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Clouds

Understanding the clouds will give you a head start in predicting the weather.

Introduction

Clouds are formed and named according to their height and shape. They are given Latin names which describe their characteristics, e.g. cirrus (a hair), cumulus (a heap), stratus (a layer) and nimbus (rain-bearing). It's an interesting fact that all clouds are white, but when viewed from the ground some appear grey or dark grey according to their depth and shading from higher cloud. 

  • high clouds - have bases above 18,000 feet
  • middle clouds - have bases between 7,000 and 18,000 feet
  • low clouds - have bases below 7,000 feet

High Clouds

These are primarily composed of ice crystals and include the following:

Cirrus are high altitude wispy clouds. They are usually quite thin and often have a hair like or filament type of appearance. The curled up ends as depicted in this picture are very common features.
Cirrocumulus are high clouds that have a distinct patchy and/or wavelike appearance.
Cirrostratus are high clouds that usually blanket the sky in ill-defined sheets. These clouds are usually optically thin and the sun and moon can usually shine some light through. Like other stratiform clouds, one usually can't detect distinct cells or sharp features. This picture shows the sun shining through a gray, diffuse cirrostratus overcast.

Middle clouds

These have many similarities to the cumuloform and stratiform high clouds. Since they are closer to a ground based observer, the cumuloform elements in particular appear larger than their high cloud counterparts. They can contain ice crystals and/or water droplets and may occasionally be associated with some light precipitation.

Altocumulus have distinct cloud elements and are either in a patchy, scattered distribution or can appear in linear bands. 
Altostratus have a more uniform and diffuse coverage where it is difficult to detect individual elements or features. In this picture a few altocumulus clouds in the foreground precede a more uniform deck of altostratus.

Low clouds

The are most often composed of water droplets, but can have ice crystals in colder climates. Some of these clouds can develop into the multi-level clouds and can go through various phases, such as, a morning stratus deck turning into late morning stratocumulus, then early afternoon cumulus, and vertical development into cumulonimbus which can produce heavy rain and possible lightning and thunder.

Cumulus are usually puffy and often have very distinct edges and usually a noticeable vertical development. They often have a popcorn-like appearance. Cells can be rather isolated or they can be grouped together in clusters.

Rain makes a fundamental difference of the wind characteristics of a cumulus cloud. The main reason for this is that the first rain to fall out of the base of the cloud evaporates into the air beneath and cools it, often by several degrees. This cooled air descends the more it is cooled the more rapidly it descends. Thus instead of air rising into a cloud we have no only rain falling out the cloud, but air as well. The drier the air beneath the cloud the more it is capable of being cooled by evaporation, and so long as there is enough rain coming out of the cloud the colder the air becomes. The cold air will literally drop out from beneath the cloud with the rain and spread out in all direction at the surface. The light wind that was moving in towards the cloud, suddenly becomes a squall rushing out and away from it. You can see the rain falling, often in grey streaks below the cloud sometimes in a dramatic arch of black cloud spreading out from the parent cloud.

Over the sea cumulus clouds are normally found in regularly spaced lines. The best examples of these are found in the trade winds where they extend for many tens of miles. These show up a pattern in the vertical movement of air which is like a horizontal roll. Between the lines of cloud you will find the stronger more gusty and slightly veered winds, and beneath the lines of cloud somewhat lighter and more backed winds.

Stratocumulus can be widely scattered (as depicted in this photo, but are usually concentrated closer together in clusters or layers and have very little vertical development. These relatively flat clouds usually lack the sharp edges and "popcorn" appearance of most normal cumulus clouds.
Stratus are usually the lowest of the low clouds. Stratus often appear as an overcast deck (as shown), but can be scattered. The individual cloud elements have very ill-defined edges compared to most low cumuloform clouds (e.g. cumulus and stratocumulus).

Multi-layer clouds

These are the heavy precipitation producers. The depth of these clouds give precipitation hydrometeors a better environment to develop and grow.

Nimbostratus are often included in many texts as low clouds, but here they are considered multi-layer clouds because their vertical extent often goes well into the middle cloud region and these clouds often have even taller cumulonimbus clouds embedded within them. The clouds are very dark, usually overcast, and are associated with large areas of continuous precipitation. If it's a gray and rainy day as shown in this photo, the sky will most likely be filled with nimbostratus clouds. (sounds like Dorothea!)
Cumulonimbus are the clouds that can produce lightning, thunder, heavy rains, hail and strong winds. They are the tallest of all clouds that can span all cloud layers and extend above 60,000 feet. her levels of the atmosphere. 

 

 

 

 

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