The science behind Europe’s Siberian chill this week

Nice out. The A48 in the snow. Image: Getty.

The so-called “Beast from the East” arrived in the UK this week, bringing unusually cold weather – about 7°C colder than the historical average for this time of year. Wind chill is making temperatures feel particularly arctic.

So how did the Siberian gusts come to arrive on Europe’s doorstep?

The movement of air across the globe, and the weather it brings with it, is governed by three major influences: gravity, the sun, and something called the Coriolis effect. The influence of gravity is simple, constantly pulling air towards the Earth’s surface.

The rise and fall of the sun dictates whether the air stays there. During the day, radiation from the sun heats the Earth, warming air directly above the surface and causing it to rise, leaving behind a region of low pressure (a low density of air particles). As the air rises, it cools and spreads outwards.

This mass of air, now denser than the air below it, sinks back down under the force of gravity, and naturally flows back towards the lower pressure region of air, creating a cycle of air circulation. These circulating patterns of wind exist on an intercontinental scale, transporting heat all the way from the tropics to the poles.

However, thanks to the Coriolis effect – the deflection of objects moving in a straight path due to the Earth’s rotation – the winds do not travel directly north or south. To illustrate this effect, imagine a spinning top. Parts of the spinning top closer to the spindle rotate at slower speeds than parts further away, as they have less distance to travel to complete a full circle. Similarly, the equator has to travel much faster than the poles do as the Earth rotates. As air travels north from the equator, its extra momentum compared to the slower rotating land that it is moving over makes it curve across to the east, while air travelling to the south pole curves westward.

In the northern hemisphere, this interaction between the Coriolis effect and the circulation systems produces the northern polar jet stream: high altitude currents of air blowing eastwards at hundreds of miles per hour, moving weather systems around the globe. This causes the UK’s prevailing westerly and south-westerly winds, which usually draw weather systems in from the relatively warm Atlantic and shield us from colder air masses to the east.

The shape of the jet streams is not rigid – it follows a meandering path, much like a slithering snake. Occasionally, the jet stream path can become so twisted that it folds back upon itself, reversing the direction of the prevailing wind, and drawing in cold air from the east.

This is exactly what just happened. In the last couple of days, the bitterly cold front combined with water vapour in the air to carpet the country in a blanket of brilliant white.

As the warmth of the sun disappears each night, the cold can feel all the more biting. But in the absence of the sun’s heat, the smaller difference in temperature between air near the ground and higher up makes air circulate more slowly. This often creates calmer conditions that might just provide a brief respite from the extra chill of the wind. For this same reason, air passengers generally experience smoother flying conditions when flying at night.

If you live in the city however, your experience of the “beast” can vary wildly from place to place. Cities continue to produce heat at night, generating their own microclimates. This man-made heat keeps air moving, and warms city dwellers up more than those in rural areas. At the same time, the ordered formation of buildings in cities creates strong wind corridors that are certainly best avoided at times like these.

Wherever you are experiencing this freezing weather, you can at least be thankful that you are here on Earth. Wind circulation patterns on other planets produce far more extreme weather than we will ever experience. Visitors to Venus, for example, would experience some serious turbulence when approaching landing, as the 500°C difference between surface and cloud generates extreme air circulation.

The ConversationHowever, if you were lucky enough to touch down and survive the experience of the crushing pressure found on Venus’ surface, you would feel nothing more than a gentle breeze, thanks to the planet’s very slow rotation, weak Coriolis effect, and dense air. You might want to seek shelter though – at close to 460°C, suddenly a cold chill doesn’t seem so bad.

Gareth Dorrian, Post Doctoral Research Associate in Space Science, Nottingham Trent University and Ian Whittaker, Lecturer, Nottingham Trent University.

This article was originally published on The Conversation. Read the original article.


Why doesn’t London build an RER network, like Paris did?

A commuter walking by a map of the RER B line at the Chatelet-Les Halles station in Paris. Image: Getty.

I’ve heard many people make many different complaints about the Parisian transport system. That it does a bad job of linking a rich, white city with its poorer, more diverse suburbs. That, even as subway systems go, it’s a hostile environment for women. That the whole thing smells distractingly of urine.

I’m familiar with all of these complaints – I’ve often smelt the urine. And I’m aware that, in many ways, London’s is the superior transport network.

And yet I can’t help be jealous of Paris – In large part, because of the RER.

Central Paris. The Metro lines are thinner, and in pastel shades; the RER lines are thicker, and in brighter colours. Image: RATP.

Paris, you see, has not one but two underground railway systems. The more famous one is the original Paris Metro, opened in 1900: that’s the one with those fancy green portals with the word “metropolitain” written above them in a vaguely kooky font.

The Metro, though, mostly serves Paris Intra-muros: the official city, inside the Boulevard Périphérique ring road, site of the city’s last set of walls. As a result, it’s of very little use in most of the city’s suburbs. Its stations are very close together, which places a limit on how fast its trains can cross town. It was also, by the mid 20th century, becoming annoyingly overcrowded.

So starting in the 1960s, the city transport authorities began planning a second underground railway network. The Réseau Express Régional – Regional Express Network – would link suburban lines on either side of Paris, through new heavy rail tunnels beneath the city. Its stations would be much further apart than those of the metro – roughly one every 3km, rather than every 600m – so its trains can run faster.

And fifty years and five lines later, it means that 224 stations in the suburbs of Paris are served by trains which, rather than terminating on the edge of the city, now continue directly through tunnels to its centre.

The RER network today. Image: RATP.

London is, belatedly, doing something similar. The Elizabeth Line, due to open in stages from later this year, will offer express-tube style services linking the suburban lines which run west from Paddington to those which run east from Liverpool Street. And Thameslink has offered cross-town services for 30 years now (albeit not at tube-level frequencies). That, too, is going to add more routes to its network over the next few years, meaning direct trains from the southern suburbs to north London and vice versa.

Yet the vast majority of suburban National Rail services in London still terminate at big mainline stations, most of which are on the edge of the centre. For many journeys, especially from the south of the city, you still need to change to the London Underground.

So, could London ape Paris – and make Thameslink and Crossrail the first element of its own RER network?

In a limited way, of course, it’s doing just that. The next big project after Crossrail is likely to be (original name, this) Crossrail 2. If that gets funding, it’ll be a new south-west to north-east route, connecting some of the suburban lines into Waterloo to those in the Lea Valley.

The proposed route of Crossrail 2. Click to expand.

But it’s not immediately obvious where you could go next – what Crossails 3, 4 or 5 should cover.

That’s because there’s an imbalance in the distribution of the remaining mainline rail services in London. Anyone who’s even remotely familiar with the geography of the city will know that there are far more tube lines to its north. But the corollary of that is that there are far more mainlines to the south.

To usefully absorb some of those, Crossrail 3 would probably need to run south to south in some way. There is actually an obvious way of doing this: build a new tunnel from roughly Battersea to roughly Bermondsey, and take over the Richmond lines in the west and North Kent lines in the east, as a sort of London equivalent of RER C:

Our suggestion for Crossrail 3. Image: Google Maps/CityMetric.

But that still leaves a whole load of lines in south and south east London with nowhere to send them beyond their current terminal stations.

In fact, there are reasons for thinking that the whole RER concept doesn’t really fit the British capital. It was designed, remember, for a city in which the Metro only served the centre (roughly equivalent of London’s zones 1 & 2).

But London Underground wasn’t like that. From very early in its history, it served outer London too: it was not just a way of getting people around the centre, but for getting them there from their suburban homes too.

This is turn is at least in part a function of the economic geography of the two cities. Rich Parisians have generally wanted to live in the centre, pushing poorer people out to the banlieues. In London, though, the suburbs were where the good life was to be found.

To that end, the original operators of some lines weren’t just railway companies, but housing developers, too. The Metropolitan Railway effectively built large chunks of north west London (“Metroland”), partly to guarantee the market for its trains, but partly too because, well, housing is profitable.

In other parts of town, existing main line railways were simply added to the new underground lines. The Central line swallowed routes originally built by the Great Western Railway and London & North Eastern Railway. The District line absorbed part of the London, Tilbury & Southend Railway.

At any rate: the Tube was playing the same role as the RER as early as the 1930s. London could still benefit from some RER-type services, so hopefully the Elizbaeth Line won’t be the last. But it doesn’t need an entire second metro network in the way 1960s Paris did.

There is another idea we could more profitably steal from Paris. Those suburban railways which aren’t connected to the RER are still run by the national rail operator, SNCF. But it uses the Transilien brand name, to mark them out as a part of the Parisian transport network, and – as with the RER – each route has its own letter and its own colour.

The Transilien & RER networks in Paris. Image: Maximilian Dörrbecker/Wikimedia Commons.

This would not have the transformative effect on London that building another half a dozen Crossrails would. But it would make the network much easier to navigate, and would be almost infinitely cheaper. Perhaps we should be starting there.

Jonn Elledge is the editor of CityMetric. He is on Twitter as @jonnelledge and on Facebook as JonnElledgeWrites

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