It is a Saturday morning as I write this while sitting in Granary Square in Kings Cross, London. I’ve just enjoyed an Ethiopean filter coffee at Caravan. If only more cafes offered the possibility of sampling single estate coffees rather than the espressos that are otherwise so popular in London.
In the square outside, people are laughing (and dancing!) in front of the old warehouses that accomodate Caravan. Amongst them all, four sets of ground-level fountains push jets of foaming water 50cm into the air, in patterns that change as you watch. There is so much physics here to observe: The white colour of the water foam, the dance of the water droplets as they emerge from the main jet of the fountain and then fall back to earth, the fact that the wet concrete around the fountains is darker than the dry concrete nearby.
Consider though one more observation. As the water shoots upwards, it is pushed by occasional gusts of wind from west to east making the fountains appear as loops rather than columns of dancing liquid. Although the direction of the wind is determined by local weather patterns, over the UK the prevailing wind direction is Westerly, that is flowing from west to east.
People have wondered about the origin of the winds from ancient times. The Greeks had four wind Gods who had authority over the winds from each direction: Boreas, god of the North wind, Notus of the South, Euros of the East and Zephryos of the West. Pliny the Elder speculated at length on the causes of the winds and yet the start of the modern conversation regarding the origin of the winds had to wait until 1686 with the publication of a work by Edmund Halley.
Halley (1656-1742) is now more famous for the comet that is named after him rather than his meteorological work but, as with many scientists of the time, he had his finger in many pies. He also seems to have been a keen coffee drinker, or at least, he regularly spent time in one of London’s coffee houses, the Grecian, discussing science with Isaac Newton, Hans Sloane and others. A pub, the Devereux, now stands on the site of the old Grecian in a little side street off of Fleet Street.
Did Halley ponder cloud formation, rain and the origin of the winds while contemplating his steaming coffee cup on cold days in 17th Century London? Regardless, Halley did recognise that the heat from the Sun was the driving force for the wind system. Halley surmised that as a parcel of air was heated by the Sun and rose upwards, the cold air surrounding it would have to flow in to its place so as to replace the risen air so “..by a kind of Circulation N.E. Trade Winds below will be attended by a S.W. above, and the S.E. with a N.W. Wind above”* The problem for Halley was that his explanation of the wind system could account for a North-South wind direction owing to the Sun’s heating the air at the equator, but not the Easterly direction of the Trade Winds near the equator nor the Westerly direction of the winds over the UK.
A few years later, George Hadley (1685-1768) suggested that it was the rotation of the earth that was responsible for the east-west component; the mass of air, being detached from the earth, would appear to flow in a particular direction as a consequence of the earth spinning below it. The idea was not new, Galileo had proposed it some years earlier while similar arguments were made later by the philosopher (and scientist) Immanuel Kant (1724-1804). At first sight, such an argument looks appealing but there are problems, as John Herschel (1792-1871) pointed out. If this were the explanation for the wind direction, the effect would be “so great as to produce not merely a wind, but a tempest of the most destructive violence”†.
Herschel suggested, as had Hadley before him, that friction could slow the wind to the speeds that we normally observe, but while this may explain the wind speed at ground level, what about the upper circulatory patterns noted by Halley: What friction could slow these down?
It turns out that this is not the reason for the discrepancy in the wind speed. Hadley’s theory was wrong on a number of issues (if you are interested, I suggest reading this article). The real driving force for the Trade Winds is the Coriolis effect which deflects the warm air rising at the equator towards the right as it travels to the North pole. The majority of this air then cools and descends at about 30 degrees latitude, circling back on itself (as per Halley) as the Easterly trade winds. However the air that continues in the westerly direction north (or south) of 30 degrees latitude becomes those prevailing westerlies of the sort that batter the shores of the UK (see here for more information).
Even if Hadley’s simple model was wrong, its contemplation did lead to an important discovery that is still relevant for us today. The question was: What was it in the upper atmosphere that could cause a friction effect that could slow the winds? The person contemplating this question was taking a walking holiday in the Alps in the first half of September in 1886. Hermann von Helmholtz (1821-1894) observed a layer of clouds which showed “whirls formed by perturbation and rolling up” of the surfaces of two neighbouring layers of air. Helmholtz had observed what became known as “Kelvin Helmholtz clouds”, a beautiful but very rare cloud type, for an example click here. Helmholtz realised that the formation of these clouds required that two layers of air rubbed against each other. In the region between the two layers, the air became unstable, wavy and finally showed the whirls which are actually a series of vortices. As these vortices developed, the two layers of air would get more thoroughly mixed and it was in this way that friction could develop in the upper atmosphere.
Such vortices and “surfaces of discontinuity” are now an important concept in many places including the coffee cup. The video “Coffee Rings” presents another manifestation of the effects of surfaces of discontinuity. So we have returned from contemplation of the wind in a late summer square in London, through a famous Coffee House and back to the coffee.
I have not yet had the opportunity for myself to see a Kelvin Helmholtz cloud. If any reader has been so fortunate please share photos with @thinking_bean. Let me know what you think and what you see around you in the comments section below and most importantly, enjoy your coffee!
*from E. Halley, An Historical Account of the Trade Winds, Transactions of the Royal Society, 1686, p. 133, via “From Watt to Clausius”, DSL Cardwell, Cornell University Press, 1971
†Quotes taken from Anders O Persson, “Hadley’s Principle: Understanding and Misunderstanding the Trade Winds”, History of Meteorology, 3, (2006) p. 17 (linked in article)