Expert comment: Questions on crazy weather answered

The UK’s weather has been up and down over the past few weeks, with mild conditions turning into record low temperatures before a rapid return to warmer weather.
 
To get a better picture of what is going on around us, we put the questions we all want answered to resident weather expert Dr Peter Inness, from the Department of Meteorology at the 色狗导航.
 
色狗导航: Why did we have such quick changes in the weather last week?
 
Dr Peter Inness: The cold weather last week was due to the air coming from a cold direction. We had a north-easterly wind for quite a long period, and that brings air down from the Arctic, Scandinavia and Siberia. If the air comes from a warm direction like from the Atlantic or from further south, over southern Europe, or from the Azores, it's going to be much warmer. These changes in wind direction can happen quickly, over a period of a day or so. On this occasion the forecast for the change was very good a week in advance.
 
UoR: How far into the future can forecasters predict the weather?
 
PI: It varies depending on what the weather is actually doing. On some occasions, it might only be two or three days ahead. On other occasions, it could be a week or more. Beyond two weeks, it's very hard to say exactly what's going to be happening at any one location at any given time. It depends on the atmosphere - if it is in an unpredictable state, we may only be able to see two or three days ahead.
 
UoR: What do forecasters need to see what’s coming?
 
PI: The main tool that forecasters use is a numerical weather prediction model, which is basically a computerised model of everything that goes on in the atmosphere. All the laws of physics, how temperature changes, how wind speed changes, those can all be written down as equations, which can be coded up into a big computer model, and solved on a massive supercomputer. It's all about writing down the laws of physics, coding them up into a computer and then running them forward in time.
 
UoR: Why is it difficult to get perfect weather forecasts?
 
PI: The atmosphere is a chaotic system. Tiny perturbations in the initial state of the atmosphere can grow rapidly with time, so sometimes the forecast can be quite different from reality, two or three days forward into the future. Our computer models are really improving, so we can put more scientific knowledge into them. We can also pin down the initial conditions better due to more and better data coming from weather satellites. But there will be a point at which we can't make any more improvements simply because of the chaotic nature of the atmosphere.
 
UoR: What weather forecasts do you use personally?
 
PI: I use the Met Office, the UK’s official provider of weather forecasts. I use their app and their website quite a lot. The amount of information they provide is what I'm looking for. I also use the website of the European Centre for Medium-range Weather Forecasts (ECMWF). They run very sophisticated computer models, looking not just at what the weather is going to be, but also the uncertainty in those forecasts. They're world leaders in looking at uncertainty and unpredictability. Their forecasts give me an idea of what other scenarios might happen. They can tell me the forecast as we see it at the moment, but also the possibility of how different the weather could be.