Expert Advice

Reading the Clouds

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If you really want to know what kind of weather is coming your way, skip the radio and TV broadcasts and look up at the clouds. Even though forecasts are a lot more reliable than they used to be, this old-fashioned way is still valuable.

With today's sophisticated devices, including satellite imagery and Doppler radar, an international network of weather experts can keep an eye on almost every breeze that stirs the earth's surface. The National Weather Service crunches all that information on huge mainframe computers, creates many different kinds of short- and long-term forecasts and sends them out every four hours in formats that include voice, text, fax, and GRIB.

Watching clouds still plays an important role in the National Weather Service's forecasting protocols, and every year the NWS trains ordinary citizens to become part of its SKYWARN spotter network. They look for clouds that have potential for creating such dangerous local weather events as thunderstorms and hailstorms, tornadoes and waterspouts. Spotters report all significant cloud formations and surface windspeeds to NWS meteorologists, who analyze the data and broadcast weather warnings.

Clouds form whenever there is moisture in the atmosphere. The type of cloud, the amount of cloud cover, and the cloud's height above the surface give important first clues to future weather.

Luke Howard, a 19th-century British amateur meteorologist, was not the first person to name the different cloud formations, but his use of Latin terminology defused the international opposition to terminology in one living language or another. The academic and scientific communities accepted his efforts enthusiastically, and he holds a well-deserved place in the history of meteorology, Howard named four basic cloud types and several subtypes.

Cirrus ("hair" in Latin) describes those wispy high-level clouds that appear at heights above 16,000 feet. Cirrus clouds are often the first sign that a major weather system is on the way.

Howard used the Latin stratus ("layer") for cloud formations that lack any specific features except one: they form in layers at a specific altitude.

Cumulus ("pile" in Latin) clouds are probably the easiest to spot. While they can be any size, they are often large, with a vertical shape that has lumpy and irregular sides. Cumulus clouds can have a low-level base, less than 6,500 feet, but on a warm day with high humidity a cumulus can top out at heights above 20,000 feet.

Nimbus ("precipitating") is a perfect name for low-level clouds that have gray tones. The more moisture in a nimbus cloud, the darker it will be.
Howard used other prefixes to describe cloud heights more precisely. Alto, for example, indicates a mid-level cloud.

When you look at cloud formations, first determine the percentage of the sky covered by clouds; this can range from clear (no clouds at all) to overcast conditions (clouds cover almost the entire sky). In general, the greater the cloud cover, particularly if the clouds are cirrus or stratus, the higher the probability that there will be a significant deterioration in the weather.

Opacity, or cloud thickness, is another clue to the coming weather. Less opacity means more moisture, and if thick clouds are headed your way, there's a chance that some moisture could fall on you.

Observing and recording cloud development over a period of hours, or even days in the case of a large frontal system, and comparing this information with what you've observed in the past can help you improve your prediction skills, particularly for your local area. For example, when the sun heats the land, the air above it rises and forms cumulus clouds, cooler air over the water comes in to replace that air. The flow, which exists almost everywhere there's a land/water interface, is called a sea breeze.

The formation of cumulus clouds in the morning is a sign that air is rising, and you can be pretty sure a sea breeze will fill in several hours later. Maximum windspeeds are determined by the water and land temperatures; in general, the higher both temperatures are, the greater the potential windspeed.

When cloud cover prevents the sun from heating the land, the chance of a sea breeze filling in drops dramatically. Knowing what type of cloud is preventing the heating makes your prediction even more precise.

The opposite flow often takes place at night, especially during the summer. Winds will blow offshore from the land, which now is cooler than the water, resulting in a land breeze.

Cumulus clouds that have a nimbus, or gray tone, are carrying a lot of moisture; take it as a signal to get out the raingear. If the cloud is very dark gray, watch your deck gear and sail trim.

Sailors, pilots, and farmers have earned a reputation for being able to make accurate weather predictions. Much of the anecdotal know-how has, of course, been passed down over the years, and there have been plenty of bad calls along the way. Familiarizing yourself with the basic cloud formations and what they mean is not only fun, but can help you get to your destination safely.

When you know what clouds are on the horizon, you've got a head start on deciding whether to head for port or to keep sailing. And if you enjoy sailing in a breeze, you might even want to head for those approaching clouds. Whether you are sailing in the Caribbean or on a small lake somewhere in the Midwest, if there's a cloud there's convection. And where there's convection, there's usually wind.


NOAA has a new Sky Watcher chart available at;
www.weather.gov/os/brochures/cloudchart.pdf
The NASA/NOAA helpful Introduction to Clouds is at;
http://science-edu.larc.nasa.gov/cloud_chart

SOME OLD PROVERBS
These just might be true, though much depends on where you live.
» Mare's tails and mackerel scales make tall-ship captains take in their sails.
» The moon with a circle brings water in her beak.
» Rain before seven, stop by eleven.
» Red sky in the morning, sailors take warning.
» Moonlit nights have the hardest frosts.
» When smoke in clear weather rises vertically, the weather will remain calm.

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