Atmospheric Pressure

Key idea: 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.

Cloudy Venus 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 of Pascals. 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.

Question: In addition to bars, atmospheric scientists also use units of millibars when measuring the pressure of thin atmospheres or upper atmospheres. How many millibars do you suppose there are in one bar???

Atmospheric pressure is sometimes expressed in terms of the number of inches in a column of Mercury that are supported by the downward push of the atmosphere. Meteorologists and weather people often do it this way. If the pressure is 1 bar, then the atmosphere will support a column of Mercury which is 29.5 inches tall. Air pressure is also sometimes measured in units of psi where 1 psi = 1 pound per square inch. The inflation limit for automobile and bicycle tires is usually expressed in psi. There certainly are many ways to express air pressure!!!

The atmospheric pressure at the surface of the Earth is about 14.7 psi. That means that the weight of a column of air which has a cross section of 1 square inch and which stretches from the surface of the Earth to space is 14.7 pounds (on the average).

Atmospheric pressure is measured with a barometer.

Question: Why do you suppose that a device used to measure air pressure is called a barometer???

Activity/demonstration: Make a barometer with a beaker and a balloon stretched over its rim.

Atmospheric pressure decreases with height above the surface of a planet because there is less total mass in the atmosphere above a reference point as the height of the reference point increases. Only the weight of the mass above the reference point contributes to the pressure at the reference height. For example, in Earth's atmosphere the pressure at a height of 5.5 kilometers is only 50% of the surface pressure. This means that one half of the mass in our atmosphere is above 5.5 kilometers in altitude (and one half is below that altitude). At a height of approximately 16 kilometers in our atmosphere, the pressure is about 10% of the surface pressure. Therefore, only 10% of the mass in our atmosphere can be found more than 16 kilometers above the surface.

Activity: For mathematically advanced students, learn about the exponential decrease of pressure with height in a typical atmosphere, and estimate the pressure at the top of Mount Everest!!!


Ideal Gas Laws

Volcanic Eruption on Io Astronomical tidbit: After inventing the telescope, Galileo turned it to the planets and discovered four large moons of Jupiter in 1610. On a clear and dark night, these moons are visible with binoculars. The nearest of these four Galilean moons to Jupiter is called Io. When the Voyager 2 spacecraft encountered Jupiter in July 1979, a photograph was taken of Io which captured the eruption of an active volcano. Scientists were astounded by this photograph!!! The volcano has since been named Loki. The photograph clearly shows a plume of gas being being released into the thin atmosphere of Io. The gas is believed to be sulfur or sulfur dioxide. Many more volcanoes and vents have since been discovered on Io. The great heat within the satellite is generated by the tremendous tidal forces that are generated by the nearby giant Jupiter.


The pressure exerted by an enclosed gas depends upon the temperature of the gas and on its volume. The enclosed gas molecules are in constant motion and they collide with the walls of the enclosure. These collisions exert a force on the container, and the pressure on the container is equal to the force divided by the surface area of the container. You have already learned in a previous lesson that gas molecules move more quickly when they are heated. These molecules exert a greater force and greater pressure on the walls of an enclosure than cooler molecules.

If gas at a constant temperature is compressed into a smaller volume then it will have a higher density and exert a greater pressure on the walls of the enclosure. If a constant volume of gas is heated to a higher temperature, then the pressure which it exerts on the enclosure will also increase. If a gas is held at constant pressure, then it will expand to fill a larger volume when it is heated and its temperature is raised. These are the laws which govern what scientists call ideal gases. All these phenomena can be explained by thinking about the collisions between the gas molecules and the walls of the enclosure and how the resulting pressure is affected by changes in the volume and temperature of the gas.

Activity/demonstration: Add heat to and remove heat from an inflated balloon. Observe how the circumference changes. Note that the gas inside the balloon expands when it is heated and its volume increases. When it is cooled, the gas inside the balloon contracts and its volume decreases.

The atmosphere is actually a fluid. If not mixed, fluids of different densities will arrange themselves into layers due to gravity. The most dense fluid ("heaviest") will move to the bottom of a container and the least dense fluid ("lightest") will move to the top. Much of Earth's atmosphere is well mixed by the winds, so that different gases are not generally separated by density. Warm air and cool air do separate, however, because warm air is less dense than cold air and warm air therefore rises.

Activity/demonstration: Layer liquids of different densities in a graduated cylinder, and observe that the liquids arrange themselves by density with the most dense liquid at the bottom and the least dense liquid at the top.

Demonstration: Add carbon dioxide to a container and observe that it is more dense than air.

Activity/demonstration: Add ice to a large container and observe that cooler air remains on the bottom while warmer air rises to the top.

Air pressure plays a major role in determining weather patterns. Generally, the weather is good when the pressure is high and the weather can be threatening when the pressure is low. Winds generally blow from areas of high pressure to areas of low pressure. The air is forced to rise when it reaches the low pressure areas. As the rising air expands and cools, water condenses out of the atmosphere. When this happens, cloudy or wet weather results. You will learn much more about this is a later lesson.

Activity: Examine a weather map and find the areas of high and low pressure. Note the weather in these areas.


Last updated: November 17, 1999
Joe Twicken / joe@nova.stanford.edu
Rob Wigand