The clouds that matter for VFR pilots

The World Meteorological Organization (WMO) describes ten cloud genera in its International Cloud Atlas. For VFR flying you should know six cloud types especially well.

Ten genera on three levels

The WMO distinguishes ten cloud genera. It divides the troposphere into three levels: high, middle and low. The levels overlap, and their limits vary with latitude.

Level	Genera	Height in temperate regions
High	Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs)	5 to 13 km
Middle	Altocumulus (Ac), Altostratus (As), Nimbostratus (Ns)	2 to 7 km
Low	Stratus (St), Stratocumulus (Sc), Cumulus (Cu), Cumulonimbus (Cb)	surface to 2 km (6,500 ft)

Most clouds stay within their level. There are three exceptions:

• Altostratus is usually found in the middle level, but it often extends higher.
• Nimbostratus is almost always found in the middle level, but it usually extends into the other two levels.
• Cumulus and cumulonimbus usually have their bases in the low level, but their vertical extent is often so great that their tops may reach into the middle and high levels.

Six clouds that matter for VFR flying

The description comes from the WMO Cloud Atlas, the effect on the flight from the FAA Aviation Weather Handbook.

Cloud	How to recognise it	What it means for the flight
Cumulonimbus (Cb)	heavy, dense cloud of considerable vertical extent, like a mountain or huge towers	contains nearly the entire spectrum of flying hazards, including extreme turbulence
Towering cumulus (TCU)	cumulus of great vertical extent, called cumulus congestus by the WMO	very strong turbulence and some clear icing above the freezing level, the first stage of a thunderstorm
Cumulus (Cu)	detached, generally dense clouds with sharp outlines, rising as mounds, domes or towers	with little vertical development some turbulence and no significant icing
Stratus (St)	generally grey layer with a fairly uniform base	little or no turbulence, but hazardous icing at temperatures near or below freezing; with fog or precipitation it becomes troublesome for visual flying
Nimbostratus (Ns)	grey layer, often dark, made diffuse by more or less continuously falling rain or snow	very little turbulence, but a serious icing problem at temperatures near or below freezing
Lenticular cloud (altocumulus lenticularis)	patch in the shape of a lens or almond, often very elongated	a sign of mountain waves and strong winds, with often moderate to severe turbulence beneath the clouds

Thunderstorm clouds: cumulonimbus and towering cumulus

A thunderstorm is always produced by a cumulonimbus cloud. It is always accompanied by lightning and thunder, usually by strong gusts of wind and heavy rain, and sometimes by hail. Because of lightning and thunder, the cloud is popularly called a thundercloud.

According to the FAA handbook, every thunderstorm has hazards for aviation, though not every one has all of them, and you cannot tell from the outside which it contains. They include low ceiling and visibility, lightning, adverse winds, downbursts, turbulence, icing, hail and rapid altimeter changes. Thunderstorms are usually too tall to fly over and too dangerous to fly through or under. The handbook advises: avoiding thunderstorms is the best policy.

In the METAR the cloud type is reported only for cumulonimbus and towering cumulus, abbreviated as CB and TCU. TCU is the ICAO term for cumulus congestus, a cumulus cloud of great vertical extent.

Lenticular clouds and mountain waves

In stable air, strong wind across a mountain range must rise on the windward side and descend along the downwind slopes. The disturbed airstream then oscillates: mountain waves form, and in moist air clouds appear at the wave crests. The WMO calls lenticular clouds an indication of strong winds in stable middle levels of the atmosphere. They do not produce precipitation.

According to the FAA handbook, the clouds indicate the position of the wave crests, but they do not necessarily tell you how intense the turbulence or the updrafts and downdrafts are. They are visual proof that mountain waves exist, but may be absent if the air is too dry. Pilots frequently report moderate to severe turbulence beneath the clouds. According to the WMO, rotor clouds indicate severe turbulence at or near the surface and present a hazard to aviation.