Sunday, August 20, 2017

Reading Clouds

August 1, 2017 by · Leave a Comment 

reading clouds title

Clouds are signposts in the sky — if you know how to read them, they are the most accurate weather forecast tools available. They are so local they can predict storms that are half a mile away and may hit your area in the next 10 minutes. Where else can boaters get that kind of real time information?

cProfessional mariners and anglers spend years accumulating cloud-spotting wisdom, but all boaters can benefit by learning to identify key clouds. No memorization necessary — skippers can buy a laminated cloud card in many marine stores or create one (make sure to waterproof it). The cloud card illustrates major formations you are likely to encounter in local waters. There’s also a small book called Instant Weather Forecasting by Alan Watts (look for the fifth edition of this 1968 book). It succinctly explains what to look for and what the clouds you see are predicting.

For quick reference, isolated wispy white clouds high in the sky usually indicate fine weather for the next 24 hours. So do those billowy dense white clouds that look like cotton candy, but not for as long as a day. On the other hand, dark heavy clouds foretell dirty weather ahead.

The so-called “King of Clouds” is a thunderhead (aka cumulonimbus), a solitary formation that builds within sight of land on a summer afternoon. Its footprint is relatively small, but the cloud can move as fast as 30 knots, so while the sun may be shining overhead, the storm hammering a nearby area may soon be upon you.

Thunderhead clouds starts out innocently, like the fluffy cotton candy (cumulus) clouds, but stay alert. If a cloud suddenly starts expanding vertically like a huge billowing tower, it can quickly turn dangerous. The tipoff is when the top of the cloud flattens out to resemble an anvil and the bottom turns dark. You can tell if the storm is coming your way by observing that anvil top — it tends to lean in the direction the wind is blowing.

aIf you spot one of these bearing down, take cover immediately as the winds will dramatically increase in strength. Soon rain will beat down in buckets while lightning flashes and thunder drowns out conversations. In 20 minutes or so, the storm will have likely passed by, but you were just in one of the most dangerous weather situations you can face in a summer of boating.

Cirrostratus clouds create a solid wall of gray covering the entire skyline and indicate the possibility of rain. The same is true of altostratus clouds, which also herald an approaching warm front. Stratocumulus clouds bring on deteriorating conditions, nimbostratus clouds produce a steady rain over a large area, and altostratus clouds are streaky and forecast rain. Another indicator of a thunderstorm in the offing is the vertical buildup of altocumulus clouds.

Fog is a low-lying cloud at ground level (rarely higher than 50 feet). It blankets coves and harbors most frequently in the spring and usually burns off by midmorning. However, depending upon the direction of the wind, fog can sometimes last all day and cover a larger area.

Courtesy NOAA

Courtesy NOAA

By William C. Winslow

The author is the Division 5 – Staff Officer Public Affairs, First District Southern Region, for the U.S. Coast Guard Auxiliary, the all-volunteer, non-military arm of the Coast Guard, teaching boating safety education and conducting search and rescue operations. Visit http://cgaux.org/ to join the Auxiliary or for class information.

webPlus_web_green1   Cloud chart and lightning video

is Division 5 – Staff Officer Public Affairs, First District Southern Region, U.S. Coast Guard Auxiliary, and a freelance writer for marine publications. His work has appeared in Sailing, Wooden Boat, and Good Old Boat; he is also the author of Cat Boat Tales.

Comments are closed.