Pressure
Atmospheric pressure is the force per unit area that is always applied perpendicularly to a surface by the surrounding gas. It is determined by a planet's gravitational force in combination with the total mass of a column of gas above a location. Units of air pressure are based on the internationally recognized standard atmosphere (atm), which is defined as 101,325 Pa (or 1,013,250 dynes per cm2). One (atm) equals 14.696 pounds per square inch (psi).
The pressure of an atmospheric gas decreases with altitude due to the diminishing mass of gas above each location. The height at which the pressure from an atmosphere declines by a factor of e (an irrational number with a value of 2.71828..) is called the scale height and is denoted by H. For an atmosphere with a uniform temperature, the scale height is proportional to the temperature and inversely proportional to the mean molecular mass of dry air times the planet's gravitational force per unit area of on the surface of the earth. For such a model atmosphere, the pressure declines exponentially with increasing altitude. However, atmospheres are not uniform in temperature, so the exact determination of the atmospheric pressure at any particular altitude is more complex.
Read more about this topic: Atmosphere
Famous quotes containing the word pressure:
“The universal social pressure upon women to be all alike, and do all the same things, and to be content with identical restrictions, has resulted not only in terrible suffering in the lives of exceptional women, but also in the loss of unmeasured feminine values in special gifts. The Drama of the Woman of Genius has too often been a tragedy of misshapen and perverted power.”
—Anna Garlin Spencer (1851–1931)
“Destiny is an absolutely definite and inexorable ruler. Physical ability and moral determination count for nothing. It is impossible to perform the simplest act when the gods say “no.” I have no idea how they bring pressure to bear on such occasions; I only know that it is irresistible.”
—Aleister Crowley (1875–1947)
“Under the pressure of the cares and sorrows of our mortal condition, men have at all times, and in all countries, called in some physical aid to their moral consolations—wine, beer, opium, brandy, or tobacco.”
—Edmund Burke (1729–1797)