Barometric Pressure - my attempt to learn

Discussion in 'The Basics, Starting Out' started by tlk, Jul 8, 2010.

  1. tlk

    tlk Well-Known Member

    Apr 11, 2008
    One of the things that has confused me lately is barometric pressure and why it is important. Here are my notes - thought this might help others who are wondering why this matters. I have included the footnotes as well.

    For you experienced hands - please let me know where this is wrong so I can get it right in my head. I tried to keep it simple, but I need it to be accurate.


    What is barometric pressure?

    • The barometric pressure is the calculation of the current air pressure. Air pressure is basically the weight of the air sitting on top of us. When we have high pressure, the air is sinking and we will see clear weather. When the air pressure drops, the air rises and helps to create clouds and rainfall. A typical high pressure reading in inches of mercury would be over 30 inches. For low pressure, you will see reading around 29 inches or lower. While it doesn't sound like a huge difference (a couple of inches), the weather can be dramatically different in that range.1
    • Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation.2
    • The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1087.0 mbar. The lowest measurable sea-level pressure is found at the centers of tropical cyclones.2
    Why is it constant at 29.92?

    • 29.92inHg (inches of mercury) is the air pressure at sea level. Most readings are done in relation to sea level for any altitude.
    • Average sea-level pressure is 101.325 kPa (1013.25 mbar, or hPa) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg).2
    • The amount of atmosphere in place over a certain area differs depending on the elevation of that point, so barometers are calibrated to show readings based on what the barometric pressure would be for that point, if it were at sea level. 7
    Why does it change with weather?

    • Basic answer: weather changes the amount of compressed atmosphere on top of you.
    • Detailed answers:
    • Hot air is less dense than cool air, which is why hot air rises and cool air falls. This can be explained by thinking about how molecules move in hot air and in cool air. Molecules in hot air are moving quickly, so they tend to bounce off of each other and move apart, creating a less dense mass of air. Molecules in cold air move more slowly, so they tend to stay together, creating a denser mass of air. 5
    • Weather changes also affect the barometric pressure in an area. Masses of warm and cold air move through the atmosphere, meeting, parting and causing changes in the atmospheric pressure below. The polar regions send out masses of cold and dry air, while masses of warm and wet air come from the equator. 6
    • Rising air pressure often occurs as the result of moisture advection. Moisture advection is a term used to describe the phenomenon of the wind blowing moist air to or from an area. When moisture advection transfers air with a high water content out of an area, the air pressure in that area rises. This change is due to the fact that dry air has a higher density than moist air, which results in more pressure on the area below. A phenomenon similar to moisture advection is warm air advection. Warm air advection is the result of winds moving a warm air mass. When a warm air mass is blown away by the wind, the air pressure will rise in that area. Cold air is more dense than warm air, so when cold air replaces the warm air mass, the air pressure in that area rises. Rising barometric pressure is a good indicator that there are no storms brewing over the following 24 hours. As long as the barometric pressure increases, and remains high, the weather forecast should be consistently dry and cool. 6
    • There are several different types of weather phenomena that are associated with a fall in barometric pressure. One weather system that results in lower barometric pressure is a low pressure trough, which is a long area of low barometric pressure. In a low pressure trough, warm air rises, and cools as it goes higher into the atmosphere. The space left behind as the portions of a warm air mass rise are filled by additional warm air, which exerts less pressure on the earth, resulting in a lower barometric pressure reading. When a low pressure trough comes into an area, or deepens, indicating lower levels of barometric pressure in the center, the atmospheric pressure in that area will fall in response to the warm air masses overhead. In addition to low pressure troughs, wind can also contribute to a fall in air pressure. When the wind blows moist air into an area, the air pressure in that area will drop in response to the change. This phenomenon is called moisture advection, and results from the fact that moist air has a lower density that dry air, and results in less pressure on the earth's surface. Warm air advection is a similar phenomenon that can also cause lower air pressure. When warm air, which is less dense than cold air, is pushed into an area by wind, the barometric pressure in that area will fall. 7
    • Watch the sky conditions outside. If you have clear skies and then notice that clouds are moving in, the pressure is most likely falling. If your skies are already cloudy, watch for precipitation to start to fall. These conditions are almost always associated with falling barometric pressure.8
    Why does it change with elevation?

    • There is less atmosphere on top of you.
    • Barometric pressure varies with weather patterns and geography. Higher elevations have lesser atmospheric pressure because they simply have less atmosphere above them. In lower elevations, of course, the opposite is true.4
    • The altitude of a location affects the barometric pressure indirectly, because altitude affects temperature. For example, the temperatures in the mountains are colder, so the mountains have a higher average barometric pressure than the temperatures at the beach. 5
    • Pressure varies smoothly from the Earth's surface to the top of the mesosphere.2
    What is its importance in shooting?

    • Basic answer: It affects bullet drop. It is a distance thing: the longer it is in flight the longer it has to affect the downward force and speed of the bullet.
    • While it isn't common for us to think of gas as weighing on anything, but as matter it does in fact have mass. Because of this and the effect of gravity upon the gas, the air above us and around us does weigh down on us. 4
    • Like all molecules, air molecules are pulled to the ground by gravity. The pressure the molecules exert on the ground is dependent on the force of gravity. For example, the barometric pressure on the moon would be less than the barometric pressure on Earth because there is less gravity on the moon. 5
    • The density of a mass of air affects the barometric pressure. If the mass of air over a specific point on Earth is more dense, there are more air molecules exerting pressure on that point. Therefore, the barometric pressure is higher. If the same mass of air is less dense, there were fewer air molecules exerting pressure on that same point, which means the barometric pressure is lower. 5
    • [Seems that there are two things to overcome: the density of the atomosphere (number of molecules in the bullet’s way) and the weight of the atmosphere on the bullet as it travels. Also seems like these two are in a 1:1 correlation and are collectively contributing to the amount of drag AND drop (please correct this if it is wrong). IOW, you got stuff in the way AND that stuff weighs down on you, in addition to the gravity of the earth.]
    • Elevation and barometric pressure can definitely have a significant effect on the accuracy of your load. Why? Because they affect the density of the air, and therefore, the amount of resistance it exerts on the bullet as it travels to the target. That’s why it’s so important for hunters to re-zero their rifles once they arrive in camp when they travel to higher elevations to hunt such big game animals as elk, caribou, bear, and moose.9
    What is station pressure?
    • The term station is the designation for the vertical point that you take your measurements; vertical meaning above (or below) sea level. The absolute air pressure is the calculated air pressure, but not corrected for altitude. It is also called the station pressure in weather reports from NOAA.3
    • (I think they mean that absolute pressure is the pressure at your point in the vertical plane and not corrected to be relative to the standard sea level)

    Last edited: Jul 8, 2010
  2. Chas1

    Chas1 Well-Known Member

    Feb 15, 2009
    tlk, good read. Looks like you put alot of time and thought into putting that together. I'm no authority on the subject but, for what it's looks pretty darn good to me. Thank you for your time, effort and for sharing.

  3. royinidaho

    royinidaho Writers Guild

    Jan 20, 2004
    Way too much reading for me!!!

    I think you're on the right track if Chas1 says so....

    Air pressure indicates air density.

    The higher the pressure the more dense the air thus more resistance to bullet flight. Just the opposite for decreased air pressure.

    Observe one of those mercury u-tube barometers and it'll come together.

    Results are the higher the elevation, the lower the air pressure, the less resistance to bullet flight thus higher ballistic coefficient.

    The hotter the air the less dense it is also. That's why crop dusters quit flying after about 1100 hrs or so.

    Just the way I look at it.....
  4. Ridge Runner

    Ridge Runner Well-Known Member

    Dec 13, 2002
    Not sure id BP actualy affects your loads accuracy or not, but in can significantly change the plotted trajectory of your chart, because BP in simple terms effectively changes the bullets BC.
    Think about it, not figureing the change in trajectory from BP would be like guessing at the BC.