This same principal explains how a barometer measures the atmospheric pressure. Since mercury is much heavier than water, the atmospheric pressure can only support a column of mercury about thirty inches high. But as the atmospheric pressure changes, so does the height of the column it can support. When the meteorologist announces that the barometric pressure is twenty-nine inches of mercury, it means that the air pressure is capable of supporting a column of mercury twenty-nine inches high. You could measure the atmospheric pressure with a column of water, but a thirty-foot water barometer would be much more cumbersome than a three-foot mercury barometer.