Barometers are instruments that measure air pressure. In today’s world this may seem fairly irrelevant to the majority of us most of the time, but it can be more important than we realize, especially if we desire to continue breathing sufficient oxygen while flying high above the ground in an aircraft. Detecting changes in cabin air pressure at high altitudes, that may indicate leaks, is the function of electronic barometers, and results in the automatic dropping of oxygen masks from the above compartments when potentially needed. Barometers are also one of the pieces of equipment our weather forecasters have used for centuries to tell us what to expect in the near future, weather-wise.
All up, the standard barometer is a quite simple device, very similar to a thermometer. Both of which generally use the same substance, mercury, to make their measurements. This is because, while being a metal, mercury is a liquid at normal temperatures and pressures. And that is where the difference between a barometer and a thermometer comes in. An increase of temperature heating the mercury in a thermometer causes it to expand, rising up the narrow vein of the calibrated thermometer disclosing the ambient temperature to us.
With a barometer, the mercury is designed to rise in the vein of the instrument in response to a decrease in the local air pressure, rather than a rise in temperature, as pressure constricts the volume a set amount of mercury will fill. This can be used to determine either altitude, as air pressure reduces the higher you are, or the amount of water vapor in the local air.
The molecular mass of water vapor is 18.02, which is lower than the average of the other components of air based on their respective percentages; dry air (all other components of air besides water vapor) has a molecular mass of 28.98. What this means is that the higher the water vapor content or humidity of air, the lower its mass, and air pressure is directly proportional to its mass; the more mass the more pressure. The more water vapor there is in the air, the less pressure that air volume applies to anything within it.
This is where the high and low pressure readings shown in weather forecast maps comes in. The higher the absolute humidity and therefore the water vapor content of the air, the lower the overall air pressure and vice versa. Low pressure regions have a high water vapor content and high pressure regions have a low water vapor content. The lower the water vapor content of the air, the less likely it is to result in precipitation, whether that be rain, sleet or snow.
In determining the probable weather forecast, the actual barometric level shown is far less important than the change in pressure over time. If the pressure reading showing on your barometer is dropping, it indicates that the water vapor content of the local air is increasing, meaning you are likely to to receive some type of precipitation; ambient temperature generally indicating whether it will be rain or something colder. If it drops fast, you are probably facing a storm front, with heavy rains and possibly thunder and lightning likely in the near future.
If the barometric pressure is rising it indicates that the water vapor content of the local air is reducing, so you are probably in for some nice sunny weather and clear skies. A plus for most of us as long as we are not currently residing in a drought-stricken locality.