Image:Himmel ueber dem Bielersee05.jpg 115px|left| An '''atmosphere''' is the layer of gases that envelope a celestial body of sufficient mass and is retained by the gravitational field of the body. Some planets consist mainly of various gases, and thus have very deep atmospheres (gas giants). In contrast, the '''Earth's atmosphere''' mostly lies within 100 kilometre km (62 mile mi) of the surface. It contains roughly four-fifths nitrogen and one-fifth oxygen, with trace amounts of other gases. The atmosphere protects life on Earth by absorbing ultraviolet solar Solar radiation radiation and reducing temperature extremes between day and night. The ''':Portal:Atmospheric sciences atmospheric sciences''' involve the study of the atmosphere, its processes, the effects other systems have on the atmosphere, and the effects of the atmosphere on these other systems. Sub-disciplines include meteorology, climatology, atmospheric physics, atmospheric dynamics, and atmospheric chemistry. '''Meteorology''' focuses on weather processes and forecasting, and '''climatology''' is the study of atmospheric changes (both long- and short-term) that define average climates and their change over time, due to both natural climate variability and anthropogenic climate variability.