As I completed a quiz Wednesday to see how badly I needed to take an online course in forecasting aviation turbulence (if I did aviation weather, I would definitely need to complete this course, believe you me!), a recent article that was directly related caught my eye.
A British atmospheric researcher at the University of Reading has postulated that climate change may be linked with an increase in clear air turbulence/CAT incidents, and that continued warming would produce a continued increase.
Dr. Paul Williams and a colleague have looked at reported clear air moderate to severe turbulence incidents, which have been increasing, and ran models used to predict CAT along with climate models to seek causes. Their hypothesis is that global warming has increased wind shear at altitudes of 6 to 7 miles.
Wind shear is a sudden change in wind speed or direction. Most of you have looked up and seen different cloud layers moving in different directions. Where those directions shift or speeds increase, there is an element of wind shear, though it may be minor.
Airliners often encounter CAT at high altitudes with little or no warning, because there are no visual or onboard radar clues to its presence. Moderate turbulence will spill your drink. Less common severe turbulence will scare the noun-of-your-choice out of you. Unbelted passengers and flight crew get violently tossed up and injured, sometimes badly. The aircraft gets shaken, but at least it, generally, is none the worse for wear.
Williams says the warming aloft – warming occurs not only at the surface and in the oceans, but at higher altitudes as well – has increased the difference in speeds between different layers within the jet stream. If that is correct, CAT would be increasing. He noted the FAA reported a doubling of major severe turbulence incidents between 1982 and 2003, even with statistical adjustments made for the increased number of flights.
So, CAT does appear to be going up. Williams states that all 21 models he ran show such an increase will continue with further increased warming aloft.
This is not an open-and-shut case. While CAT incidents have gone up, the trend has not been causally proven to be related to climate change. This is speculative science and hasn’t gone through a complete peer-review process.
In fact, Rutgers Professor Jennifer Francis says the mid-latitude jet stream has actually slowed, though that does not address the issue of shear between layers.
Regardless of whether the increase in CAT is climate-related or not, Williams notes there are now thousands of moderate to severe turbulence incidents per year.
At the other end of the scale is the good news that the forecasting and detection of low-level wind shear has improved, especially with better pilot training and the advent of both FAA terminal Doppler radar and the network of NWS Dopplers (we are fortunate to have our NWS Doppler right at the airport). That has greatly reduced weather-related hazards on takeoff and final approach, which used to be related to a number of major airline disasters.
In the meantime, both nervous and calm flyers should stay buckled as much as possible, even if the captain has turned off the seatbelt sign. Remember, the captain can’t see CAT.