Bar is a measure of air pressure. 1 Bar is typically slightly less than the atmospheric pressure around sea level.
A millibar is thus 1/1000th of a bar.
Generally, one of the signs of a hurricane is a sudden drop in air pressure, and the level of the drop can indicate the severity of the incoming storm. Thus, if you are watching out for potential weather events on the horizon keeping an eye on air pressure is quite important.
Latest Answers