This layer holds almost all of the water vapor in the atmosphere, as well as almost 3/4of its mass

Troposphere.

The troposphere contains 75 percent of atmosphere's mass - on an average day the weight of the molecules in the air is14.7 lb ... (sq. in.) - and most of the atmosphere's water vapor. Water vapor concentration varies from trace amounts in Polar Regions to nearly 4 percent in the tropics. Most prevalent gases are nitrogen (78 percent) and oxygen (21 percent), with the remaining 1 - percent consisting of argon, (.9 percent) and traces of hydrogen ozone (a form of oxygen), and other constituents. Temperature and water vapor content in the troposphere decrease rapidly with altitude. Water vapor plays a major role in regulating air temperature because it absorbs solar energy and thermal radiation from the planet's surface.