If you’ve ever tuned in to hear your local weatherman, checked the Internet or watched the skies before shoving-off—and every boater should—you’ve heard about weather fronts.

That’s because fronts—defined in layman's terms as a transition between two air masses—produce the kind of unsettled skies and rocky seas that can make life difficult for boaters.

Understanding how these weather fronts work, and their impact on clouds, rain and wind, is perhaps the best way to stay ahead of the weather. While nobody can predict Mother Nature’s whims all of the time, understanding how fronts interact will give you some indication of the weather heading your way and how to prepare. At a minimum, you will know when to break out the foul weather gear—or when to head for port.

COLD FRONTS

The air that surrounds the Earth is always changing. Air masses of differing temperature and humidity are in constant motion, and not all of them readily mix with one another.

When two dissimilar air masses meet, hot air rises above cold air causing the formation of a storm front. This front, while represented by a single line on a weather map, is actually a transition zone that can stretch from five to 60 miles. Inside the front, heavy cold air masses advance at speeds ranging from 10 to 50 knots—faster in winter and slower in summer—while warm air masses advance more slowly at speeds of 12 to 15 knots.

When a cold air mass pushes against a warm air mass, it’s called a cold front. The cold air forces the warm air skyward, where it cools and produces rain. Cold fronts are most likely to produce thunderstorms, including dangerous squalls that can appear from 100 to 300 miles ahead of the front itself. (For a story on thunderstorms, see link below.)

While no one can predict the weather with 100 percent accuracy, there are patterns that are useful to know. Normal air circulation in the northern hemisphere travels clockwise, so the wind on the western side of the warm air mass moves south or south-east. On the eastern side of the cold air mass, the wind will generally move north or north-west. Weathermen call this “anti-cyclonic” movement.

When a cold front strikes, it’s marked by a series of events that evolve over a 24-hour period. They begin as wind picks up from the south or southwest. As barometric pressure begins to fall, clouds will sink lower in the sky, thunderheads will form and wind and rain will intensify.

As the front passes, winds shift rapidly to the west or northwest; strong gusts blow through and the barometer reaches its lowest reading. When the front has passed, clouds will clear and the barometer will rise rapidly. Meanwhile, the temperature will drop and the wind will become steady from the west or northwest. After a cold front passes you can usually count on several days of good boating weather, particularly for sailboats.

WARM FRONTS

When warm air pushes against a colder air mass, a warm front is formed. As warm air rises, clouds form and precipitation falls, primarily in advance of the front.

Because warm fronts move more slowly, they can spread over a longer distance—up to 1,000 miles—and last up to 48 hours.

The first sign of the front is the presence of high cirrus clouds. A succession of lower clouds will follow as the barometer falls. The appearance of cirrocumulus clouds—sometimes called a "mackerel sky"—indicate that the warm air mass is unstable and the possibility of heavy rain and thunderstorms increases.

As the front passes, the skies clear, the temperature rises, and the winds veer, usually to the south or southwest.

OTHER ACTIVITY

Occasionally fronts stall, causing them to hang in one place. These stationary fronts behave much like mild warm fronts, often producing extended rain along their length. Winds on either side of the front will usually blow in opposite directions. Sometimes stationary fronts start moving and evolve into warm or cold fronts.

Since warm and cold fronts move at different speeds, it is also possible for a cold front to catch up with a warm front. This produces what’s called an occluded front and results in rain on both sides of the front. Occluded fronts can lead to another weather front known as an extra-tropical cyclone, which can produce severe weather.

It’s important to remember that fronts are always moving. By tracking these movements in relation to your boat, you will have some indication of what to expect. The best way to stay ahead of the weather is to keep an eye on the west. If it’s happening there it’s probably coming your way.