Professional Pilot, May 2017

Coriolis effect If we consider a balloon that starts off at the center of a high pressure aloft and moves toward a nearby low well notice that it starts to veer off course until eventually it is moving on a path exactly parallel to the isobars The reason for this is that the earth is rotating beneath the balloon and this relative movement creates an apparent force that acts perpendicular to the pressure gradient force This is called the Coriolis effect The Coriolis effect is dependent on the velocity of the air the faster it goes the greater the effect Initially the pressure gradient force is stronger and causes the air to accelerate toward the low pressure But as it speeds up the Coriolis effect increases displacing the wind away from the pressure gradient path Eventually the wind has shifted direction enough so that the Coriolis effect and pressure gradient are balanced The wind is now considered geostrophic as it is flowing parallel to the isobars having been deflected to the right in the northern hemisphere or to the left in the southern hemisphere At the surface wind direction is affected by friction an additional force that acts opposite to the pressure gradient force slowing the wind Because Coriolis is dependent on the velocity of the air friction decreases the effect while the pressure gradient remains unchanged This causes the surface wind to deflect back toward the pressure gradient path and therefore 74 PROFESSIONAL PILOT May 2017 spiral into surface lows and out of surface highs Whether above or at the surface relative wind speeds can still be estimated by the spacing of the isobars on the map Windshear For most of us the wind becomes a real problem when it shears Windshear occurs when there is a change in wind speed and or direction over a relatively short distance Because the atmosphere is a 3 dimensional fluid shear can occur vertically horizontally or in some combination of the 2 Shear can occur on many scales such as around a jet stream beneath a thunderstorm or even around the edge of a hangar shear itself is a small scale localized event Shear is often manifested in turbulent eddies that are created along the boundary between the dichotomous flow or felt as a change in wind forces as one flies across that boundary Aloft we notice it most often as the slight to moderate turbulence associated with rising thermals However in many situations shear can produce severe and damaging effects on aircraft In fact 3 of the most common shear dangers aloft come from the jet stream mountain waves and thunderstorm complexes Jet stream shear is experienced around the periphery of the jet core where wind speed may change by many knots over the span of a few hundred feet Often this flow is quasi laminar and shearing is minimized but significant eddies occasionally shear from the faster flow Eddies are most likely to shed when the jet stream is changing speed or direction rapidly and significantly such as around a deep trough or at the entrance or exit to a jet streak These eddies can be up to 1 km in diameter and may slam into an aircraft from any direction at speeds of up to 100 kts or more occasionally causing damage and injuries to unsecured crew or passengers Because it frequently occurs in clear air this type of shear is called clear air turbulence or CAT Flow over mountain ranges also produces significant and often extreme windshear as the flow is compressed and sped up and is forced into a downstream wave pattern Areas above and downwind of a mountain range are common places to encounter shear from these moun Winds normally increase speed with altitude as surface friction becomes less influential This can produce windshear aloft and help transform ordinary thunderstorms into supercells Circulations in the exit and entrance regions of an upper level jet streak Moderate static stability 90 kts 110 kts 130 kts Direct thermal circulation Indirect thermal circulation Weak static stability Jet core Windshear in storm cloud 60 kts at 21120 ft 35 kts at 15000 ft 17 kts at 10000 ft 12 kts at 5000 ft 2 kts at 0 ft Jet streaks are areas of faster air within the jet stream The acceleration of the air coupled with flow around the streak can produce severe turbulence Image courtesy Moore 2004