Figure 12-11.  Favorable winds near a high pressure system.  Figure 12-12.  Convective turbulence avoidance.  Effect of Obstructions on Wind  cooler, denser air flowing in from over the water. This causes  an onshore wind called a sea breeze. Conversely, at night land  cools faster than water, as does the corresponding air. In this  case, the warmer air over the water rises and is replaced by  the cooler, denser air from the land, creating an offshore wind  called a land breeze. This reverses the local wind circulation  pattern. Convective currents can occur anywhere there is an  uneven heating of the Earth’s surface.  [Figure 12-13]  Convective currents close to the ground can affect a pilot’s  ability to control the aircraft. For example, on final approach,  the rising air from terrain devoid of vegetation sometimes  produces a ballooning effect that can cause a pilot to overshoot  the intended landing spot. On the other hand, an approach over  a large body of water or an area of thick vegetation tends to  create a sinking effect that can cause an unwary pilot to land  short of the intended landing spot.  [Figure 12-14]  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. It is especially important to be vigilant when flying  in or out of airports that have large buildings or natural  obstructions located near the runway.  [Figure 12-15]  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 takeoff and  landing performance of any aircraft and can present a very  serious hazard. During the landing phase of flight, an aircraft  12-8 