Are cumulonimbus clouds cause for concern?

James Stevens

What would you do if you were sailing towards cumulonimbus clouds? James Stevens answers your questions of seamanship

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You are sailing towards cumulonimbus clouds, what should you do?

James Stevens answers your questions of seamanship

Question:

Vikki purchased her 10m cruiser racer, Meridian, last year.

She is an experienced dinghy sailor and has entered a few yacht races but today she is on a cruising holiday to the West Country with two friends.

She has taken the RYA shore-based navigation course but not the practical training.

It is a hot and humid day in midsummer.

They are crossing Lyme Bay on the 45-mile passage from Portland Bill to Dartmouth.

They are beating against a 12-14 knot wind from the south-west.

The forecast is south-westerly Force 4-6 with occasional thunder-storms, but at the moment Meridian is going well with one reef and the No 1 genoa.

There is, however, a blot on the horizon in the form of a monstrous cumulonimbus cloud coming straight for them.

Continues below…

There have been a few other shower clouds but this one looks menacing.

It is tall, black and likely to rain underneath.

There is other light cloud cover and some streaks of showers which seem to disappear before they hit the ground.

They look like jellyfish tentacles in the sky.

Vikki is quite an inexperienced sailor and is anxious, but her crew reckon the worst that can happen is that they get a heavy shower.

They all start putting on their wet weather gear, laughing about who is going to be on watch when it arrives.

Are they just going to get wet or is Vikki right to be concerned?

James Stevens answers:

It is going to be windy under the cloud.

One of the problems with large cumulonimbus clouds is that the wind can come at you from any direction.

With her present sail plan Meridian could easily be overpowered and experience violent wind shifts.

The weather conditions are also indicative of a bigger hazard.

Offshore passage

James Stevens, author of the Yachtmaster Handbook, spent 10 years as the RYA’s Training Manager and Yachtmaster Chief Examiner

The telltale jellyfish sky is a warning of possible microbursts.

A microburst is a very rapid downward column of air which on hitting the surface spreads in all directions.

The accompanying rain can be heavy and projected downwards at great speed.

Sailing vessels can be in real trouble under microbursts and they are particularly dangerous for tall ships.

Vikki’s best plan is to drop all the sails, start the engine and wait for the wind.

Meridian is in for a wild few minutes with very strong winds changing direction without warning.

If she keeps even a heavily reefed main up there is a good chance of a crash gybe and damage to the sail and rig.

There will be white water around the yacht and the sea state will be uncomfortable, but it is unlikely to be dangerous.

Microbursts in the UK rarely result in the storm force winds that are reported in the USA, but even without them cumulonimbus clouds need respect and the safest action is to prepare for strong, unpredictable winds.

Enjoyed reading Are cumulonimbus clouds cause for concern?

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