There are many excellent texts and online sources available  for learning more about weather that are referenced  at the appropriate point in this chapter. Much of the  following information can be found in the Federal Aviation  Administration (FAA) Advisory Circular (AC) 00-06A,  Aviation Weather For Pilots and Flight Operations Personnel,  and AC 00-45E, Aviation Weather Services, both of which  may be found at the FAA’s Regulatory and Guidance Library  (RGL) located online at rgl.faa.gov. Other online sources for weather information are also helpful.  The National Oceanic and Atmospheric Administration  (NOAA) offers a weather tutorial at www.srh.noaa.gov/srh/ jetstream/. Developed to meet the needs of educators, weather  professionals, and others interested in learning more about  weather, it provides a number of concise explanations of  weather theory. Additionally, ww2010.atmos.uiuc.edu/(Gh)/ home.rxml is a site developed as part of the Weather World  2010 project by the Department of Atmospheric Sciences at  the University of Illinois at Urbana-Champaign.  This chapter is designed to give balloon pilots a basic  knowledge of weather principles, acquaint them with the  weather information available for flight planning, and help  them develop sound decision-making skills as they prepare  for and execute a safe flight. The atmosphere is the envelope of air that surrounds the  Earth. Approximately one-half of the air, by weight, is within  the lower 18,000 feet. The remainder of the air is spread over  a vertical distance in excess of 1,000 miles. No definite outer  atmospheric boundary exists, but the air particles become  less numerous with increasing altitude until they gradually  overcome Earth’s gravity and escape into space. In addition  to the rotation of the air with the rotation of the Earth, another  type of air movement occurs within the atmosphere. This  movement of air around the surface of the Earth is called  atmospheric circulation. The Atmosphere Composition The atmosphere is a blanket of air composed of a mixture  of gases that surrounds the Earth and reaches over 560  kilometers (km), 348 miles, from the surface. This blanket  of gases provides protection from ultraviolet rays, as well as  supporting human, animal, and plant life. Nitrogen accounts  for 78 percent of the gases comprising the atmosphere,  while oxygen makes up 21 percent.  [Figure 4-1]  Argon,  carbon dioxide, and traces of other gases make up the  remaining 1 percent. Within this envelope of gases, there  are several recognizable layers of the atmosphere as defined  by altitude.  4-2 Figure 4-1.  Composition of the atmosphere. 1% 21% 78% Trace gases Oxygen Nitrogen The first layer, known as the troposphere, extends from sea  level up to 20,000 feet (8 km) over the northern and southern  poles and up to 48,000 feet (14.5 km) over the equatorial  regions. The vast majority of weather, clouds, storms, and  temperature variances occur within this first layer of the  atmosphere.  At the top of the troposphere is a boundary known as the  tropopause, which traps moisture and the associated weather  in the troposphere. The altitude of the tropopause varies with  latitude and with the season of the year which causes it to  take on an elliptical shape, as opposed to round. Location  of the tropopause is important because it is commonly  associated with the location of the jet stream and possible  clear air turbulence. To provide a common reference when discussing weather,  the International Standard Atmosphere (ISA) has been  established. To arrive at the standard atmosphere, conditions  throughout the atmosphere with respect to latitudes, seasons,  and altitudes were averaged. The standard reference point is  59 °F or 15 °C, and 29.92 inches of mercury ("Hg) or 1013.2  millibars (mb). Pressure does not decrease linearly with  altitude, but for the first 10,000 feet, 1 "Hg for each 1,000  feet approximates the rate of pressure change. There is also  a standard temperature lapse rate of 3.5 °F or 2 °C per 1,000  feet of altitude, up to 36,000 feet.  The Standard Atmosphere At sea level, the atmosphere exerts pressure on the Earth at  a force of 14.7 pounds per square inch (psi). This means a  column of air one inch square, extending from the surface up  to the upper atmospheric limit, weighs about 14.7 pounds. 