Since air is a gas it can be compressed or expanded. When air is compressed a greater amount of air can occupy a given volume. Conversely, when pressure on a given volume of air is decreased, the air expands and occupies a greater space. That is, the original column of air at a lower pressure contains a smaller mass of air. In other words, the density is decreased. In fact, density is directly proportional to pressure. If the pressure is doubled the density is doubled, and if the pressure is lowered, so is the density. This statement is true, only at a constant temperature.