One evening last week, at the end of my working day, I looked up and saw long white streaks high in the sky, thin and feathery, as if someone had brushed them across the blue with a dry paintbrush. Behind them, a whole front was rolling in, and it was spectacular. I did not need an app to tell me what that meant. Rain was on its way, and within a day or so it would arrive.
I never look at a weather app anymore. I look at the sky. For most of history, and for every sailor I have ever raced with, the forecast was written up there, and it still is. It is free, it updates itself every minute, and all it asks is that you look up once in a while. I can walk my dogs in the morning, glance at the clouds, and know what the rest of the day is going to do, and I still find that wonderful.
It helps to know where wind comes from in the first place. The sun heats the earth unevenly, most of all around the equator. Warm air there rises, a bit like a hot air balloon, cools as it climbs, and spreads out towards the north and the south. It does not make it all the way to the poles. Somewhere around thirty degrees north and south it has cooled enough to sink back down again, and where it sinks, it presses on the surface and creates great belts of high pressure. From there, the air flows back along the surface towards the equator to fill the space left by the rising air. A giant loop, running day and night.
If the earth stood still, that returning air would blow straight from north to south. But the earth spins, and at the equator the ground is moving at roughly a thousand miles an hour. Air moving across a spinning planet gets bent, to the right in the northern hemisphere and to the left in the southern one (on its own, that is enough to send the flat earth theory back to the kitchen cupboard, where plates belong). That is the Coriolis effect, and it turns that returning air into the trade winds: steady, reliable winds from the northeast above the equator and from the southeast below it, which sailors have relied on for centuries.
In my last Note I wrote about racing from Cape Town to Rio. What I did not explain is why we sailed close to four thousand miles on a course of three thousand six hundred. The South Atlantic high sat right across our way, a huge area of sinking air with very little wind in the middle. Sailing straight through it would have meant parking the yacht for days, so we went around it to find the trade winds on its northern side. And even in the trades, we watched every cloud, because under a cloud the wind is almost never the same as next to it. Usually stronger and shifting, sometimes dying away completely once it has passed.
High and low pressure are, in the end, about balance. In a high, air sinks and presses down, the sky tends to clear and the weather settles. In a low, air rises, and with it come clouds, wind and rain. Air always wants to move from high to low, the way air rushes out of a balloon, but because the earth spins it never goes in a straight line. It spirals, anticlockwise around a low in our hemisphere, and the bigger the difference in pressure, the harder it blows. Here on the Isle of Man we live in the belt where those lows come rolling in from the Atlantic one after another, which is why most of our weather arrives from the west.
And that is where the clouds come in. Cumulus are the friendly ones, the soft cotton wool clouds of a sunny afternoon, and as long as they stay small with flat bottoms, the weather is fine. When they start to grow tall, swelling upwards like cauliflower and spreading out at the top like an anvil, a shower or a thunderstorm is on its way.
I once won a free dinner with that. I was sitting on a terrace in La Rochelle during the boat show, sun shining, beautiful weather, when I started to feel puffs of cold wind on my face. The leaves on the trees would move, then nothing, then another cold puff. That is air cooled by rain, falling out of a shower cloud and spreading out ahead of it, and it means the rain is not far behind. So I asked the waiter if I could sit inside. Why, he asked. Because it is going to rain. He laughed. He had lived there for twenty five years, and it was not going to rain. I insisted, he insisted, and in the end we made a deal: if it rained, he would pay for my dinner, and if it stayed dry, I would pay double. I went inside. It rained. Dinner was on him.
Cirrus, like the ones I saw that evening, are the highest clouds of all, made of ice crystals five miles or more above your head, and they are often the first messengers of a warm front. If they are followed by a thin milky veil, sometimes with a halo around the sun or the moon, and then by a grey layer that makes the sun look as if you are seeing it through frosted glass, the rain is not far behind. Sometimes those ice crystals catch the low sun and create a so called “sun dog”, a small bright patch of rainbow light beside the sun, at the same height, like a second sun keeping it company. You can see one in the photo above. It is beautiful, but it is also a message: there is ice high up, and the weather is about to change. Rows of small white puffs, the sheep clouds as we call them in Dutch, often mean the same: change is coming. Sailors put it in an old rhyme: mackerel sky and mares’ tails make tall ships carry low sails.
Even the trails behind aircraft tell you something. When they disappear almost as soon as they form, the air up there is dry and the weather is likely to stay settled. When they linger and slowly spread into a white sheet across the sky, the upper air is moist, and that often means a change is on its way.
None of this makes a proper forecast useless, especially when you are heading offshore. But there is something different about knowing it yourself, about walking out with my dogs, glancing up, and understanding what the sky is telling me before anyone else does. The sky has been giving the forecast away for free for as long as there have been people to read it. Most of the time, we are just looking down.
