Weather

Talking Met Captain Chris Leech, Log Board member, interviews two senior aviation experts from the Met Office Darren Hardy Senior National Aviation Met Adviser Responsible for fulfilling the national aviation met requirements of the CAA through the provision of a range of aviation products Jon Dutton Head of Aviation Responsible for the provision of aviation met products and services on behalf of the CAA, including our world area forecasting and volcanic ash aviation commitments also affecting route planning. Chris: Does the prevailing wind ‘prevail’ as much as it used to? As pilots, we are particularly interested in the crosswind we experience on take-off and landing, as it makes handling the aircraft more difficult and our planes have absolute crosswind limits. Again, my perception is that, since I started flying, we are experiencing more occasions when strong winds don’t align with runway direction; obviously, a change in prevailing wind could have an impact on the planning of new runways. Jon: We have the perception that the prevalence of easterly winds is increasing, possibly linked to a phenomenon such as the strength of the North Atlantic oscillation, but it’s difficult to be sure on the wider scale. Chris: Is it more turbulent at altitude? When I started commercial flying, I don’t remember having the seat belt signs on in the cruise very often. It appears much more common now, but is this just that we are more sensitive to light turbulence in an increasingly litigious age? Jon: This may be partly because there are simply more flights; more flights mean there are more opportunities to experience turbulence. However, work by the University of Reading predicts an increase in turbulence thanks to climate change. Here at the Met Office, the current models look at wind shear potential not severity, but we are moving to a more ‘severity-based’ indicator. Darren: Also, at the moment, we use an approximately 140km grid which, since turbulence appears in pockets, means an aircraft can easily slip through the gaps, encountering turbulence where none was predicted, or missing it where it was. Greater computing capacity is allowing us to move to a resolution of 25km horizontally. Instead of the 17 vertical levels currently used, every flight level between FL050 and FL630 can be used. Chris: Is it becoming more difficult to forecast the weather? During the last wind events, the actual wind experienced was, on several occasions, greater than the maximum wind indicated on the TAF (significant if you are anticipating being close to max crosswind limits). Also, as successive TAFs were issued, there was a significant change in what was forecast for a given period. Jon: The predictability at five days is now the same as it was at three days, 20 years ago. In respect of the UK’s weather patterns, part of the problem is that the path of a weather system rapidly moving in from the southwest is critical by quite small margins. For example, at your base, if the centre crosses north Wales, then Cardiff airport could experience strong winds, but if it moves over south Wales – not a huge distance laterally – Cardiff airport could experience relatively light winds. Darren: Also, the format of the TAF is somewhat restrictive. While a TV forecaster may be able to use phrases such as “the situation is somewhat unclear”, that wiggle room isn’t available to us in aviation forecasting. Chris: Are the above trends just a cyclical blip, or part of a general change in weather patterns, such as global warming and climate change? If so, what does this mean for aviation in the UK and globally? Darren: This is ongoing work, but we are considering things such as Tarmac standards, where increased heat may affect the surface of runways. Also, at higher airports – for example, Denver – performance may become more of an issue, requiring longer runways. At the opposite end of the height scale, low-lying airports, say London City, are vulnerable to sea level change. We have seen a 17cm rise globally since the start of the 20th century, and a 16cm rise here in the UK during the same period, equating to about 1.4mm per year. Even if the airport footprint is not directly impacted, it could affect access to the airport, and drive demographic changes and effects on patterns of tourism that may significantly alter airport demand. Chris Is it getting windier? I remember some years ago, as a new captain, operating into Brussels with the wind gusting 44 knots. That stuck in my mind as, at the time, this seemed uncommon, but over the past few years there have been numerous occasions where the wind has been that strong or stronger. Or is it just a case that we are more aware of these events because they are now named Darren: There’s strong evidence, presented by the Met Office, of global temperature rise since the middle of the last century. We would expect this to lead to increased moisture and, therefore, energy in the atmosphere, and thus increased wind. Jon: This increase in the energy and moisture content of the atmosphere has other effects. More moisture means more convection, so more Cb activity and thunderstorms – a real problem in high-density airspace, such as the London TMA. It also means the troposphere - the lowest layer of Earth’s atmosphere – is getting higher. Currently, over the UK, the tropopause – the interface between the troposphere and the stratosphere – sits at around 30,000-35,000ft, which most jet aircraft can get above. If that is not the case, it will affect flight planning and optimum route utilisation in the future. Rising temperatures at the poles, too, is causing the jet streams to creep further north again, WEATHER Talking WEATHER Talking Met WEATHER Talking Met Captain Chris Leech, Log Board member, interviews two senior aviation experts from the Met Office WEATHER