Weather 101 - Cloud Formations
You wouldn't leave the dock without first checking the local weather forecast. You can get weather information from TV, radio, your VHF radio and on the Internet (see Safety Links above). While on the water, your VHF radio is the best source for weather warnings.
Even so, at certain times of the year weather can change rapidly and you should continually keep a "weather eye" out, especially to the west, in order to foresee changes which might be impending.
Clouds are a tool you can use to predict or forecast weather. The type of cloud and direction of movement can warn you of weather changes that are imminent. Clouds are categorized by the altitude at which they appear and the shape that they take.
(This is not an in-depth study of clouds, but an attempt to simplify the subject for use by recreational boaters.)
Cloud Group Cloud Height Cloud Types High Clouds = Cirrus Above 18,000 feet Cirrus
Cirrostratus
CirrocumulusMiddle Clouds = Alto 6,500 feet to 18,000 feet Altostratus
AltocumulusLow Clouds = Stratus Up to 6,500 feet Stratus
Stratocumulus
NimbostratusClouds with vertical growth Cumulus
Cumulonimbus
It is helpful to remember the following definitions of cloud shapes:
| Cumulus meaning "heap, a pile, an accumulation" | |
| Stratus meaning "spread out, flatten, cover with a layer" | |
| Nimbus meaning "rainy cloud" |
Variations of cloud types are created by combining the cloud's shape/description with the altitudinal names as a prefix or suffix.
Cirros (high) or Cirro can be used with cumulus (heap) to indicate a cirrocumulus or high, lumpy cloud. Cirrocumulus clouds, sometime called "mackerel skies," can indicate the approach of a hurricane in the tropics. It can also be used with stratus (flat, layered) as in cirrostratus to indicate a high, flat or layered cloud.
Alto can also be used with cumulus and stratus to indicate altocumulus and altostratus which are middle altitude lumpy clouds and middle altitude layered clouds respectively.
Nimbo or nimbus might be used with cumulus or stratus to indicate a cloud formation that is producing precipitation. These clouds could be either cumulonimbus which would be a lumpy, vertically-rising rain cloud or nimbostratus which would be a sheet or flat-looking rain cloud.
High clouds exist above 18,000 feet and are cirrus clouds.
| Cirrus clouds are the most common of the high clouds. They are composed of ice and consist of long, thin, wispy streamers. Cirrus clouds are usually white and predict fair weather. Sometimes called mares tails, they stream with the wind. By watching the movement of cirrus clouds you can tell from which direction weather is approaching. The appearance of cirrus clouds usually indicates that a change in weather will occur within 24 hours. |
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