Understanding weather: a quick guide for sailors

Only the foolhardy would put to sea without first checking the weather and forecast – understanding how to make use of the information available is a vital part of sailing, says James Stevens

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Cast your mind back to your first trip out of sight of land, writes James Stevens.

If you were the skipper, my guess is that your greatest worry was the weather.

Navigation can be learnt in a classroom, but understanding weather and its effects on the sea state is very difficult to learn from a book.

The real anxiety occurs during the hours before departure, with the crew armed with euros and anticipation and the skipper scanning the internet to find a forecast better than the previous one.

The decision to set off is not easy, particularly if you have to consider whether you can get back at the end of the week.

A surprising number of Yachtmaster candidates are vague about the weather. A popular perception is that the forecaster is vague also.

A synoptic chart used for understanding weather

Synoptic charts will give an indication of weather fronts, high and low pressure areas

In fact, although long-term forecasts can often turn out to be fiction, short-term ones are pretty accurate.

The Met Office usually knows if there is a gale 48 hours away and can predict its direction along with likely rainfall and visibility.

For most people with indoor jobs, the weather has to be extreme before it even slows our journey to work.

Afloat, the difference between Force 4 and Force 5 is a change from pleasant to uncomfortable.

An energy-sapping Force 6 on the nose could at the same time be an exhilarating downwind sail.

Professional sailors are generally well up to standard at studying weather patterns and likely wind directions.

A poor career move for a sailing instructor is to ring the boss from Cherbourg to explain that the boat and crew are stormbound the wrong side of the Channel.

Understanding weather: forecasts

The modern sailor has access to a huge amount of weather information.

The internet has numerous forecasts. You can also find the websites and a lot of other information in the RYA weather forecasts booklet G5, so it is worth carrying on board if you are going longer distances.

The BBC broadcasts the national shipping forecast, and local radio stations often give really useful coastal information.

The coastguard issues inshore and shipping forecasts every three hours on the VHF radio and you can pick up forecasts at marinas, harbour offices and yacht clubs, and by telephone.

With all this available there should be no reason for the coastal sailor to get caught out in a gale.

Understanding weather: depressions

We have all experienced days which start grey and misty with drizzle. The wind is usually in the south-west, the clouds are low, and although the weather is dreary it is not bitterly cold.

Later in the day, there is a heavy shower, the sky clears, the visibility improves and the rain droplets sparkle in the sunshine.

Big cumulus rainclouds give sharp showers and the north-westerly wind breeze feels cool. A cold front has passed, and while this might not make much difference to our lives ashore, it is easily identifiable.

With a bit of practice and a close look at a weather map, you can predict this quite accurately, and your land-based friends will think you are a magician for accurately foreseeing such a dramatic change.

The dismal claggy weather preceding the cold front is the warm sector.

Understanding weather: cumulus rainclouds

Cumulus rainclouds give sharp showers. Credit: Getty

This is preceded by the warm front, a much less obvious weather feature because it sneaks up with lowering and increasing greyness and drizzle.

You can tell a warm front is on the way by observing high wispy cirrus clouds, or mare’s tails. These gradually thicken from the west and form a white milky veil called cirrostratus.

The sun or moon appears hazy and there is often a halo. If you notice this at sea you need to start planning your strategy.

If the low is passing to the north from west to east, as they normally do, you can expect a freshening south-easterly or southerly wind.

Persistent rain arrives with the warm front and the wind veers, so once you are in the warm sector you can expect the rain to be hitting the sails from the south-west.

A yacht sailing under a dark cloud at sea

Understanding weather: Expect reduced visibility in persistent rain. Credit: Graham Snook/YM

The visibility will drop, but the good news is that the cold front is on its way with a dramatic improvement.

Smart navigators watch these changes closely and use the synoptic charts which show the present and future positions of the fronts, and plan their courses and destinations.

For example, if you were sailing down the English Channel towards Ushant in the warm sector you would be beating against a south-westerly.

Knowing there is a cold front and a sizeable veer ahead, the best plan is to sail on port tack until the front passes, then tack on to starboard when the wind veers to the north-west, and with luck clear Ushant.

If you get that wrong and sail south first, there is a long beat ahead.

On a synoptic chart the isobars, which are lines joining areas of equal pressure, can tell you not only the wind direction but also its force.

Air circulates anticlockwise around lows and clockwise around highs.

The closer the isobars, the windier it is.

Understanding weather: high pressure

High pressure systems are associated with fine weather and generally light airs, but if there are lows nearby it can get windy.

Earlier this year there was a persistent high over Britain with low pressure to the south. For a couple of weeks, there were strong easterlies in the English Channel.

This made a change from the prevailing westerlies caused by low pressure to the north.

When the weather is hot and the winds light, you can expect a sea breeze near the coast.

Hot weather and light winds often lead to a sea breeze near the coast

Sea breezes are onshore winds caused by air heating and rising above the land and cool air being drawn in from the sea.

In these conditions, it pays to make coastal passages close enough to the land to take advantage of the breeze.

When I worked as a dinghy instructor in the West Country the sea breeze was so predictable that we towed the dinghies to the sailing area, rigged them and made sure everyone was holding the right rope in the right place, then let them go at 1030 to coincide with the wind coming up the harbour.

Two types of fog

Even the most relaxed skipper gets anxious in fog. There are two types of fog.

Radiation fog forms on clear nights when the land cools and the air above reaches dew point.

A skipper using a fog horn on a boat

Understanding weather: Even the most relaxed skipper gets anxious in fog. Heavy fog can be completely disorientating. Credit: Georgie Stocker

It rolls down over estuaries and coastal waters and can be incredibly thick – so bad in fact that it is possible to get completely disorientated even in your familiar home port, especially at night.

Radiation fog usually burns off in the late morning and stays close to the land so if you wait, or go offshore, it is not too much of a problem.

Advection fog is formed by warm, moist air passing over cold sea, the conditions in the warm sector.

You can experience advection fog in a gale and it usually sticks around until a change of air mass – the cold front.

Knowing when the cold front is going to arrive is critical, and it is worth keeping out of shipping channels until it does.

Applying this to sailing

Sailors should know how to apply weather information to the skills of navigation and skippering a boat.

A detailed knowledge of cloud types is much less important than being able to make passage planning decisions based on forecast information.

Before leaving port, track the weather on the internet and listen to the shipping forecasts. The terms used have very specific meanings: imminent, moderate and gale all have precise definitions.

You should be able to make an informed decision, based on the type of yacht and strength of the crew.

Remember, fronts come and go and wind strengths change, but if your preplanned course stays the same, it will result in slow, uncomfortable passages.

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