The Atmosphere of the Moon: A Surface Boundary Exosphere The atmospheric pressure on the Moon is only about 3x10-15 bar, which is equivalent to 2.96x10-15 atmosphere (atm) (Stern, 1999). For comparison, the mean sea-level atmospheric pressure on Earth is 1 atm, which is equivalent to 1.013 bar.
The number of molecules in the atmosphere decreases with height.Download Image The atoms and molecules that make up the various layers of the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface, they exert a force on that surface in what we...
www.noaa.gov
The atoms and molecules that make up the various layers of the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface, they exert a force on that surface in what we observe as pressure.
Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable. Air pressure can be increased or decreased in one of two ways. First, simply adding molecules to a container will increase the pressure because a larger number of molecules will increase the number of collisions with the container's boundary. This is observed as an increase in pressure.
A good example of this is adding or subtracting air in an automobile tire. By adding air, the number of molecules increases, as does the total number of the collisions with the tire's inner boundary. The increased number of collisions increases the pressure and forces the tire to expand in size.
The second way of changing air pressure is by the addition or subtraction of heat. Adding heat to a container can transfer energy to air molecules. Heated molecules move with increased velocity, striking the container's boundary with greater force, which is observed as an increase in pressure. Learning Lesson: Heavy Air
Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions.
In the International Space Station, the density of the air is maintained so that it is similar to the density at the Earth's surface, 14.7 pounds per square inch. Learning Lesson: A Pressing Engagement Learning Lesson: Going with the Flow
Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, which means there is a decrease in air pressure. In fact, while the atmosphere extends hundreds of miles up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km).
This decrease in pressure with height makes it very hard to compare the air pressure at ground level from one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we convert the station air pressures reading to a value with a common denominator.
The common denominator we use is the sea-level elevation. At observation stations around the world, the air pressure reading, regardless of the observation station elevation, is converted to a value that would be observed if that instrument were located at sea level.
The two most common units in the United States to measure the pressure are "Inches of Mercury" and "Millibars". Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. At sea level, standard air pressure is 29.92 inches of mercury.
Millibars comes from the original term for pressure: "bar". Bar is from the Greek "báros", meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.
Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature.
Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air.
The number of molecules in the atmosphere decreases with height.Download Image The atoms and molecules that make up the various layers of the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface, they exert a force on that surface in what we...
www.noaa.gov
The atoms and molecules that make up the various layers of the atmosphere are constantly moving in random directions. Despite their tiny size, when they strike a surface, they exert a force on that surface in what we observe as pressure.
Each molecule is too small to feel and only exerts a tiny bit of force. However, when we sum the total forces from the large number of molecules that strike a surface each moment, then the total observed pressure can be considerable. Air pressure can be increased or decreased in one of two ways. First, simply adding molecules to a container will increase the pressure because a larger number of molecules will increase the number of collisions with the container's boundary. This is observed as an increase in pressure.
A good example of this is adding or subtracting air in an automobile tire. By adding air, the number of molecules increases, as does the total number of the collisions with the tire's inner boundary. The increased number of collisions increases the pressure and forces the tire to expand in size.
The second way of changing air pressure is by the addition or subtraction of heat. Adding heat to a container can transfer energy to air molecules. Heated molecules move with increased velocity, striking the container's boundary with greater force, which is observed as an increase in pressure. Learning Lesson: Heavy Air
Since molecules move in all directions, they can even exert air pressure upwards as they smash into object from underneath. In the atmosphere, air pressure can be exerted in all directions.
In the International Space Station, the density of the air is maintained so that it is similar to the density at the Earth's surface, 14.7 pounds per square inch. Learning Lesson: A Pressing Engagement Learning Lesson: Going with the Flow
Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, which means there is a decrease in air pressure. In fact, while the atmosphere extends hundreds of miles up, one half of the air molecules in the atmosphere are contained within the first 18,000 feet (5.6 km).
This decrease in pressure with height makes it very hard to compare the air pressure at ground level from one location to another, especially when the elevations of each site differ. Therefore, to give meaning to the pressure values observed at each station, we convert the station air pressures reading to a value with a common denominator.
The common denominator we use is the sea-level elevation. At observation stations around the world, the air pressure reading, regardless of the observation station elevation, is converted to a value that would be observed if that instrument were located at sea level.
The two most common units in the United States to measure the pressure are "Inches of Mercury" and "Millibars". Inches of mercury refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. At sea level, standard air pressure is 29.92 inches of mercury.
Millibars comes from the original term for pressure: "bar". Bar is from the Greek "báros", meaning weight. A millibar is 1/1000th of a bar and is approximately equal to 1000 dynes (one dyne is the amount of force it takes to accelerate an object with a mass of one gram at the rate of one centimeter per second squared). Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars.
Although the changes are usually too slow to observe directly, air pressure is almost always changing. This change in pressure is caused by changes in air density, and air density is related to temperature.
Warm air is less dense than cooler air because the gas molecules in warm air have a greater velocity and are farther apart than in cooler air. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air.
atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.
Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere
It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.
atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.
Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere
It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.