Professional Pilot, May 2018

Strong winds and gusts W ith a frantic phone call the FBO manager said You should get out here as soon as you can The squall line that just rolled through collapsed the hangar Theres substantial damage to your aircraft Such is a phone call no pilot ever wants to get from their FBO but it happens more often that we may think Wind remains at the top of the list of meteorological conditions that contribute to aviation accidents While some of these accidents are to hangared or otherwise unoccupied aircraft year after year we still manage to fill the investigation files with accidents stemming from pilots taking wind for granted at critical times in a flight Air pressure as a force of nature Wind is the force applied against an object by the movement of the air Uneven heating of the atmosphere by the sun produces differences in air density from one place to another and at scales of a few millimeters to thousands of kilometers To understand this we just need to look at a couple of basic equations that apply to the atmosphere First is that force equals mass 80 PROFESSIONAL PILOT May 2018 times acceleration Density of the air supplies the mass which is simply the number of air molecules in the volume multiplied by the individual mass of each molecule and within that same volume temperature supplies the acceleration of the molecules The resulting force is air pressure The 2nd equation is the ideal gas law which relates the temperature and pressure of a gas If temperature increases in a cube of air fixed volume it is because the air molecules are absorbing heat energy and becoming more energetic increasing acceleration and therefore the pressure force within the cube Conversely as molecules lose energy to their environment their acceleration decreases lowering the pressure force within the cube We apply fixed volumes to the atmosphere solely to satisfy these equations We know there are no cubes of air in the atmosphere but the principles remain As solar energy heats the surface the energy is absorbed by the overlying air increasing acceleration Pressure would increase as well were it not for the ability of the molecules to simply spread apart That expansion results in fewer molecules in any given cube of air so less density and less pressure force Alternatively chilling the air has the opposite effect contracting the molecules meaning more are occurring in the given cube of air with associated higher density and therefore pressure Wind speeds The atmosphere like any fluid is always attempting to achieve a state of equilibrium where density is equal throughout It does this by its molecules moving from areas of higher density to areas of lower The greater the difference in density the faster the molecules will move Because its easier to measure pressure than density and because the 2 are related we can use the pressure difference between 2 locations to determine the speed of the air flow This difference is the Pressure Gradient Force PGF and the larger the PGF the faster the wind On a weather map we can easily identify the PGF by drawing lines between areas of high and low pressure and calculating the pressure difference We can also estimate speed by looking at the spacing of the isobars on the map Closely spaced isobars mean pressure is changing rapidly over distance and so the wind will be strong Widely spaced isobars mean little pressure change and calmer winds Wind direction on the other hand is guided by 2 additional forces Coriolis and friction Because the air is moving above a spinning earth its path appears to curve relative to the surface Well above the surface the Coriolis force balances the PGF but in the opposite direction which steers the wind perpendicular to the original PGF direction and therefore parallel to the isobar pattern This flow is called the geostrophic wind Surface friction slows the wind and so weakens the Coriolis force relative to the PGF This results in steering the wind slightly back toward the lower pressure and is the reason that surface winds spiral into a low and out of a high rather than simply circulating around them as it does in the upper atmosphere Air also flows vertically in the atmosphere The density of the atmosphere decreases with increasing altitude Gravity pulls molecules toward the surface but at the same time there is a vertical PGF that encourages upward movement of the molecules At WEATHER BRIEF Powerful and changing movements of air can pose drastic dangers to pilots De Havilland Dash 8 Q300 landing in a strong crosswind at WLG Wellington New Zealand Wind remains a leading meteorological factor in aviation accidents By Karsten Shein Comm Inst Climate Scientist Photo by Phillip Capper Wikimedia Commons