Chapter 3b: Effects of Weather on Small Unmanned Aircraft Performance  elevation. For example, a station at 5,000 feet above sea level, with a reading of 24.92 "Hg, reports a  sea level pressure reading of 29.92 "Hg.  By tracking barometric pressure trends across a large area, weather forecasters can more accurately  predict movement of pressure systems and the associated weather. For example, tracking a pattern of  rising pressure at a single weather station generally indicates the approach of fair weather. Conversely,  decreasing or rapidly falling pressure usually indicates approaching bad weather and, possibly, severe  storms.  Another atmospheric hazard exists that can create problems for pilots. Obstructions on the ground  affect the flow of wind and can be an unseen danger. Ground topography and large buildings can break  up the flow of the wind and create wind gusts that change rapidly in direction and speed. These  obstructions range from man-made structures, like hangars, to large natural obstructions, such as  mountains, bluffs, or canyons. Effect of Obstructions on Wind The intensity of the turbulence associated with ground obstructions depends on the size of the  obstacle and the primary velocity of the wind. This can affect the performance of any aircraft and can  present a very serious hazard.  This same condition is even more noticeable when flying in mountainous regions.  [Figure 3-3]  While  the wind flows smoothly up the windward side of the mountain and the upward currents help to carry  an aircraft over the peak of the mountain, the wind on the leeward side does not act in a similar  manner. As the air flows down the leeward side of the mountain, the air follows the contour of the  terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The  stronger the wind, the greater the downward pressure and turbulence become.  Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area. Wind  shear can subject an aircraft to violent updrafts and downdrafts, as well as abrupt changes to the  horizontal movement of the aircraft. While wind shear can occur at any altitude, low-level wind shear  is especially hazardous due to the proximity of an aircraft to the ground. Low-level wind shear is  commonly associated with passing frontal systems, thunderstorms, temperature inversions, and strong  upper level winds (greater than 25 knots). Low-Level Wind Shear Figure 3-3.  Turbulence in mountainous regions. 23  Remote Pilot – Small Unmanned Aircraft Systems Study Guide 