March 25, 2013
BY THOMAS A. HORNE (From AOPA Pilot, March 1997.)
We should all be weather-watchers, and not just on the days we plan to fly. Knowing the basic cloud types, their movements, their shapes, and a few other basic weather variables can give us rudimentary forecasting skills. On long cross-country flights, unforecast clouds are apt to appear, forecast ones may not show up, and you may have to fly in or close to clouds for extended periods of time (assuming you're instrument-rated and IFR-current, that is). Knowing your clouds can obviously help you to formulate critical in-flight decisions. Press on or turn around? Land now or divert to an alternate? Sometimes it depends on the clouds.
The World Meteorological Organization (WMO) - the people who gave us METARs and TAFs - have come up with hundreds of different cloud types, each one with an erudite Latin name. The National Weather Service's Cloud Code Chart lists 27, arranged according to altitude. Here we'll check out the most common.
Cumulus. We all know these. If you're with the WMO, there are dozens of Latinate cumulus clouds, but I think of them as coming in one of three varieties: good, uh-oh, and bad. Good cumulus clouds are found in high pressure and are scattered. Those with flat bases usually occur after a cold frontal passage and indicate a rough ride below the cloud base. It'll be turbulent in the clouds, too, with a good chance of clear or mixed icing when temperatures are between 0 and minus 20 degrees Celsius. Above, the air is usually smooth. The bases are usually around 3,000 to 4,000 feet agl or so, and tops seldom go higher than 8,000 feet agl.
Uh-oh cumulus (cumulus congestus, calvus, and capillatus, in order of development) are clouds that are building in height. These typically appear on the back side of a high pressure system or ahead of a cold front. Bases may be lower (especially east of the Mississippi); tops are higher and can rise well into the flight levels during the heat of the day. And don't try a VFR climb or descent around them. If a scattered condition exists, it may not stay that way long. Broken to overcast skies are the rule. Expect moderate turbulence and, when temperatures are right, icing in these clouds. VFR flight beneath these clouds may be possible but uncomfortably turbulent. The problem with uh-oh cumulus is that they can turn into â€¦
Bad cumulus, which implies stronger convective activity and thunderstorms. It may be difficult to maintain VFR; the sky ahead will become darker and the ride bumpier, and precipitation may start. If in the clouds, turbulence and icing will quickly become issues. Best to turn around and land. These clouds can move quickly and may be part of a complex of violent frontal weather. Stay on the ground until they pass, then try launching in the morning, when convective activity is less intense.
Pretend that you had to run a search on these clouds in some computer database and tried the following keywords: west winds; turbulence; clear ice; smooth on top; was VFR on top, now have to descend on instruments; why is it so dark?; wish I hadn't gone in this cloud; what's VA for this airplane?; when will this be over?; is my insurance paid up?
Stratus. Stratus clouds usually are precursors to a nearby warm front; post-cold frontal instability clouds; or marine- or lake-influenced onshore layers. You often hear that stratus clouds consist of shallow layers no more than 2,000 feet thick. But I've tried to work my way through a "thin," ice-laden stratus layer that went from 25,000 feet right down to 2,000 feet agl. The ride in stratus is usually smooth. If there's any ice, it ought to be of the rime variety. Precipitation can be continuous and widespread. Ceilings can go right down to the ground when the temperature is right on top of the dew point.
VFR-only pilots can fly underneath a high stratus layer with no problems - as long as the layer stays high. Just remember: you may be flying toward a warm front in the next hour or two, with lower stratus and precipitation.
Keyword search: southerly winds aloft; smooth air; onshore flow of air; warm front to the west or south. Neat feeling breaking out on top of a shallow layer after an instrument departure or seeing clouds part on an instrument approach, needles centered, and stratus fractus (scud) sliding by as the runway appears directly ahead (instrument-rated pilots). Wish I had an instrument rating because it's so smooth and I know I'd be on top in 5 minutes (non-instrument-rated pilots). Don't need sunglasses when you fly in direction of sun and below cloudbase; cloud shield can stretch for hundreds of miles; this must be coastal California in the summertime (all pilots).
Cirrus. By definition, these clouds are found between 16,500 and 45,000 feet. But even if you're flying turbine-powered airplanes, they pose few problems. Cirrus are composed of ice crystals, and temperatures are usually so cold that ice doesn't accrete. These clouds are the earliest harbingers of an approaching warm front; they can appear 500 miles or more ahead of the front. Good VFR conditions prevail, and turbulence is rarely a problem. Winds aloft can run from a variety of directions - but the westerly and southwesterly quadrants are favored directions below 12,000 feet or so. At higher altitudes, expect westerly winds. Beware of a cirrus shield that gets progressively denser. A high overcast may form, followed by lower layers that signal the front's approach.
Keywords: Learjet country; contrails; trouble in 2 days.
Finally, a serious word about flying in any type of cloud or visible precipitation. Please remember to turn on your pitot system heat before entering these conditions. Also, please, please be on the lookout for any signs of a drop in manifold pressure or any other indications of a loss of power. These are subtle warning signs that sneak up on pilots. It's a big clue that carburetor or some other type of induction system icing is forming. It's absolutely imperative to activate your carburetor heat (carbureted engines) or alternate source of induction air (fuel-injected engines) if these signs crop up.
The alternate air doors fitted to fuel-injected engines are usually designed with a spring release, sometimes backed up with a means of manually opening the doors. When the normal air source becomes restricted, the force of the induction air automatically pulls open the spring-loaded alternate air door(s). This can all happen without any pilot action, which can be an important safety feature. However, it's important to emphasize that some airplanes don't use the springed-door arrangement, relying instead on a manually-activated door system. Also, springed doors may fail to work properly. If they don't open and the pilot fails to open the doors manually (assuming there is a manual method), the engine(s) may quit because of induction air starvation. So if you fly fuel-injected airplanes, don't forget the alternate air, as so many do. You do know how your alternate air system works and where any alternate air controls are, don't you - not the alternate static source for pitot-static instrument air, mind you, but the alternate engine air source?
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