Who has not looked up at the sky and found shapes and stories in the clouds that drift high above our heads? But clouds have their own stories to tell, if you can read them — stories that are important for all mariners to be able to understand. In essence, clouds tell you what is happening with the weather way up in the sky. This skill is not entirely necessary, but it can help you out in a tight spot and sometimes keep you from getting drenched.
Starting with the very basics, there are three levels of clouds, categorized by the altitude at which they exist.
In addition, clouds are divided into 27 categories, but you will not need to know that level of information for marine weather prediction and forecasting. Instead, this guide will focus on the clouds that are important on the water, and how you can use them to help you on your way.
For best use of this information, you want to have a barometer aboard your boat. Barometers measure changes in atmospheric pressure, and using one in tandem with an ability to read the clouds will give you a huge advantage on your boating excursions — especially when it comes to avoiding bad weather. Listening to the VHF radio for weather updates provides a further safety net, giving crucial information on wind, waves, and temperature.
As mentioned above, clouds tell you what is happening with the weather, often before it happens. These are some particular cloudy situations to look for when you are out on the water:
Say you want to go for a sail, but there is no wind either in the harbor or out to sea. You need to know if the sea breeze will fill in or not before you decide to make the trip, and there is in fact an easy way to tell. Look over land. If you see small white cumulus clouds, caused by cool sea air meeting warm land air, the sea breeze will fill in before too long.
In general, bright white cumulus clouds are fair-weather clouds. If you see them out at sea, they indicate that the weather will remain stable so you will have smooth sailing, as do high wispy cirrus clouds.
Despite their name, thunderstorms are not actually storms in the traditional sense. They have a vertical circulation of air, unlike the circular counterclockwise pattern of regular storms. Especially dangerous at sea, thunderstorms should be avoided at all costs. Fortunately, they are easy to spot and therefore easy to avoid. Cumulo-nimbus clouds, often called thunderheads, are huge high cloud formations that span all three layers of clouds. Shaped like an anvil, they are dark towards the bottom and white and puffy at the top. If you see one, head into port if you can — you do not want to be caught out at sea on what amounts to a huge lightning rod.
A squall line is another non-storm that feels just like one if you happen to be caught up in one. Squall lines are the disturbance created by a cold front meeting warmer air, and they can be quite dangerous. They can move very, very quickly, with high dark clouds similar to thunderheads. If one is approaching you, the sky and sea will turn a dead, flat black and the wind will change direction.
In case you had not seen the line approaching and are caught up in it, follow these directions:
Even on a calm day with only five knots of breeze, a squall line can bring gusts of up to 40 knots, followed by an enormous downpour. They only last 5-10 minutes, but failing to ease out on your sails can result in ripped sails, capsizing, or injury.
Ever heard the phrase “lowering sky?” Though it is now perhaps more a literary term, it is in fact literal. When rain clouds form, it is the result of clouds getting thicker and lower in the sky, turning a heavy gray or gray-blue color. Sometimes this can be seen from far off, but sometimes it sneaks up on you, as the formation can take hours. Though just rain will not cause your boat much damage, keep in mind that rain without a storm will mean there is no wind in the affected area.
Storms are a different story. Wind and rain, often accompanied by large waves, tear over the ocean in circular patterns. They look similar to rainclouds, and often the only difference is that storms are windy and just rain is not.
Called mackerel sky by sailors, cirro-cumulus clouds are a common sight at sea, often light up in bright colors by the rising or setting sun. The saying red sky at night, sailor’s delight; red sky at morning, sailors take warning may be part of seagoing superstition but it can also work as a real-world weather indicator. Weather systems in general move west to east, so the presence of clouds in the east lit by the setting sun in the west meant in general that the clouds would be gone by morning. Clouds lit in the west by the rising sun in the east, on the other hand, meant that the cloudy weather was incoming.
Remember, this is only a basic guide. If you see any unusual cloud formation, accompanied by a rapid rise or fall in barometric pressure, watch out. Both factors are a sure indicator of bad weather, and the quicker the change the more severe the weather. Learn to read clouds, become a better mariner, and enjoy your new knowledge of the sea!