Clues in the clouds
An understanding of what lies ahead may be written in water vapor.
Ominous mammatus fill the sky above AVL (Asheville NC) above ragged scud. Mammatus often appear in the upper regions of cumulonimbus clouds and indicate the potential for severe storm conditions.
The cold also limits the moisture, and therefore these high clouds are normally very thin. Although small vertical currents can generate cumulus development, these cirrocumulus clouds—also known as a mackerel sky when the clouds appear in long rows—are less common than the translucent coverage generated by cirrostratus.
Cirrus clouds are similar to cirrostratus, but tend to occur even higher, and in strong winds aloft, where the individual clouds are drawn out into wispy tails that provide a good indicator of turbulence at that altitude.
Midlevel clouds occur between about 6000 and 20,000 ft and are usually thicker than those found at higher levels. Middle clouds are given the prefix "alto," and again are separated into stratus and cumulus.
Both cloud types tend to have widespread coverage, but altocumulus will have a more mottled appearance, while altostratus is a bit more uniform and may be thin enough to allow a "watery" sun to appear through it.
These clouds indicate a good amount of midlevel moisture, and the presence of cumuli, especially in the morning, means that the vertical motion of the air could result in scattered thunderstorms later in the day. Similarly, altostratus often precedes warm fronts and widespread precipitation as the bases lower.
Low clouds—those below about 6000 ft—are generally the richest in terms of moisture and therefore tend to be thickest. Although the prefix for clouds at this level is "strato," it is not used only with cumulus clouds at this level. However, not all cumuli at this level are called stratocumulus.
That term applies to widespread, almost continuous coverage by low cumulus. Precipitation rarely falls from these clouds, but they are often the remains of larger cumuli, such as cumulonimbus. When the cumuli are larger or more spread out, they are simply cumulus clouds.
(They will probably have a secondary name to further distinguish them from other cloud types.) Low stratus will almost always appear as a solid gray overcast, often fairly close to the surface. These clouds are usually caused by moist air flowing in at low levels or cool air flowing over a warmer surface layer. They often follow altostratus in a runup to precipitation from a warm front.
The final primary classification of clouds is according to whether they are producing precipitation. When stratus clouds become thick and moist enough that rain, snow or ice pellets fall from them, they are called nimbostratus. Cumuli are treated a little differently. A cumulus cloud that produces precipitation is not automatically a cumulonimbus. Convection currents are often able to pump tons of water vapor tens of thousands of feet into the air.
This means that what begins as an ordinary cumulus cloud at 2000 or 3000 ft may quickly grow into a cumulus congestus (or towering cumulus), and eventually a cumulonimbus. Cumulus congestus will often produce showery precipitation. It is differentiated from cumulonimbus in that a cumulus congestus has a distinct cumulus top.
Cumulonimbus clouds (Cb for short) will lose that well-defined top as upper-level winds shear the cloud. The shear may be coupled with the cloud flattening out as it reaches the temperature inversion at the base of the stratosphere.
This incus, or anvil top, spreads out downwind and is a clear identifier of a Cb. Cb often also exhibit other cloud characteristics, such as mammatus (pouchlike protrusions beneath the anvil or rear flank), a wall cloud beneath the main updraft core, or roll clouds out ahead of and beneath the main cloud—all of which indicate severe weather.
What they mean
"Red sky at night..." Reddish hues of the setting sun reflecting off the clouds overhead are generally indicative of improving weather in the middle latitudes, where the atmospheric flow is from west to east.
Widespread high clouds are often precursors of an approaching front. Moist air is being lofted by the front and carried out ahead by the faster upper-level winds. Due to their altitude, the red and orange rays of a rising or setting sun often reflect off the cloud bases, giving the sky an overall reddish hue.
Since midlatitude weather systems almost always approach from the west, this characteristic gave rise to the rhyme "Red sky in morning, sailor's (or shepherd's) warning. Red sky at night, sailor's (or shepherd's) delight." Red light from a sunrise reflecting off high clouds to the west usually signaled that the weather would worsen, while red light scattered from easterly clouds meant the threat was past.
A similar night-time saying—"Ring around the Moon, rain soon"—referred to the ice crystals of cirrostratus cover at night creating a halo around the Moon, which indicated the presence of moisture moving into the area. Cirrostratus is often the first indication of an approaching warm front. At any time, widespread high-level clouds moving in from the west often signal that the weather is likely to deteriorate over the next few days.
If high-level clouds give way to midlevel clouds, this is another indication that the weather is likely to worsen. The same goes for a transition from mid to low-level clouds. The thickness and extent of the clouds at each level can give you a pretty good idea of how much moisture (and thus latent energy) is present in the atmosphere.
If you are used to reading the clouds and knowing what the next phase will likely be, you can get a pretty good idea about the degree to which conditions may deteriorate.
While all clouds are categorized by the cloud classification scheme, some are more unusual than others. For example, altocumulus lenticularus are, as the first name implies, midlevel clouds produced by vertical motion. The second name is the one by which they are more commonly known—lenticular clouds.
They are normally midlevel in that they occur over and downwind of the tops of mountains and ridges as the air is forced into turbulent waves. The upward motion of the wave cools the air, creating the cloud. As the air descends after the wave crest, it warms, re-evaporating the liquid droplets.
This gives the cloud its lens shape, which appears stationary despite strong winds. It also indicates the potential for severe turbulence. Often, the wave creates a horizontal vortex beneath the wave crest. As the spinning air is dragged upward, the vortex may produce a rotor cloud, ragged and spinning. Rotors show clearly the presence of dangerous windshear and turbulence.