Atmosphere of Earth

The atmosphere of Earth is the layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

The common name air (English pronunciation: /ɛər/) is given to the atmospheric gases used in breathing and photosynthesis. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere. Air content and atmospheric pressure vary at different layers, and air suitable for the survival of terrestrial plants and terrestrial animals is found only in Earth's troposphere and artificial atmospheres.

The atmosphere has a mass of about 5.15×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km (75 mi). Several layers can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

Enter

Enter or ENTER may refer to:

See also

Boeing 737 Next Generation

The Boeing 737 Next Generation, commonly abbreviated as 737NG, is the name given to the −600/-700/-800/-900 series of the Boeing 737 airliner. It is the third generation derivative of the 737, and follows the 737 Classic (−300/-400/-500) series, which began production in the 1980s. They are short- to medium-range, narrow-body jet airliners. Produced since 1996 by Boeing Commercial Airplanes, the 737NG series includes four variants and can seat between 110 to 210 passengers.

A total of 5,419 737NG aircraft have been delivered by the end of May 2015, with more than 6,800 ordered. The -700, -800, and -900ER have unfilled orders and are being produced, but orders for -600 and -900 have been filled. The 737NG's primary competition is with the Airbus A320 family. Upgraded and re-engined models in development as the 737 MAX series will eventually supplant the 737NG.

Design and development

Background

Prompted by the development of the Airbus A320, which incorporated ground-breaking technologies such as fly-by-wire, in 1991 Boeing initiated development of an updated series of aircraft. After working with potential customers, the 737 Next Generation (NG) program was announced on November 17, 1993. The 737NG encompasses the −600, −700, −800 and −900 variants, and is to date the most significant upgrade of the airframe. The performance of the 737NG is essentially that of a new airplane, but important commonality is retained from previous 737 generations. The wing was modified, increasing its area by 25% and span by 16 ft (4.88 m), which increased the total fuel capacity by 30%. New quieter and more fuel-efficient CFM56-7B engines were used. These improvements combine to increase the 737's range by 900 nmi, permitting transcontinental service. A flight test program was operated by 10 aircraft: 3 -600s, 4 -700s, and 3 -800s.