THE WEATHER
Understanding the weather is an essential thing to grasp when you’re planning a stint of out-of-season sailing.
There are four challenges to contend with as sailors: frontal depressions, cold polar air, low sea temperatures and fog. Depending on whether you’re in the autumn, winter or spring their frequency will vary, but you need to be prepared for them at any point.
The seasons are created by the sun’s annual migration from 23° south in mid-winter, to 23° north in mid-summer.
It’s an extraordinary journey covering almost 3,000 miles in six months. The sun drags the weather systems with it and the patterns we experience shift dramatically, giving us our seasons.
If we’re lucky, during the summer months the British Isles lie under the protective embrace of the Azores High, which is pushed north towards us. It creates stable, dry weather which can be rather windless unless a sea breeze kicks in.
FRONTAL DEPRESSIONS
As the sun slips back across the equator at the time of the autumnal equinox we must fend for ourselves against the trademark weather feature of the North Atlantic: frontal depressions. These systems are generated on the boundary between two different types of air mass: cold, dry polar air and warm, wet maritime air. The boundary forms two distinct fronts. The warm front is the leading edge of the low and brings a grizzly, grey day of drizzle and rain. It precedes the warm sector, which is a wedge of maritime air that has infiltrated the surrounding polar air mass. Behind the warm sector is the mighty cold front. Though more compact in stature than its sprawling warmer brother, it packs quite a punch. Towering cumulonimbus clouds bring heavy, often thundery, downpours and gusty conditions. Although it heralds the beginning of the end of the low pressure system this is often where the strongest winds are found. This, combined with a marked veer in direction, makes for tricky conditions on the water. Often a secondary low is spawned on the trailing edge of a cold front, perpetuating a cycle of unsettled conditions.
Although these systems are generally well forecast these days, it pays to take heed of synoptic charts. Simon Rowell, meteorologist for the British Olympic Sailing team, explained that ‘the autumn is particularly unstable as it’s the time of year when sea temperatures are at their highest and it’s peak hurricane season in the Caribbean. Some of them will come our way as deep lows.’
SEA TEMPERATURE
One of the most under-appreciated changes through the winter is sea temperature, both on a local and global scale. ‘The temperature difference between the poles and the equator increases dramatically through the winter months,’ explains Rowell. ‘This imbalance is a key driver and one of the reasons behind stronger trade winds and more extreme weather in the winter.’
On a more local scale, the temperature differential between the land and sea which creates summer sea breezes, weakens. A windless day in the summer can become a Force 4 by early afternoon thanks to the convection created by rising temperatures on shore, but that won’t be a feature in the winter months. A windless day will most likely remain a windless day.
One of the biggest differences between spring and autumn sailing is the sea temperature. It’s still relatively high in October, but hits rock bottom around the end of February just as bright spring days and longer hours of daylight lure us out on our boats. What may seem pleasant weather on land is very different offshore, and the invisible chill of the water should not be underestimated.
COLD POLAR AIR
In my early twenties I worked on the Fasttrack Yachtmaster Programme at UKSA which ran all year round. One of the most senior skippers gave me a stark warning at the beginning of winter: ‘Beware the easterlies,’ he said with dread in his voice. He was right of course. At least with a chain of depressions you get a let up at some point. But if high pressure establishes itself over Scandinavia then expect relentless, bitter winds of Force 6 and above. It’s what caused the Beast from the East in 2018 and it’s a time for armchair sailing only.
FOG
Fog is an often-overlooked feature of winter sailing. Technically it’s visibility of less than 1,000m, although anything less than 2 miles probably requires a change in the way you’re operating on board. As a rule of thumb if you can’t see the horizon from the cockpit of a small yacht, the visibility is 2 miles or less.
It’s easy to fall into the habit of just checking the wind forecast on an app and forgetting visibility. In the summer months fog is less likely, but out of season the risk increases. It’s an integral part of the Shipping forecast though, so pay attention to it.
There are two types of fog: advection, or ‘sea’ fog, and radiation or ‘land’ fog. The ‘land’ and ‘sea’ refer to the cold surface on which the fog forms when warm, damp air drifts over it. Although they look the same from the inside, it’s important to differentiate them as their impact on sailing differs.
Land fog is a local phenomenon which tends to occur in the autumn when warm land radiates heat and moisture to the air above overnight. This causes the morning mists which gather in estuaries, rivers and other low-lying areas. It’s a purely coastal phenomenon which usually burns off by late morning or by heading a mile or so offshore. You shouldn’t need to cancel a whole day of sailing for land fog, but you might want to hit the snooze button a few times.
Sea fog is much more widespread, usually in spring when sea temperatures are at their lowest. Large areas of relatively warm, damp air drift in from the Atlantic and condense when they reach cold water. The resulting fog is pernicious – it doesn’t disperse even with strong winds and requires a whole new air mass to arrive before clearing up.
AIR DENSITY
When I’ve sailed in the high latitudes I’m always amazed by how forceful the wind is. Cold air seems to hit the sails much harder. There isn’t a definitive explanation for this, with theories ranging from the increased density of the air to more complex hypotheses. Whatever the cause, you’ll need to be more conservative about the sail plan.