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When reading the weather on a passage, don't rely solely on weather reports. Learn to read the weather with books, and by observing, and studying the sky.

Our barometer

Barometer

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

How they work: 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 clock-wise as the pressure within the bag changes.

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 trong, the anvil will sheer off in the direction the wind is blowing.

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.

Cirrocumulus clouds

Cirrocumulus clouds, or 'mare's tales' 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:

Reduction in visibility

Direction of wind, shifting from N > E, or from E > S (cold front).

Clouds moving in different direction, at different heights indicate shifting winds and approaching storm fronts.

Cumulus begin to develop vertically and darken

Sky is dark and threatening to the West.

Clouds increasing in numbers, moving rapidly across the sky.

Other indicators of impending strong winds:

Yellow sunset

Light, scattered clouds alone in a clear sky

Sharp, clearly defined edges to clouds

Unusually bright stars

Major changes in the temperature

Indicators of Impeding Precipitation:

Distant objects seem to stand above the horizon

Sounds are very clear and can be heard for great distances

Transparent veil-like cirrus clouds thicken, ceiling lowers

Hazy and sticky air. Rain may occur in 18-36 hours

Halo around the sun or moon

Increasing South wind with clouds moving from the West

Wind (especially North wind) shifting to West and then South

Steadily falling barometer

Pale sunset

Red sky to the West at dawn

No dew after a hot day

Indicators of Continuing Fair Weather:

Early morning fog that clears

Gentle wind from the West or Northwest

Barometer steady or rising slightly

Red sky to East with clear sky to the West at sunset

Bright moon and light breeze at night

Heavy dew or frost.

Clear blue morning sky to West

Clouds dot the afternoon summer sky

Indicators of clearing weather:

Cloud bases rise

Smoke from stacks rise

Wind shifts to West, especially from East through South

Barometer rises quickly

A cold front has passed in the past 4 to 7 hours

Gray early morning sky shows signs of clearing

Morning fog or dew

Rain stopping and clouds breaking away at sunset

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

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).

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