The natural Navigation: The Right Course, Naturally

I took this picture a few minutes ago. For those of you interested in these sorts of cloudscapes then here is a good tip for finding them:

Take a really keen interest in the sky whenever bad weather is forecast after a good spell. Don’t wait for the change to become obvious though, the interesting higher clouds: the wispy cirrus, mares’ tails, mackerel skies etc. will all be found just before most people notice that the weather is changing, so you need to remain aware.

It works the other way round too of course, if you don’t have access to forecasts, or prefer to do your own, then this sort of sky after settled weather is a strong sign that a big change is on its way. More tips on weather lore here.

(There should be a great opportunity to see some moonlit cloudscapes tonight hopefully too.)

Venus and the brighter stars, like Arcturus, appeared shrouded in a veil this morning.

This was not mist close to the ground – horizontal visibility was excellent – but thin layers of high cirrus clouds. This effect has been used by navigators and travellers the world over as a sign that the a front may be approaching and a weather change is likely.

Cirrus on its own is not a guarantee of anything, but when followed, as it so often is, by cirrostratus and altostratus it is a strong indicator of an approaching warm front.

In my book I give the example of the frequent Greenland traveller, Gretel Ehrlich, who noted during a dog sled trip with a local hunter that a ring circling the sun in the morning signalled bad weather. Similar examples are to be found in deserts, on Pacific islands and among students of weather…

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I was just ‘tweeted’ by Anne who had spotted what initially appeared to be an unusual light phenomenon appearing in some cirrus clouds. I think it is just a small arc of a standard ‘primary rainbow’, but part of me desperately wanted it to be a ‘fire rainbow’ which I have never knowingly seen.

Fire rainbows (see photo) are very rare and form in cirrus ice crystals at high altitudes; their coloured arcs are near horizontal and parallel to the horizon. Fire rainbows can only come into being if the sun and atmospheric conditions meet very stringent criteria, another of nature’s beautiful balancing acts. There is more background to these extraordinary light shows on the National Geographic website.

As this website explains, fire rainbows can only occur when the sun is high enough in the sky (58 degrees or more) and sadly that rules them out from…

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A weekend of sailing has just passed, one that was full of meteorological kindness.

Nature was generous in two ways over the weekend – perhaps as a reward for my having set up a page of weather lore?

Firstly, blue skies, fair winds and warm sun ruled over the Solent. Secondly, an approaching warm front signalled its intent, in line with meteorological science and common folklore, by sending a team of mares’ tails ahead. These cirrus clouds would be followed by cirrostratus, altostratus and then the rain-bearing nimbostratus. A show was being promised, and then put on.

Our good fortune continued on Sunday, when we were given a ringside seat to watch the nimbostratus, but never had to get too close. The dark grey clouds advanced slowly and pressed down to the west of us. The rain fell barely a mile from our small Contessa 32, but by the…

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The northerly winds were carrying high cirrus and contrails down towards the coast this morning. They have brought colder air, as forecast yesterday. This gave us our first frost of the season. The feel and even the sounds of the grass underfoot have a relationship with the direction the air is moving.

I took this picture a week ago. It shows the lower fair-weather cumulus clouds against the upper cirrus clouds. It is not at all unusual to watch the lower clouds and upper clouds move in different directions and to feel a third wind direction on your face at the same time.

This morning’s sun was a strong enough clue, but if we wanted to know which way was southeast then these aircraft contrails are pointing the way to the continent.

It looks like a particularly busy morning for aircraft, but this is just a reflection of atmospheric conditions. The hydrogen-rich jet fuel has mixed with oxygen, reacted in the engines and formed, among lots of other lovely and not so lovely things, water. In certain temperatures and humidity levels this water freezes into ice crystals. The high cirrus clouds that we normally see are also composed entirely of ice.

The length of time that a contrail survives depends on the humidity, if the air is dry it will sublimate away, but if saturated they will last as long as other cirrus clouds.

Tonight’s early evening sky is a feast of cloud types. Cumulus, passing low and darkened by the shade, perhaps the last of the fair-weather clouds for a bit. Higher there is cirrus, cirrostratus and altostratus all heralding the approaching warm front. Thrown in for a bonus there are some contrails from aircraft heading to and from the continent.