On a boat, the weather becomes your most constant companion, dominating your days and nights, your moods and comfort.

So what happens when you are in the middle of a long passage and your state-of-the-art electronics offer conflicting predictions, or a single forecast covers a 1,000-mile swath of ocean? With a look to the sky, you can begin to decode the behavior of your seemingly unpredictable escort.

While all sailors can spot a squall on the horizon, a true cockpit meteorologist will be able to predict changing weather days in advance. The key lies in the clouds.

Clouds form whenever warm, moist air encounters cool, dry air. This can happen when air heats up over land or water. The warm, moist air rises, cools and condenses to form clouds. It can also happen when a warm air mass encounters a cold air mass, creating a front.

LIN CHAMBERS/S’COOL/NOAAPThick sheets of altostratus clouds usually mean rain may be coming soon.CLOUDS AND FRONTS

Because warm air is less dense than cold air, when a warm air mass overtakes an area of cold air, it rises up and over the cold air in front of it. As the warm, moist air rises it cools, forming clouds and eventually rain. The incoming air mass creates an elongated wedge of warm air above the cold air. As the wedge of warm air passes overhead different clouds will develop depending on the height of the boundary between the warm and cool air.

If you can identify the different clouds associated with the warm front, you will be able to predict the changes in weather.

Cold fronts, on the other hand, form when a mass of cold air overtakes a mass of warm air. In the mid-latitudes, cold fronts often follow right behind warm fronts. Because cold air is denser, it forces itself under the warm air, causing an abrupt change in weather. While warm fronts might linger and slowly progress, cold fronts swiftly force the warm air up causing rapid condensation, convection and cloud building. Even though cold fronts pass through much more quickly, the clouds will still offer telltale signs of its approach.

If you have watched the clouds progressively thicken and drop in elevation, you will have a good idea of how fast the front is approaching. When you see the thick sheets of altostratus (see photo above), it is time to snug down the hatches as the rain is coming soon. Altostratus form at middle- to low-elevation and are thicker and darker than their cirrostratus cousin, so much so that they can obscure the sun at times.

NOAAWhen the top is broad and looks wind swept, it is called an anvil.Again, altocumulus clouds precede the front, but by only 120 miles or so with a cold front. These gradually thicken and lower to become a dark band of towering cumulonimbus. Sometimes the cumulonimbus are obscured by lower-level clouds, but if you can see them you will observe dark, thickly packed balls that appear to be churning or mushrooming and have defined edges.

While the bottoms are dark and ominous, the tops can be brilliant white in the sunlight. When the top is broad and looks wind swept, it is called an anvil (see photo). An anvil is a sign that the cloud is dying and gusts will be more pronounced. Cumulonimbus form with cold fronts because the temperature differential between the warm air and the incoming cold wall of the front cause significant updrafts that rapidly cool and condense as they rise.

Squall lines of cumulonimbus can precede cold fronts by several miles, and a line of dark, towering cumulonimbus on the horizon indicates it is time to shorten sail and don foul weather gear. The size of the cloud, color and definition of its edges offer clues to how much wind and rain are coming.

Directly preceding the front are stratus clouds, becoming nimbostratus. Stratus are uniformly sheetlike with no shape. They are low and gray, and while they might emit a little drizzle, the real rain comes with the darker nimbostratus clouds that come with the passage of warm fronts.

As the warm front passes, the rain often decreases, cloud cover begins to break and the wind shifts. However, the weather often does not clear, and the humidity is still high.

Because of the way lows form, a cold front usually follows close on the heels of a warm front. Unlike warm fronts, cold fronts pass through quickly and with greater severity. While cold fronts can be violent, they are often followed by clear skies, cooler temperatures, a decrease in humidity, a wind shift and often sharply defined cumulus clouds that give way to softer, more cottony fair weather cumulus.

DOUG STODDARD/S’COOL/NOAA

RALPH F. KRESGE/NOAA On warm days in the tropics, fair weather cumulus, top, can grow vertically (above) and become squalls by the afternoon (below).

NOAA

TROPICAL SQUALLS

However, as anyone who has sailed in the tropics knows, squalls and cumulonimbus clouds do not have to accompany cold fronts but become one of the regular features of life on board. In an anchorage, you find yourself planning your daily tasks and dinghy runs around afternoon squalls, and at sea, the afternoon and late night watches often seem to need an extra reef and foulies.

Like clockwork, a beautiful clear day can turn cloudy by noon and yield passing squalls in the afternoon. Squalls form when there are unstable weather conditions caused by significant temperature differences in the atmosphere as you go aloft. Obviously cold fronts bring these kinds of temperature differences, but so do crystal clear days in the tropics.

On days without cloud cover, there is no insulating effect to keep the heat in the atmosphere, so as you move aloft, the air gets drier and cooler. Couple cool, dry air aloft with moist air at the surface heated by the warm ocean, and you have an unstable atmosphere that is perfect for squalls.

The warm air rises and condenses as it encounters the cooler air at higher elevation. As the warm air continues to rise, it creates a tall cloud. Eventually, the condensation becomes precipitation. As the rain falls through the drier air, it cools the air in the cloud. This dense, cool air sinks with the precipitation, causing wind and rain.

Because air at the surface must be warmer than the air above it, squalls are common in the afternoon when surface water is at its warmest and at night when the ocean keeps the air at the surface warm after the air aloft has cooled.

A couple of tips to keep in mind as you forecast squalls from your cockpit: Taller clouds will release more energy, and the larger the temperature drop before the squall, the more wind to expect. Because clouds are the simple result of warm air aloft condensing, they form in a variety of circumstances. The key to using clouds to decode the coming weather is to pay attention to the trends and distinguish between cottony clouds with soft edges and dense clouds that are thickening or growing, so you don’t get stuck under one of these monsters!

CARL WOZNIAKCirrus clouds such as these are the first sign that some weather might be coming your way. Cirrus form at 45,000 feet and are composed of ice crystals. Often they precede a warm front by 500 to 600 miles. As cirrus clouds mix with the leading edge of a warm front the clouds develop hooks that look like tendrils falling toward the ground.		DOUG STODDARD/S’COOL/NOAACirrostratus clouds might look like wispy sheets such as these, or they may be much thinner as a barely distinguishable haze or a halo around the sun or moon. Cirrostratus form slightly lower than cirrus clouds and are the next phase in an approaching warm front, preceding the front by 300 to 400 miles. If you track the change from cirrus to cirrostratus clouds, you can tell how fast a warm front is approaching.
RALPH F. KRESGE/NOAACirrocumulus clouds look like barely defined puffy balls at high altitude and can resemble fish scales. They are an indication of an approaching front, and as such have earned the rhyme, Mackerel sky, not long wet, not long dry, suggesting the changeable weather.		LIN CHAMBERS/S’COOL/NOAAPAltocumulus clouds can form in fair weather, or they can indicate the approach of either a warm front or cold front. The clue lies in whether the clouds are thin and light or are thickening and dark, and what sequence of clouds, if any, preceded the altocumulus. A distinct edge, like in the picture, is usually associated with a front.