The dials of older barometers often feature artistically-drawn backgrounds to indicate weather conditions like storms, strong winds, and clear skies. For all their aesthetic, these backgrounds can be misleading. The movement of the barometer needle has far more bearing upon the coming weather. If you have an old, traditional mercury tube-type barometer, then you may need to watch the meniscus: the highest curve of the liquid mercury that fills the cylinder.
In many barometers, you can manually set a needle to mark a point on the pressure gauge. The needle will remain at this point to help you gauge recent trends in the pressure.
14. 7 psi is a rough average of barometric pressure at sea level and at STP (standard temperature and pressure. ) STP is an internationally-accepted “general state” of the atmosphere. The 14. 7 figure has been averaged over a large number of measurements, all either taken at sea level or corrected to sea level. Atmospheres are rarely used in meteorology. [7] X Research source The millibar is an especially convenient unit of pressure for atmospheric studies. 1033 millibars is the pressure equivalent to one atmosphere, 14. 7 psi, or 30 inches of mercury. Most weather maps and all aeronautical weather charts are in millibars, and the pressure at sea level is usually very close to 1000 millibars. Nearly all barometers in the U. S. are graduated in units of inches of mercury. The barometer is read to the nearest hundredth of an inch, such as “29. 93 inches. " Similarly, aircraft altimeter settings are universally given by control towers in inches of mercury corrected to sea level, regardless of the altitude of the airfield.
Convert from inches of mercury (read from the barometer) to millibars: if you know the inches of mercury, simply multiply by 34. 433. 1033/30=34. 433{\displaystyle 1033/30=34. 433} Convert from psi to mm of mercury: multiply the psi by 51. 7. 760/14. 7=51. 7{\displaystyle 760/14. 7=51. 7} Convert from psi to inches of mercury: multiply the psi measurement by 2. 041. 30/14. 7=2. 041{\displaystyle 30/14. 7=2. 041} Convert from inches of Hg (mercury) to mm Hg: multiply the measurement in inches by 25. 33. 760/30=25. 33{\displaystyle 760/30=25. 33}
