Tuesday, August 31, 2010

Hiker's Guide to Clouds

Storms can approach suddenly, especially in the mountains, but more often than not the skies are bad at bluffing and they will tip their hand if you know their tells. Innocuous high clouds that barely catch the eye can in fact be the leading edge of a major front, or just innocent little puffy clouds. Knowing bad weather is on the way is important but does not necessarily mean it can be avoided. Weathering the storm is equally important to having a safe backcountry experience.

Cloud Types

Clearing Storm over the Eastern Sierra
Clouds fall into two basic categories, sheet clouds called Stratus clouds and puffy cumulus clouds. Clouds are formed when pockets of air reach 100% relative humidity. The type of cloud that is formed though is dependent on the local atmospheric conditions. Even though there is complex meteorology going on it's simple enough to remember a few basic types to watch out for. Clouds are effected not only by atmospheric conditions but also by topography. The reason storms seem to approach faster in the mountains is because steep topography raises the relative humidity intensifying storms, and creating clouds. Often valleys behind mountains are said to be in their "rain shadow" because the mountains will squeeze so much out of the storm when it reaches the valley, with a lower relative humidity, there is nothing to release. This is why for example the Owens Valley on the eastern side of the Sierra Nevada mountain range is so dry, the 10,000ft difference between peak and valley leaves little for the Owens valley in terms of precipitation.

Cumulus & Cumulonimbus

Cumulus clouds are typically fluffy, gentile clouds which roll harmlessly individually across the sky. When we go to draw a clouds, we are drawing cumulus clouds. They are well spaced from each other, lightly colored, and low to the ground. They are not indicators of foul weather. Though they may cause some precipitation if they become especially dark it is uncommon and typically will not last long. However the most dangerous cloud of all is a variation of the cumulus. The cumulonimbus or "thunderhead" billows up high into the sky like a giant cauliflower. It may taper near the top becoming wispy and anvil shaped. These clouds may nest in beds of other clouds, or in especially severe storms may form in groups. The cumulonimbus is large, darkly colored and the base with a puffy white top, it releases heavy rain, hail, thunder, lightning and even tornado's in certain parts of the united states. Because they often "nest" in the middle of storms of other clouds it is not safe to assume that because you do not see them, there is no chance of thunder, their billowy tops may simply be obscured.

Stratus

Low, sheets of clouds. San Franciscans are well aquatinted with the stratus marine layer which moves in each night during the summer, the famous san francisco fog. It is the only cloud which comes to join us on the ground. On the east coast and abroad these clouds can also form in flat areas during times of intense humidity. Typically these clouds move in, or form slowly, however in certain environments they can move in very quickly. On my Murietta Falls hike this winter a bank of Stratus clouds moved in within 10 minutes totally surrounding us. Though they may bring cold air they typically will not bring precipitation especially low to the ground. Higher sheets of clouds may be nimbostratus, or altostratus clouds.

Cirrus

High layered clouds, silky and whispy looking. They are sheets of regularly spaced small globular "cloud-lets". Though harmless themselves they are the best indicator of strong incoming weather as they run in the front of storms. They are by far the easiest way to identify an incoming storm. Recognizing these clouds typically gives you 2-6 hours of warning of an oncoming storm. They are followed by similar looking but lower formations of Cirrostratus and Altostratus clouds. Thickening altostratus clouds are the final warning before rain. Cirrus clouds also trail a storm, so after a day of rain, cirrus clouds can be the signal the end is upon you. They won't tell you the severity of the weather but you can expect at least gusty winds shortly after their arrival.

Nimbostratus

Rain clouds. When featureless, black cloud formations descend upon you, rain, snow or hail will follow. While potentially any cloud formation may yield some form of precipitation these mid-layer monsters are what we think of as storm clouds. They can have Cumulonumbus clouds stuck in them as well which will also unleash Thunder and Lightning. Nimbostratus clouds typically persist for awhile.



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