A Sailor’s Guide to Weather

Summarize

Weather is the present state of the air all around you (in contrast to climate, which is the long-term state). The main element of weather is wind caused by air moving from places of high pressure to low pressure. It moves in curved lines owing to rotation of the earth: the coriolis effect. This is why storm systems, such as hurricanes, rotate counterclockwise (in the northern hemisphere).

In high pressure places the colder and denser air is falling and absorbing moisture. These places have clearer skies, lighter or steadier wind, and more stable weather. In low pressure areas, the warmer and less dense air is rising and releasing moisture. These places have clouds, stronger or shifting winds often with rain, and unsettled weather. If you stand with your back to the wind, low pressure is on your left and high pressure is on your right (in the northern hemisphere). The boundary between two such places is called a front.

Fronts

A cold front is where a moving mass of cold air meets a mass of less dense warm air and pushes under it. The temperature drops sharply, the wind freshens and shifts dramatically, and there might be gusts or squalls. Clearer air follows once the cold front has passed by. On a weather map, the line marking the front will have triangles on one side.

A warm front is where a moving mass of warm air meets a mass of more dense cold air and rises over it. This tends to bring gradual warming, widespread layered clouds, and steady rain or drizzle as the rising warm air releases moisture. On a map, the front will have half circles on one side.

A stationary front between a warm and a cold mass of air that are not moving much usually brings days of clouds and rain, variable winds, and weather that is slow to change. On a map, the front will have triangles on one side alternating with half circles on the other side.

An occluded front is where two masses of air meet and the colder, denser air lifts the warmer air without sliding under it. This brings complex weather, often with rain and wind. On a map, the front will have triangles alternating with half circles on the same side.

High thin clouds—especially cirrus or cirrostratus—are often the first sign of an approaching cold front or, sometimes, a warm front. Growing puffy clouds can mean daytime heating, instability, sea breezes, and possible showers. Dark towering clouds predict rough seas with potential squalls, thunderstorms, sharp gusts, perhaps lightning. Low gray overcast means steady rain or drizzle with reduced visibility.

The high clouds include cirrus (wispy streaks or thin horsetails) that mean fair weather now but a change is coming, usually a cold front. If they thicken or multiply, expect the wind to shift and the weather to deteriorate. Also cirrostratus clouds (a white veil covering the sky and a halo around the sun) that mean a warm front is approaching.

Middle clouds include altostratus (a gray film covers the sky and dims the sun), which signal heavier conditions with rain and rising wind. Also altocumulus (cotton ball clusters or wavelike patterns). If they are seen in the morning, afternoon thunderstorms are likely. If they grow taller, expect gusty, shifty winds.

Low clouds signal a quick change bringing wind and rain. They include stratus (low, flat, gray overcast) that bring low visibility and light wind with fog and possible drizzle. Also stratocumulus (broken gray layer with gaps of blue showing) that signal moderate winds, often steady, and somewhat unsettled but generally stable conditions. Nimbostratus clouds form a thick, dark sky with steady rain and shifty wind as a front passes.

Vertical clouds are the most important to sailors. Cumulus clouds (puffy cotton balls) bring light to moderate breezes, often steady but with gusts under the clouds. Watch for them to grow taller into cumulonimbus clouds (tall towers with anvil tops) that bring sudden squalls, violent wind shifts, and sometimes lightning or hail.

If clouds are getting higher and thinner, a front is approaching and weather will get worse. If they are getting lower and thicker, the front has arrived bringing rain or storms. When cumulus clouds grow taller, afternoon thunderstorms or squalls are likely. When they are dark and uniform, expect a long period of rain with steady, shifting winds.

Standard Cloud Name Abbreviations

Cloud Shapes

Strong updrafts of warm air make the clouds grow taller as energy is stored. Under the right wind shear conditions, rapid vertical growth builds towering cumulus clouds with sharp tops and flat bottoms.

When the clouds form into cumulonimbus—thunderstorm clouds—rain starts to fall, thunder begins, and downdrafts join the updrafts. This drags air downward, cooling it and causing gusty shifting winds at the surface.

As rain continues, downdrafts begin to dominate and updrafts collapse. Thunder stops. Rain becomes lighter as the clouds begin to break apart. The barometer rises as falling cool, dense air displaces inrushing warm air and the storm is ending.

During hurricane season—the six months from June 1 to November 30—on average 14 tropical storms form in the Atlantic, half of which become hurricanes. The season builds to a peak in September and then declines. El Niño (warm surface in the Pacific) suppresses hurricanes by increasing wind shear; however, this makes any storms that do form more severe. La Niña (cold Pacific) has the opposite effect.

A hurricane has an eye at the center where the weather is sunny and mild with no wind. All around the eye, a violent eyewall of intense wind funnels warm, moist air upward. Spiraling rainbands of clouds and thunderstorms release energy as water in the rising air cools and condenses. This energy drives rising winds and a continuous supply of storm clouds until landfall. Able to grow to more than 1,000 miles in diameter, tropical cyclones can unleash not just high winds, the main concern of sailors, but also several feet of rain and storm surge, the major risk ashore.

Hurricane conditions are worst on the dirty side—the right front quadrant—where the hurricane’s motion and its rotation combine to move air in the same direction. Upon making landfall, these winds pull water from the ocean onto land, causing storm surge. Extreme wind shear—the differing speed of wind at various heights—can cause tornadoes.

There is usually ample notice that a hurricane is coming, often several days in advance. The best place for a sailboat to be then is hauled and blocked ashore. The next best place is at sea, using storm tactics under little or no canvas and well off of any lee shore. The worst place to be is at dock where lines should be doubled and storm surge allowed for. In any case, remove and stow below all sails (except storm sails if at sea), all canvas, and anything carried on deck, such as fenders. Lash the dinghy securely upside down on the foredeck.

With moderate wind shear, downdrafts are pushed away from updrafts and the storm both grows stronger and propagates. There will be repeated bursts of heavy rain and thunder with strong wind gusts and possible hail.

Strong wind shear blows the tops of the towering clouds away from the updrafts. The downdrafts on the outflow side of the cloud column then fall alongside the updrafts on the inflow side and do not neutralize them. The storm is then self-sustaining and might last for hours.