percent is life sustaining atmospheric oxygen. At sea level,  atmospheric pressure is great enough to support normal  growth, activity, and life. By 18,000 feet, the partial pressure  of oxygen is reduced and adversely affects the normal  activities and functions of the human body.  The reactions of the average person become impaired at an  altitude of about 10,000 feet, but for some people impairment  can occur at an altitude as low as 5,000 feet. The physiological  reactions to hypoxia or oxygen deprivation are insidious and  affect people in different ways. These symptoms range from  mild disorientation to total incapacitation, depending on  body tolerance and altitude. Supplemental oxygen or cabin  pressurization systems help pilots fly at higher altitudes and  overcome the effects of oxygen deprivation.  Air flows from areas of high pressure into areas of low  pressure because air always seeks out lower pressure. The  combination of atmospheric pressure differences, Coriolis  force, friction, and temperature differences of the air near  the earth cause two kinds of atmospheric motion: convective  currents (upward and downward motion) and wind  (horizontal motion). Currents and winds are important as  they affect takeoff, landing, and cruise flight operations. Most  importantly, currents and winds or atmospheric circulation  cause weather changes.  In the Northern Hemisphere, the flow of air from areas of  high to low pressure is deflected to the right and produces  a clockwise circulation around an area of high pressure.  This is known as anticyclonic circulation. The opposite  is true of low-pressure areas; the air flows toward a low  and is deflected to create a counterclockwise or cyclonic  circulation.  [Figure 12-10]  High-pressure systems are generally areas of dry, descending  air. Good weather is typically associated with high-pressure  systems for this reason. Conversely, air flows into a low- pressure area to replace rising air. This air usually brings  increasing cloudiness and precipitation. Thus, bad weather  is commonly associated with areas of low pressure.  A good understanding of high- and low-pressure wind patterns  can be of great help when planning a flight because a pilot can  take advantage of beneficial tailwinds.  [Figure 12-11]  When  planning a flight from west to east, favorable winds would  be encountered along the northern side of a high-pressure  system or the southern side of a low-pressure system. On  the return flight, the most favorable winds would be along  the southern side of the same high-pressure system or the  northern side of a low-pressure system. An added advantage  Convective currents are particularly noticeable in areas with a  land mass directly adjacent to a large body of water, such as an  ocean, large lake, or other appreciable area of water. During  the day, land heats faster than water, so the air over the land  becomes warmer and less dense. It rises and is replaced by  Convective currents cause the bumpy, turbulent air sometimes  experienced when flying at lower altitudes during warmer  weather. On a low-altitude flight over varying surfaces,  updrafts are likely to occur over pavement or barren places,  and downdrafts often occur over water or expansive areas  of vegetation like a group of trees. Typically, these turbulent  conditions can be avoided by flying at higher altitudes, even  above cumulus cloud layers.  [Figure 12-12]  Plowed ground, rocks, sand, and barren land absorb solar  energy quickly and can therefore give off a large amount  of heat; whereas, water, trees, and other areas of vegetation  tend to more slowly absorb heat and give off heat. The  resulting uneven heating of the air creates small areas of  local circulation called convective currents.  While the theory of circulation and wind patterns is accurate for  large scale atmospheric circulation, it does not take into account  changes to the circulation on a local scale. Local conditions,  geological features, and other anomalies can change the wind  direction and speed close to the Earth’s surface.  is a better understanding of what type of weather to expect  in a given area along a route of flight based on the prevailing  areas of highs and lows.  Figure 12-10.  Circulation pattern about areas of high and low  pressure.  12-7  Convective Currents  Wind Patterns  Wind and Currents 