between –5 ° C and +5 ° C. In addition, an aircraft flying in the  clear air near a thunderstorm is also susceptible to lightning  strikes. Thunderstorm avoidance is always the best policy. Wind shear can be defined as a change in wind speed and/or  wind direction in a short distance. It can exist in a horizontal  or vertical direction and occasionally in both. Wind shear can  occur at all levels of the atmosphere but is of greatest concern  during takeoffs and landings. It is typically associated  with thunderstorms and low-level temperature inversions;  however, the jet stream and weather fronts are also sources  of wind shear. As  Figure 10-17 illustrates, while an aircraft is on an  instrument approach, a shear from a tailwind to a headwind  causes the airspeed to increase and the nose to pitch up with  a corresponding balloon above the glidepath. A shear from  a headwind to a tailwind has the opposite effect, and the  aircraft will sink below the glidepath. A headwind shear followed by a tailwind/downdraft shear is  particularly dangerous because the pilot has reduced power  and lowered the nose in response to the headwind shear. This  leaves the aircraft in a nose-low, power-low configuration  when the tailwind shear occurs, which makes recovery more  difficult, particularly near the ground. This type of wind  shear scenario is likely while making an approach in the  face of an oncoming thunderstorm. Pilots should be alert for  indications of wind shear early in the approach phase and be  ready to initiate a missed approach at the first indication. It  may be impossible to recover from a wind shear encounter  at low altitude. To inform pilots of hazardous wind shear activity, some  airports have installed a Low-Level Wind Shear Alert  System (LLWAS) consisting of a centerfield wind indicator  and several surrounding boundary-wind indicators. With  The probability of lightning strikes occurring to aircraft is  greatest when operating at altitudes where temperatures are  Weather radar, airborne or ground based, normally reflects  the areas of moderate to heavy precipitation (radar does not  detect turbulence). The frequency and severity of turbulence  generally increases with the radar reflectivity closely  associated with the areas of highest liquid water content of  the storm. A flightpath through an area of strong or very  strong radar echoes separated by 20 to 30 miles or less may  not be considered free of severe turbulence. Figure 10-16. A thunderstorm packs just about every weather hazard  known to aviation into one vicious bundle. Wind Shear 25  25  Tailwind Shearing to Headwind or Calm  Headwind Shearing to Tailwind or Calm F r o n t F r o n t OM(outer marker) OM(outer marker) Figure 10-17. Glideslope deviations due to wind shear encounter. 10-25