Wind and weather for sailors.

Deciding whether conditions are safe to sail in is a judgment that depends on the boat, the crew, the place and the forecast. This page does not make that decision for you. It explains the scales and warnings that official forecasters use, and what experienced sailors writing for training bodies and sailing magazines say about reading the wind. Weather is also part of formal courses: ASA offers a dedicated course, ASA 119 Marine Weather, and RYA Day Skipper theory includes meteorology.

The Beaufort scale

Marine forecasts often give wind as a Beaufort force. The scale was devised in 1805 by Francis Beaufort, a Royal Navy officer and hydrographer, according to the Royal Meteorological Society. It was officially adopted in the 1830s while he was Hydrographer of the Navy, and was first used on HMS Beagle under Robert FitzRoy.

Source: Met Office. The Met Office also gives probable wave heights in the open sea, rising from about 0.1 m at Force 1 to 1 m at Force 4, 3 m at Force 6, 5.5 m at Force 8, 9 m at Force 10 and 14 m or more at Force 12. It notes these figures describe well-developed open-sea conditions, and that waves can lag behind changes in the wind. An interactive version is on the Beaufort scale tool.

How that compares with course limits

Entry-level courses set wind limits. US Sailing's Basic Keelboat course is aimed at light to moderate conditions of about 5 to 15 knots, and ASA 101 at winds up to 15 knots, which sits within Force 4 on the Met Office scale. ASA's higher courses step up: ASA 103 Basic Coastal Cruising goes to 20 knots and ASA 104 Bareboat Cruising to 30 knots.

Official marine warnings

United States

The National Weather Service defines its marine warnings by wind speed (these definitions apply outside tropical cyclones):

Because Small Craft Advisory thresholds differ by region, check what your local forecast office uses. One office, NWS Melbourne in Florida, ties Beaufort Force 4 (about 14 to 16 knots) to "small craft exercise caution" and Forces 5 to 7 to Small Craft Advisory conditions; we could not confirm that this is national practice.

NOAA Weather Radio broadcasts National Weather Service information continuously on seven VHF frequencies between 162.400 and 162.550 MHz, from more than 1,000 transmitters covering all states and adjacent coastal waters.

United Kingdom

The Shipping Forecast is issued four times a day (at 2300, 0500, 1100 and 1700 UTC) for 31 sea areas, and gives wind direction and force, sea state, weather and visibility. Gale warnings are issued as needed for Force 8 or more. The Met Office prepares it on behalf of the MCA. The Met Office also points out that the internet is not part of the official Maritime Safety Information system, and directs sailors to GMDSS, NAVTEX and SafetyNET for the latest official information.

Planning before you go

In the UK, SOLAS Chapter V requires voyage planning on all vessels, including pleasure craft, according to the RYA. The MCA's best-practice advice in MGN 599 includes checking the weather and tides before going out and telling someone ashore what you plan to do. North Sails' advice on reefing is short: when in doubt, reef, and do it early rather than waiting for the gusts to arrive. How reefing works is on reefing and depowering.

Reading wind on the water

These observations come from coaches and training sources and are presented as their views.

• Dark water. Moose McClintock, writing in Sailing World, says darker patches of water generally mean more breeze, while flat water with no ripples signals a lull.
• The edge of a puff. Luther Carpenter, in a US Sailing coaching paper, says the leading edge of a puff is the least stable wind and the core of the gust is steadier.
• Apparent wind. Sailtrain, a UK training site, notes that telltales and wind vanes show apparent wind, and show true wind only when the boat is stopped. See how sailing works.

Sea breezes

Ed Adams, writing in Sailing World, explains that a sea breeze forms when the land is warmer than the water: heated land lowers the air pressure over it, air flows in from the sea, and a bigger temperature difference gives a stronger breeze. He describes four types (pure, corkscrew, backdoor and synoptic), depending on the direction of the morning gradient wind relative to the shore.

Squalls

Robin Urquhart, in Sail Magazine, cites NOAA's definition of a squall as a sudden increase in wind of at least 16 knots, sustained at 22 knots or more for at least a minute. He says the strongest gusts usually arrive before the rain, and advises reefing early and expecting wind at least 50 percent above the prevailing conditions. Squall wind shifts can also cause an accidental gybe.

Tides and currents

NOAA describes tides as long-period waves driven by the moon and sun. A flood current flows toward land, an ebb current flows toward the sea, and slack water is the brief period of no current between them. NOAA says tidal currents are strongest around full and new moon (spring tides) and weakest at the first and third quarter (neap tides), and can reach several kilometers per hour near estuary entrances, narrow straits and inlets. Tides and charts are covered on navigation.