The molecules consist of two atoms. O 2 is required for biological processes. Without it, we die. Oxygen reacts with foods during metabolism to produce CO2 and water. This is an example of a combustion reaction.
Humidity is a description of the amount of water vapor in air.
Atmospheric pressure is the force with which the atmosphere presses down on a given surface area. Atmospheric pressure decreases with increasing altitude, while the percentages of the different components remain the same. Atmospheric pressure is measured with a barometer. The troposphere is our immediately surrounding atmosphere. Above that is the ozone layer, stratosphere and, at the top, the mesosphere.