What is "atmospheric pressure"

Atmospheric \At`mos*pher"ic\, Atmospherical \At`mos*pher"ic*al\, a. [Cf. F. atmosph['e]rique.]

1. Of or pertaining to the atmosphere; of the nature of, or resembling, the atmosphere; as, atmospheric air; the atmospheric envelope of the earth.

2. Existing or occurring in the atmosphere.

   The lower atmospheric current.
   --Darwin.

3. Caused, or operated on, by the atmosphere; as, an atmospheric effect; an atmospheric engine.

4. Dependent on the atmosphere. [R.]

   In am so atmospherical a creature.
   --Pope.

   Atmospheric engine, a steam engine whose piston descends by the pressure of the atmosphere, when the steam which raised it is condensed within the cylinder.
   --Tomlinson.

   Atmospheric line (Steam Engin.), the equilibrium line of an indicator card. Steam is expanded ``down to the atmosphere'' when its pressure is equal to that of the atmosphere. (See Indicator card.)

   Atmospheric pressure, the pressure exerted by the atmosphere, not merely downwards, but in every direction. In amounts to about 14.7 Ibs. on each square inch.

   Atmospheric railway, one in which pneumatic power, obtained from compressed air or the creation of a vacuum, is the propelling force.

   Atmospheric tides. See under Tide.

Pressure \Pres"sure\ (?; 138), n. [OF., fr. L. pressura, fr. premere. See 4th Press.]

1. The act of pressing, or the condition of being pressed; compression; a squeezing; a crushing; as, a pressure of the hand.

2. A contrasting force or impulse of any kind; as, the pressure of poverty; the pressure of taxes; the pressure of motives on the mind; the pressure of civilization.

   Where the pressure of danger was not felt.
   --Macaulay.

3. Affliction; distress; grievance.

   My people's pressures are grievous.
   --Eikon Basilike.

   In the midst of his great troubles and pressures.
   --Atterbury.

4. Urgency; as, the pressure of business.

5. Impression; stamp; character impressed.

   All saws of books, all forms, all pressures past.
   --Shak.

6. (Mech.) The action of a force against some obstacle or opposing force; a force in the nature of a thrust, distributed over a surface, often estimated with reference to the amount upon a unit's area.

7. Electro-motive force.

   Atmospheric pressure, Center of pressure, etc. See under Atmospheric, Center, etc.

   Back pressure (Steam engine), pressure which resists the motion of the piston, as the pressure of exhaust steam which does not find free outlet.

   Fluid pressure, pressure like that exerted by a fluid. It is a thrust which is normal and equally intense in all directions around a point.
   --Rankine.

   Pressure gauge, a gauge for indicating fluid pressure; a manometer.

n. (context physics meteorology English) The pressure caused by the weight of the atmosphere above an area.

n. the pressure exerted by the atmosphere [syn: air pressure]

Atmospheric pressure, sometimes also called barometric pressure, is the pressure exerted by the weight of air in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air in cross-section, measured from sea level to the top of the atmosphere, has a mass of about and weight of about . That force (across one square centimeter) is a pressure of 10.1 N/cm or 101,000 N/m. A column in cross-section would have a weight of about or about 65.4 N.