What is a barometer and how is it useful for weather forecasting?

A barometer calculates the pressure, or force, exerted by the weight of the atmosphere column above it.

Since temperature and water content are the only two variables that can truly alter the atmosphere (all other variables are dependent upon these two variables), this information is helpful for forecasting. Pressure measures both temperature and water content because warmer air has higher pressure (Boyle's law) and moisture lowers air pressure (N2 is heavier than H2O).

A barometer is a scientific instrument used to measure atmospheric pressure. It is useful for weather forecasting because changes in atmospheric pressure often indicate changes in weather patterns. A rising barometer typically indicates clear or improving weather, while a falling barometer indicates the possibility of inclement weather, such as rain or storms. By monitoring changes in atmospheric pressure, meteorologists can predict upcoming weather conditions and issue warnings accordingly.