Atmosphere is layers of gases that envelope a planet and is held in place by the gravity of the planetary body. The upper atmosphere lies above the mesopause. Here temperatures can no longer be measured directly. A planet retains an atmosphere when the gas molecules cannot achieve escape velocity and the temperature of the atmosphere is low which maintains low velocity of gas molecules. A stellar atmosphere is the outer region of a star which includes the layers above the opaque photosphere. Stars of low temperature might have outer atmospheres containing compound molecules. The atmosphere of Earth is composed of many dry gases. Nitrogen and oxygen are the most common. Carbon dioxide and argon are also present in small number. Most organisms use oxygen for respiration. Lightning and bacteria perform nitrogen fixation to produce ammonia that is used to make amino acids. Plants along with algae and cyanobacteria use carbon dioxide for photosynthesis. The layered composition of the atmosphere minimises the harmful effects of sunlight such as ultraviolet radiation and the solar wind or cosmic rays to protect organisms from genetic damage. The current composition of the atmosphere of the Earth is the product of billions of years of biochemical modification of the atmosphere by living organisms. As measured with a barometer atmospheric pressure is the force perpendicular to a surface which is determined by the weight of the vertical column of atmospheric gases. For such an atmospheric model the pressure declines exponentially with increase in altitude from the planetary surface. Atmospheric pressure decreases at high altitude because of the diminishing mass of gas above the point of measurement. The density of the atmosphere also decreases with increase in the altitude making breathing difficult as we climb up the mountains. The atmosphere of Earth consists of a number of layers that differ in properties such as composition and temperature or pressure. The lowest layer is called the troposphere which extends from the surface of Earth to the bottom of the stratosphere. Three quarters of the mass of our atmosphere resides within the troposphere. It is the layer within which the terrestrial weather of Earth develops. The stratosphere is extending from the top of the troposphere to the bottom of the mesosphere. It contains the ozone layer. The ozone layer is the layer where most of the ultraviolet radiation from the Sun is absorbed. The mesosphere is the layer wherein most meteors burn up. The thermosphere contains the ionosphere which is a region where the atmosphere is ionized by incoming solar radiation. The ionosphere increases in thickness and moves closer to the Earth during daylight and rises at night. It can reflect radio waves sent from Earth allowing certain frequencies of radio communication over a greater range. The exosphere interacts with the planets magnetosphere. Each of the layers has a different lapse rate defining the rate of change in temperature with height. Initial atmospheric composition is generally related to the chemistry and temperature of the local solar nebula during planetary formation and the subsequent escape of interior gases. The original atmosphere of the planets originated from a rotating disc of gases which collapsed onto itself and then divided into a series of spaced rings of gas and matter that which later condensed to form the planets of the Solar system.