This article explains the process of "calculating" the barometric pressure for weather analyzes or forecasts. Conversions are of practical use. Perhaps it should be explained from the outset that you will not "calculate" the barometric pressure: you will measure it; then you will convert it to units of measurement that are more comfortable to use.
Steps
Step 1. Look for the trend
To evaluate trends and weather analysis, the absolute value of the pressure is in no way significant as his own trend. That is to say, is it rising or falling or just holding steady? Older barometers have very well-drawn artistic backgrounds on the dial indicating strong winds, storms, sunny weather, etc., and are pleasantly decorative, but deceiving nonetheless - - because it's the movement of the barometer needle (or the spherical cap also known as meniscus, if you happen to have an old type of tube barometer, very traditional mercury), which affects much more on the arriving time.
Step 2. Remember that the pressure of the atmosphere varies exponentially with altitude
This means that a barometric pressure that would have sent someone straight into a sea-level storm cellar along the Costa Rican coast would be perfectly trivial in the dead of summer in a city at 1,600m above sea level like Denver.
Step 3. Watch the reading
To determine the trend in a barometer, you need to know what the reading was, say, an hour ago and then compare it to what it is now - - in many barometers, this is done with a needle that you can manually set at each reading and will remain there permanently to serve as an indicator of recent pressure trends.
Step 4. Remember that pressure, and this specifically includes air pressure, is a measure of force per unit area
When measuring atmospheric pressure, it is more conveniently expressed in pounds per square inch or "p.s.i.", and is, in fact, very close to 14.7 p.s.i. at sea level and this value is known as "standard temperature and pressure" - - an internationally accepted and agreed state concerning the atmosphere in general, an average resulting from a large number of measurements, but all taken at sea level, or related to it.
Step 5. Know that atmospheric pressure can also be expressed in "atmospheres", ie in units of 14.7 psi each
However, this is almost never done in meteorology. Thus an atmosphere is 14, 7 p.s.i.
Step 6. Note the precedents regarding the terminology used for the measurement
Since the invention of Torricelli's original barometer was based on the fact that the average pressure of the atmosphere is able to "move" 76 centimeters or 760 millimeters of mercury (Hg), a liquid metal at standard temperature and pressure, inside of a vacuum-packed glass column or tube, we have traditionally - - and still today - - described atmospheric pressure in terms of millimeters of mercury.
- In the United States, it is common to speak of air pressure using the expression "inches of mercury" and almost all barometers in the United States are graduated in units of inches of mercury and give readings down to the nearest hundredth to an inch, for example. example "29.93 inches".
- Similarly, settings for aircraft altimeters are universally provided by sea-level corrected inch mercury control towers, regardless of airfield altitude.
Step 7. Therefore, to convert from p.s.i
to mm of mercury, multiply by 760/14, 7 = 51, 7:
- –– psi to inches of mercury, multiply by 30/14, 7 = 2.041
- –– inches of Hg to mm, multiply by 760/30 = 25.33.
Step 8. Note that air pressure is most commonly expressed in meteorology as "millibars"
A millibar is exactly one dyn (gm-cm / sec ^ 2) per square centimeter in the c.g.s. (stands for centimeter, gram, second). This has long since become a universally recognized and practical unit for expressing pressure in atmospheric studies. Turns out this converts to 1033 millibars of pressure, the same as the atmosphere or 14, 7 psi or 30 inches of mercury, and you'll find that most weather maps and all air force weather charts are in millibars. and that the measurements are generally very close to, or so, 1000 millibars at sea level.
Step 9. To get the pressure for any position in millibars, if you know the inches of mercury, just multiply by 1033/30 = 34, 433
Advice
- Unfortunately, we have not yet reached the stage where we can evaluate the barometric pressure from the study of clouds or from the color of the sky or through any other system that is not direct measurement with a sensitive device such as an aneroid barometer.
- There are sources of information on how you can predict the weather by looking at the values of a barometer during the hours and, by combining these with the knowledge of the direction and strength of the wind, on how the direction of the wind will move over time.
