To understand a barometer's readings, it is necessary either to understand the relationship between air pressure, sea level and weather or to know what certain barometric pressure readings correspond to in terms of anticipated weather. Using the second method, one will only know what weather to anticipate; with the first one, someone is likely to understand the reason for the weather prediction.
Barometers measure air pressure in the area immediately surrounding the instrument. The two factors influencing air pressure are elevation and weather. Because air pressure decreases with distance above the ground, the barometric readings on an airplane in flight will indicate something vastly different than the same readings at sea level. In most cases, barometric readings are adjusted to indicate what they would be if at sea level, so that the elevation factor can be removed from the equation. Typical barometric readings range from below 29.80 to above 30.20 inches of mercury. The higher the number, the less stormy the weather is. Readings of 30.20 or higher indicate continued fair weather. Readings lower than 29.80 indicate stormy weather, especially if the numbers are continuing to fall. Between 29.80 and 30.20, it is important to see whether the numbers are rising or falling. If they are rising or steady, the weather will not change in the near future. If they are falling, rain or snow should be predicted.