The lighter side of humidity

Step off a plane in Miami or Calcutta and you may feel as if you've been hit by a tonne of hot, moist bricks. When air is extremely muggy, it feels thicker and heavier than usual. Actually, though, tropical air is less dense than its cold, dry counterpart when both are at the same air pressure. One reason is the way that temperature works. The molecules in warmer air are moving more energetically, so they tend to take up more space at a given pressure. What's more, a molecule of water vapour only weighs about half as much as a two-atom molecule of either oxygen or nitrogen – the main components of dry air. If you packed some water vapour into a closed container of dry air, you'd be adding weight to the contents, but in real life air moves and mixes. This means that water vapour ends up muscling out some of the dry-air molecules and occupying their space. The result is less atmospheric mass and lighter air. The only thing that's become heavier is the perspiration-drenched shirt on your back.