Content

• Steps
• Tips

The article describes how to calculate pressure using a barometer. This is necessary in order to analyze and predict the weather. The tips are for practical use. Perhaps it should be noted from the outset that pressure is not "calculated" but measured with a barometer; then the value is converted into units that are more convenient for understanding.

Steps

1. 1 Watch for patterns. When analyzing a weather forecast, the absolute value of pressure is a particularly important factor in determining patterns. Namely, does it rise, fall, or remain unchanged? The old barometer panel is decorated with beautiful images that visualize strong winds, storms, sunny weather, etc. This is very illustrative, but misleading nonetheless. It is the movement of the barometer needle (or meniscus, if you are the owner of a very old model) that has much more to do with the upcoming weather.
2. 2 Recall that atmospheric pressure more or less decreases with increasing altitude. This means that pressure indicative of a major storm off the coast of Costa Rica is completely normal in mid-summer for the city of Denver, which is one mile above sea level.
3. 3 Watch the readings. To determine the weather from a barometer, you need to know what it was reading, say, an hour ago. They must be compared with the current barometer readings. Many barometers have a pointer that can be positioned at a specific division on the panel. He will remain motionless. This will help you remember your recent barometer pressure reading.
4. 4 Recall that pressure, which is mainly air pressure, is a force applied to a unit area. It is most convenient to measure pressure in pounds per square inch (or kg per cm). At sea level, the pressure is very close to 14.7 psi. inch. This value is known as "standard temperature and pressure" - accepted and agreed at the national level. It describes the state of atmospheric pressure in general. The value was obtained by numerous measurements that were taken at sea level. Measurements carried out under different conditions were reduced to those.
5. 5 Remember that atmospheric pressure is also measured in "atmospheres." However, this is rare in metrology. So one atmosphere is 14.7 psi. inch.
6. 6 Pay attention to the meteorological terminology. Due to the fact that the barometer, which was invented by Torricelli, consisted of a tube of mercury, and the normal pressure is similar to the pressure of 76 cm or 760 mm of mercury on the walls of a sealed glass tube, there is still a tradition to describe pressure in such terms.
   • In the USA, it is customary to measure pressure in "inches of mercury" and almost all barometers are graduated on such a scale. Pressure is measured to the nearest hundredth of an inch, for example "23.93".
   • In addition, the parameters for the aircraft altimeter are transmitted from the flight control point precisely in inches of mercury, corrected for sea level, regardless of the height of the aerodrome.
7. 7 Thus, in order to convert pounds per square inch to millimeters of mercury, you need to multiply by 760 / 14.7 = 51.7:
   • –– From pounds per square inch to inches of mercury - multiply by 30 / 14.7 = 2.041
   • –– From inches of mercury to millimeters, multiply by 760/30 = 25.33.
8. 8 Note that atmospheric pressure in meteorology is most often expressed in terms of millibars. A millibar in the CGS measurement system is one dyne (g-cm / sec ^ 2) per square centimeter. It took a long time for such units to become generally accepted and convenient for measuring pressure. It turns out that a pressure of 1033 millibars is also 14.7 psi.inch or 30 inches of mercury. You will notice that pressures are reported in millibars on most weather charts and on all aviation charts. At sea level, the values ​​are usually very close to 1000 millibars.
9. 9If you know the pressure, measured in inches of mercury, then in order to convert it to millibars, you just need to multiply by 1033/30 = 34.433

Tips

• Unfortunately, we have not yet reached the level to determine atmospheric pressure based on information about clouds and the color of the sky or in any other way without direct measurement using a sensitive barometer.
• So, you can predict the weather by observing the movement of the barometer needle for hours, and comparing this data with the direction and strength of the wind.