As sailors, we like to have as much insight into the weather as possible. Some wind is good, too much is not as nice, and being becalmed is quite frankly, not fun. Close to shore, we can check the weather on a phone app, or listen to the weather radio, but those options are not available out at sea.
Being able to understand cloud formations and what they could indicate in terms of weather can be a big help for sailors. Clouds not only show the stability of the atmosphere, but they also indicate where you may be relative to a weather system (high or low pressure, warm or cold front) and whether that system could bring rain or changes in the wind strength and direction.
At first it can seem confusing with what looks like so many cloud types – and indeed there are hundreds – but these myriad cloud types can basically be grouped into ten main cloud groups (genera) which are divided into three types; cumulus, stratus, and cirrus. They are then distinguished further by the height at which they are found; cirro (high-level clouds), alto(medium-level clouds) and stratus (low-level clouds). There is only one other major cloud distinction and that is nimbus, which means precipitation.
HIGH CLOUDS (between 18,000 – 40,000 ft.)
Latin: cirrus – lock or tuft of hair
Cirrus clouds are wispy in nature and are formed because of thin layers of ice crystals in the upper atmosphere (above 20,000 ft). They generally get their start from ice crystals being carried away by the jet stream from the top of thunderstorms or tropical cyclones. Cirrus clouds by themselves, are a sign of fair weather, “the higher the clouds, the finer the weather.” However, if cirrus clouds are combined with other types of high-level clouds or a backing wind (anticlockwise direction from west to south), they could indicate approaching bad weather. “Mares’ tails and mackerel scales make lofty ships carry low sails” refer to the combination of cirrus and altocumulus clouds.
Latin: cirrus – lock or tuft of hair;
cumulus – heap
These high-level white, fleecy looking clouds can also appear as thin wave-like sheets. Cirrocumulus clouds are formed by a shallow pattern of upper-air convection and can sometimes be an indicator of an afternoon or evening thunderstorm. If cirrocumulus clouds are increasing from the west, and begin to thicken into altocumulus clouds, it is a generally an indicator of an approaching warm front and the possibility of rain within 18 to 36 hours.
Latin: cirrus – lock or tuft of hair;
stratus – flattened, spread out
These form as cirrus clouds thicken. They are often distinguished by a thin hazy sheen or veil in the sky. At times the cirrostratus clouds may not be readily visible, but there will be a halo around the sun or moon as the light passes through ice crystals in the upper atmosphere. “When halo rings sun or moon, rain’s approaching on the run” is another old mariner’s proverb and is a good sign of approaching storms within 8 to 24 hours. Keep an eye on the barometer and the faster it falls the sooner the approaching depression will arrive. If the barometer remains stable, chances are the front will pass to the north or south.
MID-LEVEL CLOUDS (between 2,000 – 22,00 ft.)
Latin: altus – high; stratus – flattened,
These mid-level clouds can be gray or blueish and appear as sheets or layers with few features. Although the sun can be often be seen weakly through these clouds, some layers can be opaque. These clouds are formed through a combination of ice crystals and water droplets. Out at sea, thickening altostratus clouds generally indicate a warm front approaching and the possibility of strong winds and waves. A good rule of thumb, is the length of time between cirrus clouds arriving to losing the sun (with altostratus clouds), is usually about equal to the time from losing the sun to rain falling.
Latin: altus – high; cumulus – heap
Appearing as white or gray patchy sheets or layered clouds, altocumulus clouds are the most common mid-level cloud type. They can also appear as heaps or rolls of clouds. Typically these clouds form as a warm air mass is pushed up by an advancing cold front (think of a wedge as a cold front advances). These clouds are the “mackerel scales” in the mariner’s proverb. When they are accompanied by high-level cirrus clouds, it is a good indicator that a front could pass within 6 to 8 hours. Ahead of a cold front, winds typically back (move anticlockwise).
Latin: nimbus – rain; stratus – flattened,
Although classified as a mid-level cloud where they typically begin to form (often from altostratus clouds), their base is actually found at low altitude. These clouds are distinguished by a thick, solid appearance, sometimes with small cloud ragged fragments (scud) below. Nimbostratus are found along warm or occluded fronts and are associated with periods (can be days) of moderate to heavy rain (or snow) over a wide area.
LOW-LEVEL CLOUDS (0 – 6,500 ft.)
Latin: cumulus – heap
Known as fair weather clouds, cumulus clouds appear as white, fluffy clouds, similar to cotton balls or cauliflower. They are formed by warm air rising and condensing as it cools. Although generally associated with fair weather, cumulus clouds can sometimes continue to grow and turn into cumulonimbus clouds or thunderstorms. Although on land, cumulus clouds tend to form in the early morning and disperse by late afternoon or early evening, at sea, cumulus clouds tend to be strongest in the late hours of the night. For sailors, larger cumulus clouds can be asociated with gusty and shifting winds around the clouds.
Latin: stratus– flattened, spread out;
cumulus – heap
These clouds are low-level clouds identified by their grey or whitish color and often appear as rounded clouds or clumps which can be joined together in layers, or form rows of clouds. Stratocumulus clouds can be found in all types of weather conditions.
Latin: stratus– flattened, spread out
These clouds are very low-level, and can often appear as fog or mist at ground or sea level. Stratus clouds are identified by their grey, uniform appearance. Although they can appear as dark uniform clouds, similar to nimbostratus clouds, they are not as thick vertically, thus do not produce any meaningful precipitation.
Latin: cumulus – heap, nimbus – rain
Cumulonimbus clouds begin as fluffy cumulus clouds which expand as updrafts carry warm air aloft where it cools and condenses. Cumulonimbus clouds are your classical thunderstorm and are distinguished by the height it reaches and sometimes the appearance of an anvil top. The good news is that these thunderstorms tend to be localized and are not typically associated with long-lasting low pressure systems and sustained winds. The disadvantage is that the winds can be intense in the vicinity, especially with the downdrafts associated with a shelf cloud preceding the thunderstorm.
WARM AND COLD FRONTS
To understand the impact of cloud types on sailing weather, it’s also helpful to have an understanding of warm and cold fronts. Without going into too much detail in this post (indeed, chapters have been written about this), warm fronts tend to be preceded by cirrus, then cirrusstratus, altostratus and finally nimbostratus clouds which then bring the rain. As the warm front approaches winds begin to increase and back (changing in an anticlockwise direction), and barometric pressure falls. As the warm front passes, winds veer (changing in clockwise direction) and pressure stops falling. After the passing of a warm front, wind direction tends to remain steady.
Cold fronts tend to move more quickly (sometimes more than twice as fast) than warm fronts and are thus generally associated with higher surface winds. As the cold front passes, winds can veer suddenly and are often accompanied by squalls, and lines of cumulonimbus clouds (thunderstorms) can form. Pressure will typically rise quickly. Behind the cold front, winds can be quite strong and gusty.
This is just an introduction to cloud formations and the impact on weather for sailors. For those who are interested in expanding their knowledge of meteorology for sailors, there are a number of great resources. The Royal Yachting Association (RYA) has a great book on weather from a sailor’s perspective. North U (of North Sails) also has a good book and CD-ROM. The National Weather Service (NOAA) offers an online weather course, as does the COMET Program, which is operated by the University Corporation for Atmospheric Research (UCAR).