re: #130 claire

Agree it could have been clearer.

The ability of the atmosphere (at any given height/pressure) to hold water in a vapor state as opposed to liquid (even the tiny tiny drops of a cloud) is due to temperature.

Thus the atmosphere at higher temperatures will hold more water vapor.

Water in the gaseous state is a greenhouse gas. The more greenhouse gases in the atmosphere the more opaque the atmosphere will be to outgoing radiation.

As one goes up in elevation the temperature drops, and the amount of water the atmosphere can hold decreases. As the temperature drops below the freezing point of water the amount of water vapor in the atmosphere decreases significantly. CO2 however remains as a gas. By the time we are up to the transition from the troposphere to the stratosphere the ratio of water vapor to CO2 in the atmosphere changes radically from what it is at the surface.

From what I understand, even as AGW proceeds the mean relative humidity over the surface of the Earth is expected to remain essentially unchanged. Any additional evaporation of water (say from the oceans) as temps rise will lead to increased precipitation.