2.12 Marine Weather & Meteorology

Marine Weather and Meteorology

Weather is the one variable a small-vessel captain cannot control, only anticipate. You are not expected to be a meteorologist, but you must read the barometer, the sky, and the forecast well enough to make the go/no-go decision and to see trouble coming while there is still time to react.

Pressure and the barometer

Atmospheric pressure is measured by a barometer, in inches of mercury or millibars. The trend matters more than the reading:

• A falling barometer signals an approaching low and deteriorating weather; a rapid fall warns of strong wind soon.
• A rising barometer signals building high pressure and improving weather.
• The steeper the pressure gradient (isobars packed close together on a weather map), the stronger the wind.

Highs and lows

In the Northern Hemisphere, air spirals clockwise and outward around a high - generally fair, settled, sinking air - and counterclockwise and inward around a low - rising air, cloud, wind, and precipitation. Buys Ballot's Law gives a quick fix: stand with your back to the wind, and the center of low pressure lies to your left. Knowing where the low sits tells you how the system will track and how the wind will shift.

Air masses and fronts

A front is the boundary where two air masses of different temperature and moisture meet:

• A cold front moves fast and steep; cold air shoves under warm air, throwing up a line of cumulonimbus, squalls and thunderstorms, then clearing to cooler, drier air behind. The wind veers (shifts clockwise) as it passes and the barometer, having fallen, begins to rise.
• A warm front advances slowly, warm air riding up over cold; it brings lowering cloud - cirrus, then stratus - and prolonged, steady rain, followed by warmer, muggier air.
• An occluded front forms where a cold front overtakes a warm one; a stationary front brings persistent unsettled weather over one area.

Fog

Fog forms when air is cooled to its dew point (a small air-temperature/dew-point spread means fog is likely):

• Advection fog - warm, moist air flowing over cold water - is the thick, persistent sea fog of coastal passages, and wind does not reliably burn it off.
• Radiation fog forms over land on clear, calm nights and usually clears after sunrise.
• Sea smoke (steam fog) forms when very cold air moves over warmer water.

Wind, sea state and the Beaufort scale

The Beaufort scale runs from force 0 (calm) to force 12 (hurricane) and links wind speed to the look of the sea - ripples, whitecaps, spray, streaks of foam, wave height - so you can estimate wind from the water and judge the water you should expect from a wind forecast.

Reading the sky

Clouds forecast change:

• High wispy cirrus ("mares' tails") often precede an approaching warm front and a change within a day.
• A lowering, thickening cloud deck and a ring (halo) around the sun or moon warn of moisture aloft and coming rain.
• Towering cumulonimbus means thunderstorms - gusty squalls, lightning, hail, and possible waterspouts.
• The old rhyme holds a kernel of truth: red sky at night, sailor's delight; red sky in morning, sailor take warning.

Local winds

Near shore in fair weather, daytime heating draws a cool sea breeze onshore; at night the flow reverses into a land breeze. These daily winds can add to or oppose the gradient wind and kick up an afternoon chop.

Thunderstorms and squalls

Summer thunderstorms and line squalls (often just ahead of a cold front) bring sudden, violent gust fronts, blinding rain, dangerous lightning, and a wind that can jump from calm to gale force in minutes. When you see a dark, hard-edged cloud line advancing, or hear the static of lightning on the AM radio, reduce speed, close the boat up, and prepare before it arrives.

Getting the forecast

Sources for the six-pack captain:

• NOAA Weather Radio - continuous VHF weather broadcasts on the WX channels.
• VHF Sécurité broadcasts from the Coast Guard for hazards and storm warnings.
• Internet and app forecasts and weatherfax before departure.

Check the forecast before leaving the dock and keep the weather radio on underway. A captain carrying paying passengers who ignores a posted small-craft advisory is inviting both danger and liability.

Lightning and squall safety

If a thunderstorm cannot be avoided, keep passengers low and inside the cabin if there is one, away from metal and the rigging, and reduce the electrical bonding risk by staying off the radio unless you need it. Slow to steerageway, point the bow into the wind and the seas, and ride out the squall - most pass in fifteen to thirty minutes. Watch for the classic cloud sequence that warns of a change many hours ahead: high cirrus thickening to a milky cirrostratus (with a halo), then lowering altostratus, then rain - the signature of an advancing warm front.