Forecasting the weather is essential for anyone who is interested in the outdoors, especially sailors. However, out at sea forecasts are hard to come by and are often unreliable. You could always buy the newest equipment, which allows you to access forecasts from miles offshore but this still doesn’t give you an accurate forecast for exactly where you are. The best, even in this technologically advanced era, is to use the clouds. They can tell you just how many minutes until it will rain to whether a dangerous squall is approaching. I have therefore listed my five most important clouds for weather forecasting, what weather they indicate and why.
Clouds come not only in all shapes and sizes but also at different heights. Cloud height is important as it indicates how long until different weather arrives. The highest clouds, known as cirrus, indicate that poor weather is about a day away but the lowest pannus clouds indicate that rain is coming in just a few minutes. Each cloud type is distinctly different and even with a small amount of knowledge you can identify these clouds. However, to make it easier to recognise them I have created a cloud identifier and forecaster at the end of this blog post. It not only identifies your cloud but it also tells you exactly what weather will come next.
Cloud Types and Heights
My top clouds for weather forecasting:
My first cloud is not actually a single cloud, but a series of different clouds which indicate the approach of a warm or occluded front in a depression.
The approach of a depression
Depressions are areas of low pressure which bring rain, clouds and strong winds and move around the world in a West to East direction. In their more extreme forms they wreak havoc to sailors as they often bring gale force or stronger winds, so you must think about reducing sail before it gets too bad. However, they never come without warning. There is always a series of clouds which march through the sky, flying the flag of an impending depression.
Lowering Cloud Base Before a Depression – Cirrus clouds at the top, cirrostratus in the middle and altostratus at the bottom
The first set of clouds you will see before the warm front in a depression arrives, are cirrus. They are wispy ice-crystal clouds that are the highest in the troposphere. They often form about 12-24 hours before a warm or occluded front approaches, so if you see these cirrus clouds building up then a warm front is coming. And if you couple this with a falling barometer then you can be sure of rain later and a freshening wind.
The next clouds that are normally present before a warm front approaches are cirrostratus. They are also made up of ice crystals, are sheet-like and often cover the whole sky. You often see halos formed in cirrostratus, like the photo below and the sun always casts shadows through the cirrostratus. They indicate that a warm or occluded front is about 10-15 hours away. This sheet of cirrostratus clouds slowly lowers and thickens until it becomes altostratus cloud.
A Halo in a Cirrostratus Cloud
Altostratus clouds are relatively featureless, but the sun can usually be seen through them. They are lower than cirrostratus clouds and are now made up of water droplets instead of ice. On their own, they don’t always indicate that a front is coming. However, if some of the clouds I mentioned earlier were seen before the altostratus arrived it almost certainly means that a front is approaching. These altostratus clouds gradually thicken and lower into rain-bearing nimbostratus. When the altostratus clouds are first spotted the warm or occluded front is probably about 8 hours away. Next, the nimbostratus clouds arrive and they bring increasing amounts of rain for generally about 4 hours until the warm or occluded front has passed.
Fair weather cumulus clouds
These clouds are great to see and if they are seen after mid-morning it indicates that there will be fair weather for the rest of the day as the clouds won’t develop into cumulonimbus shower clouds. Cumulus humilis, to call them by their scientific name, are cumulus clouds that are less tall than they are high and they often form on sunny days due to rising air currents known as updrafts. As the air rises in these updrafts, it cools and when it is too cold for the air to hold any more water, the vapour condenses into clouds. Cumulus clouds have a flat bottom, as the height where air condenses and is usually very level, but they have bulging cotton wool shaped tops because the updraft is strongest in the middle, causing it to bulge highest.
Fair Weather Cumulus
There is a daily cycle to these fair weather cumulus clouds. In the early morning, the sky is completely clear and there are no clouds in sight. By mid-morning shallow cumulus clouds form that are a couple of hundred meters across. By early afternoon they are at their largest. Then, by mid-afternoon, the clouds start decreasing in size and become smaller and more fragmented. Finally, just after sunset, the sky is clear again.
These clouds don’t normally form over water but rather over areas of rising air from the rapidly heating land. There is also often a line of cumulus clouds over the land, called a sea breeze front, where the clouds start over the land. This line of clouds is important as it means that a sea breeze is present (I delve deeper into sea breezes in my book The Message of the Clouds).
A Sea Breeze Front (Its Made Up of Cumulus Clouds)
Pannus clouds are probably my favourite cloud, not because they are particularly attractive but because they are extremely accurate weather forecasters. They are grey fragmented or wispy clouds which form below the main cloud base of precipitating clouds (clouds producing rain or snow). The falling rain or snow moistens the air until so damp that these clouds form. They can also be known as virga or fallstreaks when they are in their wispy form.
Pannus Clouds Beneath a Cumulonimbus Cloud
They are often fragmented due to turbulent winds and they appear to rapidly move and change shape. Pannus clouds indicate that the clouds above are already raining and that there will be rain on the ground in the next ten minutes. These clouds increase in numbers as the rain falls until they cover the whole sky. They form below cumulonimbus clouds and nimbostratus clouds, although you will mainly see them fall below the latter.
More Pannus Clouds
Pannus clouds are useful because they almost always indicate rain in the next few minutes, so if you see these clouds remember to wear waterproofs and pack up the picnic! But most importantly for sailors is that if pannus clouds are seen upwind, then the strong downdraft from a cumulonimbus cloud could be coming. These downdrafts can reach up to 40 knots so it is essential you prepare for any cumulonimbus clouds, by reducing sail or at least making sure the crew is ready to reef.
These clouds are especially important for sailors. Altocumulus clouds are light grey or white clouds which often form in sheets. If there are patches of altocumulus clouds it means that the air is unstable, possibly causing rain or thunderstorms later. This cloud is associated with changeable weather but never imminent rain. Cirrocumulus clouds, along with the altocumulus clouds, can indicate storms later.
They can also come in bands before a front approaches. Check whether there are other clouds which indicate a front approaching, first like cirrus, cirrostratus and altostratus, before making any predictions.
If you see these clouds you should be extra careful, especially if the air is moist, as thunderstorms may be coming, which could prove devastating for a yacht. When you are sailing, I would advise you to confirm with another forecast to see if there are thunderstorms coming and seek shelter if possible.
More Altostratus Clouds (There were 250,000 lightning strikes in the hours after I saw these ones)
Cumulonimbus clouds are tall cumulus clouds which cover all three cloud levels, they are the classic storm and shower cloud and they often produce thunder. They are bigger than cumulus clouds in width as well as height. They often have a base of around 20km across, which means that their showers last around 30 minutes.
A Cumulonimbus Cloud
Cumulonimbus clouds often have anvil-shaped tops. These anvil shapes occur because the rising water vapour in the cumulonimbus cloud reaches a height where it cannot rise any further and it spreads out forming the anvil. Cumulonimbus clouds will always have fibrous tops as the top section has turned to ice. Pannus clouds spotted below a cumulonimbus cloud also confirm that rain is coming.
An Anvil Topped Cumulonimbus Cloud
Cumulonimbus clouds are particularly dangerous to sailors as the wind will often reach 30-40 knots when they pass overhead as the rain drags the wind down with it. Due to the rain forcing cold air down the temperature will also decrease. But then once the cumulonimbus cloud is overhead the wind often drops to significantly below its original strength.
To see if the cumulonimbus cloud will reach you, you can use a hand bearing compass to check if the bearing of the cloud changes or not. If it stays steady then get ready to batten down the hatches and reef the sail, as some strong winds are coming. You will not find that the sea state is impacted by the cumulonimbus cloud, as their squall never lasts for more than about half an hour.
Armed with the knowledge of these five clouds you should be well equipped to deal with most of the skies that you see. However, if you can see a cloud that I haven’t included above, or if you are struggling to work out which one it is, then use the cloud identifier below to not only find out what cloud you are seeing but also to predict the coming weather.
If you would like more information about weather forecasting using the clouds, then please check out my book The Message of the Clouds. It teaches you not only what weather is coming next, but why that weather is coming.