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.

Batak

Batak is a collective term used to identify a number of ethnic groups predominantly found in North Sumatra, Indonesia. The term is used to include the Alas, Kluet, Singkil, Karo, Pakpak, Simalungun, Toba, Angkola, and Mandailing which are distinct but related groups with distinct - albeit related - languages and customs (adat).

In North Sumatra, Toba people typically assert their identity as 'Batak', while other 'Batak' may explicitly reject that label, preferring instead to identify as specifically 'Mandailing' and 'Karo' people.

Prehistory

Linguistic and archaeological evidence indicates that Austronesian speakers first reached Sumatra from Taiwan and the Philippines through Borneo and/or Java about 2,500 years ago, and the Batak probably descended from these settlers. While the archaeology of southern Sumatra testifies to the existence of neolithic settlers, it seems that the northern part of Sumatra was settled by agriculturalists at a considerably later stage.

Although the Batak are often considered to be isolated peoples thanks to their location inland, away from the influence of seafaring European colonials, there is evidence that they have been involved in trade with other neighbouring kingdoms for a millennium or more.