Why there isn’t a Cloud in the Sky?

The Great Lock-down Weather

The lock-down is becoming tedious, the kids are growing restless and stuttering Zoom calls are becoming the norm. The major consolation is that the weather is great. You have probably remarked on the same phenomena but said “aren’t we lucky the weather is good”. Amazingly, virtually every day has clear blue skies and there has only been only one day of rain since the lines of 2-metre spaced shoppers started forming outside Tesco.

Usually at this time of year we might get the occasional view of the sun but here around London it is habitually interminable grey skies. Often the cloud base is so depressingly low it almost seems to touch the roofs of the houses and we would be fortunate, in recent years, to have even a couple of weeks sunshine before the dreary English weather closes back-in.

What is going on? Are we indeed just “lucky”, is the weather a coincidence or is there a more insidious process occurring? I have a simple but leading hypothesis - the lack of planes. Gatwick is quasi closed, Heathrow activity is 90% down, Easy-jet is grounded and BA is struggling. Usually, living near London, I can see at any time several planes whirling around in the sky at various altitudes but now there are virtually none.

The fact that planes might influence our weather is controversial. If you asked any airline official they would say “impossible”. Any TV weather forecaster would say “baloney”, of course planes don’t impact the weather and we have just been enjoying a high-pressure region or some such. Most meteorologists (weather scientists) would say it was just a coincidence and there exists no proof planes impact short-term weather. However, deep in their hearts they would know that the jury is out on the subject and there is a paucity of scientific research on the issue.

Exhibit B: Troublesome Eyjafjallaj?kull

The ideal scientific method requires a hypothesis and then validation by testing in the form of replicable experiments by independent groups of scientists. Here there is of course little experimental proof since we only have one weather system and few occasions when the relentless growth of the airline industry was “switched off” to carry out any form of scientific experiment.

However, if we consider the current weather pattern (which occurred at exactly the same time as the lock-down started) as Exhibit A for the jury, then there also exists an Exhibit B. Exhibit B was the eruption of a troublesome Icelandic Volcano called Eyjafjallaj?kull in 2010. Don’t worry I can’t pronounce that either. The microscopic ash projected into the atmosphere by the volcano was thought to interfere with the jet engines of commercial aircraft and for over a week around 20 countries closed their airspace and the flights of over 10 million people were disrupted.

During this peaceful period, I recall, no aircraft seemed to fly and every day suddenly had a crystal blue sky with no clouds in sight. At the time I mentioned to my totally disinterested colleagues that it might be due to the lack of planes. As soon as the planes started flying again the London weather reverted to its usual dismal state. Interestingly, Exhibit B probably rules out everyday air-pollution as a significant factor for cloud formation since industry was still pumping out fumes and the M25 was still at a standstill.

Macroscopic Slugs

Unfortunately, there is no Exhibit C to corroborate these hap-hazard observations so I started looking at the scientific literature on the impact of planes on the weather. A few years ago, I was interested in how the computer models for global warming (now relabelled climate change) were conceived. I was slightly staggered to learn that the effects of cloud cover and cloud formation were largely ignored and I realised there was a simple explanation for this. Scientists (especially those who build computer simulations) don’t really understand clouds and cloud formation in sufficient detail.

This problem arises due to two thorny issues. Firstly, clouds are large, macroscopic phenomena in the atmosphere and cannot be readily experimented on in the confines of a laboratory. Second, cloud formation is based on complex microscopic phenomena involving something called thermodynamics of which the conditions and physics for clouds are still vague.

Science is notoriously bad at linking small scale physics and chemistry to large scale emergent phenomena even when the complexity is low. For example, knowing about electrons and chemistry (small scale) does not immediately allow derivation of the ‘laws of civil engineering’ to aid building a bridge (large scale). Knowing how a brain neuron works does not even allow us to predict the behaviour of a slug possessing only ten thousand neurons. Clouds are complex systems, and although trivial compared with the slug’s brain, it is difficult to fully know the ‘actions’ of a cloud under different conditions, particularly when they are far from natural equilibrium and a jet plane is speeding through them.

Those Annoying Contrails

The meagre scientific research that does exist on aircraft cloud formation is thus very recent. Clouds are typically formed when water vapour saturates air when the moisture increases and cooled below the ‘dew point’. However, the condensation of water vapour into droplets is encouraged by attaching to particles in a process called ‘nucleation’. These ‘seed’ particles can be dust, soot or pollution in the atmosphere of sub-micron (millionth of a metre) size or below. Specially created seed particles are used in creating artificial rain in drought-stricken regions and in a similar process in snow cannons in ski resorts but nobody has a reliable view on how they work.

Thus, atmospheric pollution we might consider to have a key role in nucleation and hence cloud formation. Aircraft spewing out carbonised exhaust and unspent fuel might provoke nucleation centres at high altitude. This is now a generally well accepted theory and that more precisely water vapour quickly condenses around the soot from aircraft exhausts to form those long streaming clouds we can see behind aircraft high in the sky. These are technically called contrail cirrus clouds. We perceive these to be high altitude and short lived and not too troublesome if you want to sunbathe.

However, the truth concerning aircraft and contrail cirrus is more subtle and worrying. They can persist for many hours and have a greater influence than expected. Back in 2010 scientists at the US National Centre for Atmospheric Research published research that as aircraft climb or descend under subfreezing temperatures they can inadvertently alter and seed mid-level clouds causing precipitation. This effect is caused as air is cooled over jet wings and can even freeze droplets to provide snow under low enough temperatures.

More recently, in 2019 University of Helsinki researchers noticed that the contrails caused by high flying aircraft could produce much heavier rain or snow if other aircraft passed within distances up to 10km. It was postulated that aircraft provoke a ‘chain reaction’ as they pass through a cloud by causing changes in air pressure that can freeze droplets which in turn freeze others around them. Astonishingly this increased precipitation and associated clouds could extend over hundreds of km behind aircraft across fairly narrow bands of sky and these high-altitude catalysts could influence much lower cloud layers.

Since the UK is only a few hundred km across and there are many aircraft flying at different altitudes and directions we might expect that such an effect could influence, under the right conditions, the cloud formation and rainfall across much of the UK. Importantly, these effects are unrelated to pollution and even if we had emissions-free, all-electric aircraft we cannot readily avoid them.

Finally, every climate warrior is alarmed by aircraft CO2 emissions and pollution but actually perhaps these are secondary considerations for climate change compared to clouds formed by aircraft. Low level clouds are considered to have a cooling effect on the planet in climate models. However, study in Nature 2011 and subsequent work from the German Aerospace Institute for Atmospheric Physics showed that the high altitude aircraft contrails contain ice crystals and act to trap heat in the atmosphere. Mainly the contrails are confined to the northern hemisphere where aviation traffic is greatest but their influence is global. The contrail effect is predicted to contribute more to global warming than all the CO2 emissions ever produced by the aviation industry.

Sunshine Breaks

Despite the sketchy, scientific evidence a mild consensus is gradually forming. Aircraft do have an impact on our weather through cloud formation but more research is required. Unfortunately, it is not in the economic interest of airlines, aircraft manufacturers and governments to research the possible destruction of a (usually) profitable industry and major employer.

By jetting off to Spain for our next sunshine break we are ironically probably ruining the weather back home in the UK. The recent hiatus in mass aviation will soon come to an end but our penchant for overseas travel is probably deteriorating our climate and our short-term weather. So best sit back, have a beer and enjoy the sunshine since it will grey, cloudy business as usual when the coronavirus crisis is over.

An opinion by Dr. Richard D. Bateson who once upon a time was a physicist at Cambridge, CERN, Institute Laue Langevin (ILL), the European Synchrotron Radiation Facility (ESRF) and UCL London Centre of Nanotechnology.