By learning some simple rules of thumb, you can sometimes continue working offshore although the clouds are looking threatening.
Why some clouds look dark
Clouds that produce precipitation look dark because of the large number of water droplets that are scattering the sunlight. In thunderstorm clouds, this effect is especially notable, because of the larger size of the droplets (larger due to vertical movement of the droplets). Also, the great vertical extent of these cumulonimbus clouds is creating a shadow effect.
However, not all dark clouds will result in precipitation or thunderstorms. If the lower part of the atmosphere is unstable (relative warm air below colder air), air will rise until it becomes saturated and cumulus clouds are forming. If there is a temperature inversion (where the temperature rises with height instead of falls) higher up in the atmosphere, typical in 1-2km height, the clouds will flatten out and sometimes cover most of the sky. These clouds are called stratocumulus or altocumulus clouds - depending on the height of the cloud base.
Especially the lower stratocumulus clouds can sometimes look dark, due to shadows from other clouds and water droplets gathering near the inversion layer, causing a denser cloud with no direct light passing the cloud. When the stratocumulus clouds are thick enough, and the cloud top is above the freezing level, they can also produce light precipitation.
Below are three rules of thumb, that will help you decide whether the dark clouds are potential thunderstorms, or just harmless stratocumulus clouds.
Rule 1: Relationship between horizontal and vertical extent
If you think the clouds look threatening, it is a good idea to look at the relationship between the vertical and horizontal extent of the clouds – if possible. If the horizontal extent is larger than the vertical, the clouds will most likely only block the sunlight and perhaps produce some very light precipitation, especially during the winter time. On the other side, if a cloud has a large vertical extent, it could potentially be an approaching thunderstorm.
Rule 2: Satellite interpretation
If the clouds are covering most of the sky, you can use infrared satellite images to determine what type of cloud it is. Because the top of the stratocumulus clouds will be almost the same over a larger area (near the height of the temperature inversion), the color will be rather uniform. The top of the stratocumulus clouds is also much lower than the cloud top of potential thunderstorms, and therefore the cloud top temperature is much higher.
Different satellite channels will present the stratocumulus clouds differently, but commonly they will be grey or yellow. In contrary, thunderstorm clouds will be mainly white and sometimes with different color tones (shadow effect).
In daytime when using a visual image, the difference between the stratocumulus clouds and the convective clouds will be hard to tell. It is therefore recommended to use an infrared image also in the daytime in order to best determine the height of the clouds. See the example below.
A satellite image can off course only be used when there are no clouds above the stratocumulus layer.
On above images, stratocumulus clouds are covering almost the whole UK. A weak warm front with no risk of thunderstorms is located over the western part of the North Sea, and over Scandinavia, some isolated cumulus clouds with high cirrus clouds above. A front with embedded thunderstorms is located just east of Finland, and over the Baltic States showers and thunderstorms.
The first image is a visual channel image (no color difference between high and low clouds), where it is hard to see the difference between the lower cloud tops over the UK, and the high cloud tops over and east of the Baltic States.
The second image is an infrared image where it is easier to spot the different cloud tops. By using an infrared image, it is thus possible to differ between harmless stratocumulus clouds and cumulonimbus clouds which may generate lightning and thus become a thunderstorm.
Rule 3: The cloud bases
If the cloud base looks fairly uniform, it also indicates low risk of thunderstorms. In a situation of showers and thunderstorms, there are areas with updraft and other areas with downdraft due to the large vertical movements inside and near a cumulonimbus cloud. This will create nonuniform cloud formations - many times with different types of clouds within the same cloud system. Please see the difference in below two images.
Gusty winds are also associated with thunderstorms, due to the turbulent nature of a thunderstorm. This is normally not the case for an area with stratocumulus clouds overhead.
In nature there are of course always exceptions from these rules, but hopefully these rules of thumb may be a helpful guide in situations where you are uncertain or want to confirm your evaluation of the weather situation.