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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 

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. 

## 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. 

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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. 

## The Standard Atmosphere 

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. 

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. 

