I’ve written to you about these before: it was 19/07/2010 18:34, and I gave this excellent link (which is still available):
The photo above was taken recently in Scarborough, Yorkshire at night-time, and shows the sun shining on the underneath of some noctilucent clouds. The summer is the time to see noctilucent clouds; you need a clear night sky, although the weather scientists have very little idea as to why they form.
Noctilucent clouds form at 80/85km (50 miles) high, which is close to the edge of space (there is very little air at that height). Most clouds are much less than 11km (7 miles) high, although Cirrus clouds (the very wispy clouds) and Thunder-Heads (the anvil-headed cumulus clouds) are at that height. The Thunderheads in particular get their shape because they push up against the Tropopause (explained below).
Here is an astonishing photo of the tropopause in action (2007, Wyoming, USA - copied from local paper):
And here is a picture of Cirrus clouds (St. Petersburg, Florida, USA photographed by Maria Ferraro on November 24, 2008):
Almost all clouds form in the region of air called the “Troposphere” (‘tropos’ is from the Greek & means ‘changes’; almost all our weather occurs within the troposphere). The troposphere is characterised by the fact that air cools as it rises; that’s why snow tends to cap the top of very tall mountains. However, there is a peculiarity: that trend (air being colder higher up) reverses at the top of the troposphere and, for this reason, the very top of the troposphere is called the “Tropopause”.
The Tropopause occurs at about 7 miles high. Below it, warm air rises. Above it, warm air tends to fall; in fact, everything above the tropopause is arranged in strata, and that region is called the “Stratosphere” for that reason (and the original jet airliners were called “Stratocruisers”, but the name never caught on).
Jet airliners now fly in the stratosphere as much as possible, because the air is very much thinner there and there is zero weather (with one big exception). In the skies above Nottingham are masses of jet-trails all the time, every day of the year. That means that those planes are flying 8 miles high (which is where the name of the song came from). If you watch jet-trails carefully you will see that they expand horizontally (in strata, you see) and that there are very rarely any strong winds at that height. Which is very misleading...
There are just 3 mountains that poke up into the stratosphere; Mount Everest is one of them. People that climb Everest need to take compressed air with them so that they have something to breathe; most of the planet’s air is within the troposphere. However, there are *very* strong winds at the very top of Everest at certain times of the year, and that is due to a river of air known as the “Jetstream”.
View from the NASA Shuttle, April 5, 1984 at about 200 miles high: this is a Westerly jetstream travelling across the Red Sea from the Sudan to Saudi Arabia:
The Jetstreams were completely unknown 50 years ago. In fact, it was a puzzling loss of the “Star Dust” (an early South American airliner) that fixed the jetstream in people’s minds. In addition, it is now thought that the jetstream is the driver of our weather.
The Star Dust was a British South American Airways Avro Lancastrian airliner. In 1947 it’s crew--fully experienced aircrew with recent combat experience & medals for bravery--simply vanished whilst flying from Brazil to Chile. 50 years later (1998) parts of the plane began to turn up then, later still, parts of human bodies. By 2002, the bodies of five of the eight British victims had been identified through DNA testing. The physical evidence suggested that the plane had flown straight into the face of Mount Tupungato (7,300m high), and dislodging some of the Tupungato Glacier which buried it from view. 50 years later it had all been carried by the glacier to the point where it melted & released some of the remains to view.
So why did G-AGWH Star Dust (pictured below) vanish?
British pilots began to experience the Jetstream regularly for the first time in the 2nd World War. The jets are only about 5km (3 miles) thick vertically, but may be hundreds of km wide and, essentially, thread their way around the entire world. The air within them streams typically at about 100 km/hour (60 mph), although 400 kph / 250 mph has been measured, and it is not unusual for the jet to blow in the opposite direction to lower, tropospheric winds. The northern jet moves East to West, whilst that in the south goes in the opposite direction, circum-navigating Antartica. The Jetstream blows just under the Tropopause, with it’s vertical height ranging from 7 to 12 kilometre.
The most likely explanation for Star Dust’s loss, then, is that they were flying directly into the Jet-Stream but did not know & therefore their navigator would not have plotted their course accurately. Modern planes have access to Geostationary satellites, but such things never existed in 1947. He would have thought that their airspeed indicated that they were close to the airport, without realising that relative to the ground it was much, much slower and they were facing a cliff-face. He sent a Morse-Code essage: “ETA SANTIAGO 17.45 HRS STENDEC” (landing in 4 minutes). They were never heard from again (and everyone asks “what did STENDEC” mean?).
The final item about the Jetstream is that it moves between banks of hot & cold air.
In broad terms, the north pole is covered with a mass of cold air, whilst the equator is covered with a mass of warm air. The two meet & interact, causing ‘weather’ (and especially in the tropics). It is known that the Jetstream flows along at the boundaries of these masses of air, and it is observed that the Jetstream is both affected by & also affects the weather. What the weather scientists have not yet managed to work out yet is the mechanics by which it all happens, nor the precise interactions. However, it does involve one of my favourite words: the CORIOLIS effect. But that is for another day.
--------- Alex Kemp