Since air pressure decreases with altitude in the atmosphere, the 500 millibar height is just the altitude above sea level at which a barometer would read 500 millibars. Typically this is around 18,400 feet. However, the average temperature of the column of air can affect that height. If the air in the column is warm, the 500 millibar level will be higher. If the column is cool, the 500 millibar level will be lower. When a meteorologist says that the 500 millibar heights are dropping over a particular location, that means that colder air is moving into the area. Learn more about air pressure on this USA TODAY resource page.