The natural Navigation: The Right Course, Naturally

We may never know the exact method that the earliest explorers used to find their way, but there is a friendly finger of suspicion that gets pointed regularly at the birds.

Some of the routes used by the pioneers of the Pacific match the migratory routes of the birds exactly.

The route used by the Maori fleet that sailed from Tahiti to New Zealand sometime in the fourteenth century and settled there is the same as that taken by the Long-tailed Cuckoo each September.

I like to think of these earliest navigators. I imagine them gazing up as flocks of birds head uniformly over the horizon in one direction only to repeat the exercised in the opposite direction half a year later. It does not take great leaps of the imagination to deduce that the birds are not doing this great exercise for fun, QED, there must be something in…

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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.

Venus and Sirius both beamed at me this morning during my pre-dawn shiver outside. The aircraft were painting a pink path to the continent to escape the autumnal cold. They are of course heading southeast, which I’m sure you checked from the tiny crescent of the moon. Speaking of crescents, this morning calls for a hot croissant.

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If you look closely at these two pictures you can see how the aircraft contrail cuts the lower cumulus clouds at a slightly different place in the second picture. It is not dramatic, but one clue that the upper winds at the altitude of the aircraft, probably about 35,000 ft, may be moving in a different direction to the lower winds moving the clouds. The upper wind direction is likely to be the more consistent and can be gauged by lining contrails or cirrus clouds up against fixed features like high landmarks, or even by lying under a tree.

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.