Our favourite weather service is Windy, and while underway, we download weather maps(grib files) through the saildocs service with our satellite phone which we then process with PredictWind. We pick our weather carefully on passage, no sense in taking unnecessary risks. Sailing with a schedule is dangerous.

For information on storms brewing in the Pacific and Atlantic, as well as passage advice in the South Pacific, we enjoy reading MetBob.

sky reading

When reading the weather on a passage, don't rely solely on weather reports. Learn to use a barometer, and observe and study the sky.

When clouds appear like rocks and towers, the Earth’s refreshed with frequent showers.

Anvil top cloud: Named for their likeness to an iron anvil, anvil clouds are the icy upper portions of cumulonimbus thunderstorm clouds that are caused by a rising of air in the lower portions of the atmosphere. When the rising air reaches 40,000-60,000 or more feet, it tends to spread out in a characteristic anvil shape. Generally, the taller the cumulonimbus cloud, the more severe the storm will be.

The anvil top of a cumulonimbus cloud is actually caused by it hitting the top of the stratosphere—the second layer of the atmosphere. Since this layer acts as a "cap" to convection (the cooler temperatures at its top discourage thunderstorms (convection), the tops of storm clouds have nowhere to go but spread outward.

At sea, keep your eyes on the clouds. If you notice a cumulonimbus cloud with an anvil top cloud forming, and that it is pointing toward you, reef your sails, as the weather could get very nasty. If the wind is strong, the anvil will shear off in the direction the wind is blowing.

A ring around the sun or moon, means that rain will come real soon.

A ring around the moon: A halo forming around the sun, or moon, is caused by the refraction of sunlight, or moonlight, by ice crystals in high altitudes. Moisture forming at such heights is indicative that is moving downwards, and that an active weather system is coming.

Mackerel skies and mares’ tails, make tall ships carry low sails.

Cirrocumulus clouds: Cirrocumulus clouds, or 'mare's tails' and 'mackerel scales', indicate an approaching warm front, with veering winds (clocking around from NE and E over to SW and W) with rain.

Red sky at night: In the northern hemisphere, as the sun sets, light shines through the lower atmosphere which is full of dust, salt, smoke and pollution. The particles scatter the shorter wavelengths of light at the blue end of the spectrum, leaving only the longer wavelengths (at the red end of the spectrum) to shine through. In a high-pressure area, air sinks and holds air contaminants near the earth, making the sunset even more red.

Red sky in the morning: A red sky in the eastern morning sky occurs for the same reasons as a red sky at night, but then the high pressure region has already passed from West to East, and may indicate an area of low pressure is coming. Low pressure systems bring clouds, rain and storms.

Other indicators of deteriorating weather:

Indicators of impending strong winds:

Indicators of impending precipitation:

Indicators of continuing fair weather:

Indicators of clearing weather:

Source: Indicators of weather courtesy of Larry and Susan Macdonald. Their wonderful website came offline this year (2020), and we've retrieved some of the content using Wayback Machine.


A brass barometer on a bulkhead in a sailboat When the glass falls low, prepare for a blow; when the glass is high, let your kites fly.

A good barometer is a lifesaver on a boat. At sea, record the barometer readings every hour. If you see any sudden changes, this is telling of a change in weather. The larger the barometric change, the more extreme the weather change.

Aneroid barometers: Inside an aneroid barometer (aneroid = without fluid) is a small capsule. This capsule has had the air pumped out of it. When the air pressure rises, the sides of the capsule are compressed. The capsule is attached to levers which move a needle as the air pressure squeezes the capsule. A dial behind the needle tells you the air pressure and altitude or weather forecast. They are less accurate than mercurial barometers.

Wonder if your barometer is working? You can find out using a plastic bag. The idea is that if you seal it in with a pocket of air, and press on the bag, if the unit isn't faulty, the needle will move clockwise as the pressure within the bag changes.

beaufort scale

The Beaufort Scale of Wind Force was developed in 1805 by Admiral Sir Francis Beaufort as a means for sailors to gauge wind speeds through visual observations of the sea state.

The scale runs from Force 0 (calm) to Force 12 (Hurricane).

Force Wind (Knots) Classification Wind Effect
0 <1 Calm Sea surface smooth and mirror-like.
1 1-3 Calm Scaly ripples, no foam crests.
2 4-6 Light Breeze Small wavelets, crests glassy, no breaking.
3 7-10 Gentle Breeze Large wavelets, crests begin to break, scattered whitecaps.
4 11-16 Moderate Breeze Small waves 1-4 ft. becoming longer, numerous whitecaps.
5 17-21 Fresh Breeze Moderate waves 4-8 ft taking longer form, many whitecaps, some spray.
6 22-27 Strong Breeze Larger waves 8-13 ft, whitecaps common, more spray.
7 28-33 Near Gale Sea heaps up, waves 13-20 ft, white foam streaks off breakers.
8 34-40 Gale Moderately high (13-20 ft) waves of greater length, edges of crests begin to break into spindrift, foam blown in streaks.
9 41-47 Strong Gale High waves (20 ft), sea begins to roll, dense streaks of foam, spray may reduce visibility.
10 48-55 Storm Very high waves (20-30 ft) with overhanging crests, sea white with densely blown foam, heavy rolling, lowered visibility.
11 56-63 Violent Storm Exceptionally high (30-45 ft) waves, foam patches cover sea, visibility more reduced.
12 64+ Hurricane Air filled with foam, waves over 45 ft, sea completely white with driving spray, visibility greatly reduced.

This is the scale that we use in our book busy doing nothing.