falling from a thunderstorm, some of the falling rain evaporates into the air inside the storm, cooling it. The cooled air is denser than the surrounding air and descends. When the descending air hits the ground it can travel for miles as a gust front that causes shifting winds and can trigger the formation of new thunderstorms.

Pyrocumulus clouds form over large wildfires when rising warm smoke carries enough humidity aloft to create a cloud when the humidity condenses. Water vapor can be a product of combustion, but fires seldom add enough vapor to create fire-dousing rain. In the photo at center left, smoke rising from a wildfire near Boulder, Colorado, is beginning to create a pyrocumulus cloud-notice the area on the right that's beginning to take on the cauliflower-like appearance of a fast-growing cumulus cloud. You should stay far away from this type of cloud. The cloud itself might not be particularly dangerous, but the FAA normally restricts the airspace around large wildfires; it could be needed by four-engine aerial tankers dropping chemicals on the fire, helicopters ferrying firefighters to remote locations, or maybe the parachutes of smoke jumpers.

Mammatus clouds protrude from the underside of a cumulonimbus cloud, often from the anvil that stretches out to one side near the top of such a cloud. They are not a sign that a tornado is on the way, but the air around them could be turbulent. In fact, they often form after a thunderstorm has produced a tornado, large hail, or damaging straight-line winds. Cool downdrafts falling into warmer air below create mammatus.

One of the joys in learning to fly is gaining new ways to see the world, including clouds-which you may not have really looked at closely until you were in the sky with them.

Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author of Hurricane Watch: Forecasting the Deadliest Storms on Earth.


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