Weather at Sea
What about cloud types? High? Medium? low?
There are two types – flat stratus cloud, and puffy cumulus cloud.
Stratus cloud occurs in layers at various heights, and indicates that the atmosphere is stable, with little vertical air movement.
Cumulus cloud occurs when there is a vertical movement of air, and the atmosphere is unstable. Small cumulus cloud can occur at any height.
Glider pilots and soaring birds look for the upcurrents under large cumulus cloud. A large cumulus cloud with a dark base is likely to cause strong upcurrents with large wind shifts around the base. They are best avoided by sailors and even bigger boats.
Low-level clouds (base 0 – 2 km high)
- Stratus (S) – extensive, featureless, shallow cloud sheet, can yield drizzle or light rain
- Stratocumulus (Sc) – shallow cloud sheet, broken into roughly recurring masses of cumulus, may drizzle or snow
- Cumulus (Cu) – separate, hill-shaped puffy clouds, with level bases. Usually fair, but may bring showers after a cold front.
- Cumulonimbus (Cb) – very large, high (up to10km) cumulus, with dark bases and anvil shaped top. Can bring thunder, lightning, squalls and heavy rain
Medium-level clouds (base 2 – 4 km high)
- Altocumulus (Ac) – shallow cloud sheet with roughly regular patches or ripples of small rounded clouds. Usually fair weather
- Altostratus (As) – featureless, thin, translucent cloud sheet. Usually fair weather.
- Nimbus (Ns) – extensive, very dark cloud sheet, usually yielding precipitation
High clouds (base 5 -15 km high)
- Cirrus (Ci) – streaky, white, feather-like cloud. Indicates an approaching depression
- Cirrocumulus (Cc) – shallow, more or less regular patches or ripples of cloud. Fair weather.
- Cirrostratus (Cs) – shallow sheet of largely translucent cloud. Fair weather.
The passing of a weather depression
Cirrus cloud is the high, white, curled, streaks of cloud known as mares’ tails. With a falling barometer, this usually indicates the approaching warm front of a depression with associated veering wind and rain.
This is followed by lower levels of altostratus, stratus and nimbostratus cloud. Usually this brings rain; nimbus just means bearing rain.
After the warm front there is broken cloud, sometimes with light rain and poor visibility.
Then comes the cold front, which at worst may be thundery cumulonimbus with heavy rain and squalls.
Finally after the cold front we are back to cumulus, with clear skies and showers, and stronger veering winds.
Wind direction and strength
The Beaufort wind force scale is measure that relates wind speed to observed conditions at sea or on land.
The scale was devised in 1805 by Irish-born Francis Beaufort (later Rear Admiral Sir Francis Beaufort), a Royal Navy officer, while serving in HMS Woolwich.
In the early 19th century, naval officers made regular weather observations, but there was no standard scale and so they could be very subjective – one man’s “stiff breeze” might be another’s “soft breeze”. Beaufort succeeded in standardizing the scale.
The scale did not reference wind speed numbers but related qualitative wind conditions to effects on the sails of a frigate, then the main ship of the Royal Navy, from “just sufficient to give steerage” to “that which no canvas sails could withstand”.
The scale was made a standard for ship’s log entries on Royal Navy vessels in the late 1830s and was adapted to non-naval use from the 1850s, with scale numbers corresponding to cup anemometer rotations. In 1916, to accommodate the growth of steam power, the descriptions were changed to how the sea, not the sails, behaved and extended to land observations. The measure was slightly altered some decades later to improve its utility for meteorologists.
Wave heights in the scale are for conditions in the open ocean, not along the shore.
The scale is used in the Shipping Forecasts broadcast on BBC Radio 4 in the United Kingdom.
“Small Craft Warning” indicates winds of Beaufort force 6 (mean wind speed exceeding 22 knots) are expected up to 10 nautical miles offshore.
“Gale Warnings” are issued if winds of Beaufort force 8 are expected
“Strong Gale Warnings” are issued if winds of Beaufort force 9 or frequent gusts of at least 52 knots are expected.
“Storm Force Warnings” are issued if Beaufort force 10 or frequent gusts of at least 61 knots are expected
“Violent Storm Force Warnings” are issued if Beaufort force 11 or frequent gusts of at least 69 knots are expected
“Hurricane Force Warnings” are issued if winds of greater than 64 knots are expected.
Wind direction and strength is indicated on weather maps my arrows or “darts” showing the direction the the wind is coming from and the number of “feathers” indicates the wind strength.
Half a feather represents one beaufort wind force and a full feather represents two wind forces – so it is simply adding them up to find the total wind strength,
|Calm||Light Air||Light Breeze||Gentle Breeze||Moderate Breeze||Fresh Breeze||Strong Breeze||Near Gale||Gale||Strong Gale||Storm||Violent Storm||HurricaneForce|
|Light Winds||High Winds||Gale-force||Storm-force||Hurricane-force|
|Beaufort number||Description||Wind speed||Wave height||Sea conditions||Land conditions||Sea state photo|
|0||Calm||< 1 km/h||0 m||Sea like a mirror||Calm. Smoke rises vertically.|
|< 1 mph|
|< 1 knot||0 ft|
|< 0.3 m/s|
|1||Light air||1–5 km/h||0–0.2 m||Ripples with the appearance of scales are formed, but without foam crests||Smoke drift indicates wind direction. Leaves and wind vanes are stationary.|
|1–3 knots||0–1 ft|
|2||Light breeze||6–11 km/h||0.2–0.5 m||Small wavelets, still short but more pronounced; crests have a glassy appearance and do not break||Wind felt on exposed skin. Leaves rustle. Wind vanes begin to move.|
|4–6 knots||1–2 ft|
|3||Gentle breeze||12–19 km/h||0.5–1 m||Large wavelets. Crests begin to break; scatteredwhitecaps||Leaves and small twigs constantly moving, light flags extended.|
|7–10 knots||2–3.5 ft|
|4||Moderate breeze||20–28 km/h||1–2 m||Small waves with breaking crests. Fairly frequent whitecaps.||Dust and loose paper raised. Small branches begin to move.|
|11–16 knots||3.5–6 ft|
|5||Fresh breeze||29–38 km/h||2–3 m||Moderate waves of some length. Many whitecaps. Small amounts of spray.||Branches of a moderate size move. Small trees in leaf begin to sway.|
|17–21 knots||6–9 ft|
|6||Strong breeze||38–49 km/h||3–4 m||Long waves begin to form. White foam crests are very frequent. Some airborne spray is present.||Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic bins tip over.|
|22–27 knots||9–13 ft|
|50–61 km/h||4–5.5 m||Sea heaps up. Some foam from breaking waves is blown into streaks along wind direction. Moderate amounts of airborne spray.||Whole trees in motion. Effort needed to walk against the wind.|
|28–33 knots||13–19 ft|
|62–74 km/h||5.5–7.5 m||Moderately high waves with breaking crests forming spindrift. Well-marked streaks of foam are blown along wind direction. Considerable airborne spray.||Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.|
|34–40 knots||18–25 ft|
|9||Strong/severe gale||75–88 km/h||7–10 m||High waves whose crests sometimes roll over. Dense foam is blown along wind direction. Large amounts of airborne spray may begin to reduce visibility.||Some branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over.|
|41–47 knots||23–32 ft|
|89–102 km/h||9–12.5 m||Very high waves with overhanging crests. Large patches of foam from wave crests give the sea a white appearance. Considerable tumbling of waves with heavy impact. Large amounts of airborne spray reduce visibility.||Trees are broken off or uprooted, structural damage likely.|
|48–55 knots||29–41 ft|
|11||Violent storm||103–117 km/h||11.5–16 m||Exceptionally high waves. Very large patches of foam, driven before the wind, cover much of the sea surface. Very large amounts of airborne spray severely reduce visibility.||Widespread vegetation and structural damage likely.|
|56–63 knots||37–52 ft|
|12||Hurricane force ||≥ 118 km/h||≥ 14 m||Huge waves. Sea is completely white with foam and spray. Air is filled with driving spray, greatly reducing visibility.||Severe widespread damage to vegetation and structures. Debris and unsecured objects are hurled about.|