Aviation Weather: Clouds, Fog and Low Visibility We all know what a cloud is. We have been looking at them for most of our lives. The beautiful reds and oranges of a sunrise, the green flash of an ocean sunset, the foreboding of approaching dark clouds and the fear of a boiling, rolling cloud base on a thunderstorm. But can you identify the clouds and their classes? If not, then that is what this tutorial will teach. First we will talk about the classification of clouds. There is no snooty scientific verbiage involved here. Clouds are classified by what we see, their height, appearance, and/or origin. Let's take a look at height or altitude as it applies to clouds. High Clouds have bases of over 18,000 feet, while Middle Clouds lie between 7,000 and 18,000 feet, Low Clouds have bases below 7,000 feet and finally Fog is a cloud in contact with the ground. You may also hear of Multi-Level Clouds, which as you expect span several vertical levels and Orographic Clouds, caused by the interaction of winds aloft and mountainous terrain. As for appearance, there is virtually no limit to the words which could be used to describe the way clouds look. While this does not help with classification, origin can help, as fronts, troughs and other weather phenomenon have distinctive clouds associated with them. But since vertical structure and altitude are the most measurable of all the classification schemes, we will use it for this study. Most of the images used here are courtesy the Plymouth State College Meteorology Program Cloud Boutique. High Clouds - are composed mostly of ice crystals. The Cirrus are those high wispy clouds with curled edges. Cirrocumulus are high clouds with a wavy or patchy look. Cirrostratus are thin sheet-like clouds which often blanket the sky. Click on the image for an enlarged photo.
Middle Clouds - are similar to the cumuloform and stratiform high clouds. The are normally composed of ice crystals, some water droplets and may even produce light drizzle. Basically, they are high clouds dropped down closer to the observer and suffering the effects of the lower altitude. Altocumulus are fairly flat clouds, which can appear interlocked, as seen here, scattered, or in long bands like the cirrus seen above. Altostratus have a similar appearance to the cirrostratus, though they may be thicker. The sheets are fairly uniform with no distinct components. Click on the image for an enlarged photo.
Low Clouds - are composed mostly of water droplets, though they may contain ice crystals in colder conditions. These are also the clouds which can form the base for growth of multi-layer weather phenomenon. TheCumulusare the puffy white clouds so familiar during the summer months. Children, and adults, can often find a variety of images as the popcorn like clouds merge and grow. Stratocumulus can be scattered or merge to develop that flat sheet look common to stratus clouds. These, however, are lower than those above and do have slight vertical component. Stratus clouds are the lowest of the low clouds, often appearing as dark mottled overcast decks Click on the image for an enlarged photo.
Multi-Layer Clouds - are the heavy rain producers. Nimbostratus are sometimes considered as low clouds, but considering they can often grow to 10,000 feet or more, they are more appropriately multi-layer. These are the heavy rainmakers which often accompany fronts. Cumulonimbus are famous for their towering heights, anvil tops and deadly power. These are the thunderstorms. They can often punch upwards through the tropopause and into the stratusphere, up to 60,000 feet above sea level at time. Click on the image for an enlarged photo.
Orographic Clouds - are formed as winds flow against and over mountains. If those winds are not very strong, amazing Cap Clouds, like the one seen here on Mt. Rainier, can be produced. If the winds are stronger, the capped shape clouds are displaced from the mountain tops forming Lenticular Clouds, which look like flying saucers huddled near the mountain tops. Click on the image for an enlarged photo.
Fog - is basically stratus clouds which lower to the ground. When the fog "lifts," it rises from the surface as a deck of stratus clouds. Fog can also "burn off," or dissipate to leave clear skies behind. Click on the image for an enlarged photo.
Overall, clouds should not pose that big a threat to flight. Do you homework in pre-flight and make-sure you are licensed and equipped to fly into the weather conditions forecasts and there should be few problems. Fly into conditions you are not trained for and the story can be much different. Even the highest rated of pilots must also know when to turn back. In fact, that is how most of them survived long enough to reach the higher ratings. The only two cloud related conditions which might pop up on a pilot are summer thunderstorms and fog. The thunderstorms, while scattered and not forecast, are pretty easy to pick out from the other puffy white summer clouds and thus avoidable. Fog is also avoidable if you have done your homework. If there is any chance of fog at your primary destination, pick an alternate which should not have that problem and make sure you have fuel on board to make the alternate. If not, you may be stuck looking down on that left photo above, wondering where the airport is among the mountain peaks. |
