Nature’s weather indicators are invaluable to sailors
A cloud, as the venerable Mr. Webster so succinctly informs us, is nothing more than “a visible mass of condensed water vapor suspended in the atmosphere, consisting of minute droplets or ice crystals.” It’s not the most romantic description, but it’s accurate.Water vapor is present in most air masses, but it’s invisible in its natural form. It’s forced to come out of hiding, and condense, when the ambient temperature falls.Air that flows into a stormy, low-pressure system is forced skyward into the cooler upper reaches of the atmosphere, where the water vapor must condense and form clouds. In this way, nature gives us a visible warning that a storm is on the way.Not all clouds are associated with bad weather, of course. Those little cotton-puff trade-wind clouds seem to ride along merrily overhead without ever obscuring the sun and those high, streaked clouds that glow red at sunset are harbingers of fine settled weather. But there’s no mistaking the ever-thickening, ever-lowering layers of dark gray cloud in the vanguard of a storm.Clouds are commonly divided into the following four groups:

• high clouds at and above 20,000 feet, including cirrus, cirrostratus, and cirrocumulus

• intermediate clouds between 6,500 and 20,000 feet, including altostratus and altocumulus

• low clouds, below 6,500 feet, including stratus, stratocumulus, and nimbostratus

• tall vertical clouds, such as cumulus and cumulonimbus

Clouds are named according to the way they look and how

Cumulus with little vertical extent

Cumulus with moderate vertical extent

high they float. They are often combinations of two or more types. The Latin prefix alto, for example, means high; stratus is the word for a layer. Cumulus means heaped, or fluffy. Cirrus refers to wispy tendrils or feathery tufts. Nimbus is the turbulent black rain cloud. Cumulonimbus is, therefore, the imposing thunderstorm cloud with the cauliflower-shaped top and the black menacing base. You’ll often see the top being blown away by high-speed upper-atmosphere winds into the shape of an anvil.Clouds are important weather indicators for mariners, especially on the open ocean or in areas where meteorological forecasts are not available. The cloud types, together with wind speed and direction,

Cumulonimbus with no anvil

Stratocumulus from spreading cumulus

Stratocumulus not from spreading cumulus

Stratus in sheet or layer

Stratus fractus/cumulus fractus of bad weather

Cumulus and stratocumulus at different levels

Cumulonimbus

Altostratus, translucent, sun or moon visible

Altostratus, opaque, sun or moon hidden

Altocumulus, semitransparent

Altocumulus, semitransparent, multilevel

Altocumulus in layers

Altocumulus from spreading cumulus

Cirrus filaments, strands or hooks, not expanding

Dense cirrus in patches

Cirrus, anvil remaining from cumulonimbus

Cirrus hooks or filaments, increasing

Cirrostratus covering whole sky

Anvil Clouds. When a mature cumulonimbus cloud rises to the tropopause inversion boundary, the top spreads out in an anvil. This nasty-looking cloud can bring heavy rain, hail, and thunderstorms, perhaps with downbursts.

Puffy Clouds (cumulus mammatus). Spawned from cumulonimbus, mammatus clouds indicate thunderstorm activity somewhere in the vicinity. If the clouds are large and headed your way, you may be in for some severe weather.

Wave Cloud. Formed by a wavelike transport of air into and out of an atmospheric layer wherein condensation occurs, these clouds, unlike roll clouds, do not augur squalls.

Roll Cloud. A roll cloud precedes a squall line, which in turn may precede a cold front by a hundred miles or more.

and a record of the barometer’s movements will provide a reliable picture of the weather to come for at least 12 hours.At sea in the tropics, it’s common for walls of cloud to pile up menacingly on the windward horizon, but more often than not they are harmless puffs of white cumulus clouds. The key to this illusion is that we have no way of knowing their true size or their distance from us, so our brains place them at equal distances along the curved “dome” overhead. However, our straight line of sight makes them crowd together at the horizon and acquire the appearance of vertical stacks.See also Air Masses; Weather Fronts; Downbursts.

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