What is the cause of atmospheric pressure
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Atmospheric pressure is caused by the weight of the atmosphere pushing down on itself and on the surface below it. Pressure is defined as the force acting on an object divided by the area upon which the force is acting.
 Astronomical tidbit: The atmosphere of Venus is extremely thick, and the planet suffers from a runaway Greenhouse effect. The temperature at the surface is hot enough to melt lead, and the atmospheric pressure at the surface is 90 times that at the surface of the Earth!!! Because the atmosphere is so dense and so cloudy, it is not possible to see the surface of Venus from outside its atmosphere. The image at the left was taken by an ultraviolet camera aboard the Pioneer Venus Orbiter spacecraft in 1979. Only the clouds of Venus are visible in the photograph. The surface of Venus has been mapped with imaging radars aboard orbiting spacecraft. The radio signals used by these radar systems are able to penetrate the thick atmosphere, and the images produced by these instruments have given us spectacular views of the surface of Venus.

Pressure
A gravitational attraction exists between the mass of a planet or natural satellite and the gas molecules and particles in its atmosphere (if it has one!). The force of gravity tends to pull the molecules and particles toward the center of the body. The weight of the atmosphere pushing down on itself and on the surface of the planet creates atmospheric pressure.
Activity/demonstration: Balance inflated balloons on a dowel and observe what happens when the air is released from one of the balloons.
Pressure is equal to the force acting on an object divided by the area upon which it is acting. The pressure exerted by an air mass therefore depends on the mass of the air and on the area below the mass. For a given air column, if the density of the air increases then the pressure exerted on the surface by the air column also increases. That is, of course, because the higher density means that the air column has greater mass and exerts a greater force on the area below it.
Pressure is generally measured in units ofPascals. Atmospheric scientists, however, like to express pressure in units of bars. For the record, 1 Pascal = 1 Newton per square meter, and 1 bar = 100,000 Pascals. The average atmospheric pressure at the surface of the Earth is approximately 1 bar. That is why atmospheric scientists like to express pressure in units of bars. If someone told you that the atmospheric pressure at the surface of Venus was 90 bars, then you would know that it was about 90 times greater than that of Earth.
 Astronomical tidbit: The atmosphere of Venus is extremely thick, and the planet suffers from a runaway Greenhouse effect. The temperature at the surface is hot enough to melt lead, and the atmospheric pressure at the surface is 90 times that at the surface of the Earth!!! Because the atmosphere is so dense and so cloudy, it is not possible to see the surface of Venus from outside its atmosphere. The image at the left was taken by an ultraviolet camera aboard the Pioneer Venus Orbiter spacecraft in 1979. Only the clouds of Venus are visible in the photograph. The surface of Venus has been mapped with imaging radars aboard orbiting spacecraft. The radio signals used by these radar systems are able to penetrate the thick atmosphere, and the images produced by these instruments have given us spectacular views of the surface of Venus.

Pressure
A gravitational attraction exists between the mass of a planet or natural satellite and the gas molecules and particles in its atmosphere (if it has one!). The force of gravity tends to pull the molecules and particles toward the center of the body. The weight of the atmosphere pushing down on itself and on the surface of the planet creates atmospheric pressure.
Activity/demonstration: Balance inflated balloons on a dowel and observe what happens when the air is released from one of the balloons.
Pressure is equal to the force acting on an object divided by the area upon which it is acting. The pressure exerted by an air mass therefore depends on the mass of the air and on the area below the mass. For a given air column, if the density of the air increases then the pressure exerted on the surface by the air column also increases. That is, of course, because the higher density means that the air column has greater mass and exerts a greater force on the area below it.
Pressure is generally measured in units ofPascals. Atmospheric scientists, however, like to express pressure in units of bars. For the record, 1 Pascal = 1 Newton per square meter, and 1 bar = 100,000 Pascals. The average atmospheric pressure at the surface of the Earth is approximately 1 bar. That is why atmospheric scientists like to express pressure in units of bars. If someone told you that the atmospheric pressure at the surface of Venus was 90 bars, then you would know that it was about 90 times greater than that of Earth.
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as air has mass, so the air above us exerts pressure on us which gets nullified by our blood pressure.
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