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The atmosphere surrounding our planet is a mixture of gases, including water vapor. The weather that we experience every day is almost entirely caused by physical changes to these gases. Energy from the Sun causes water molecules to change phase and evaporate into the air. This water vapor moves around, sometimes changing phase again and forming clouds, rain, ice, snow, dew, fog, and so on.

By studying small samples of gas in containers, we can learn more about how gases in the atmosphere behave. The kinetic theory of gases explains the behavior of individual gas particles under varying conditions. For example, when a gas in a rigid container is heated, its pressure increases because the molecules move faster, increasing the force and number of collisions with whatever they contact. When a sample of gas is squeezed into a smaller volume, it also exerts more pressure. This is because the same number of rapidly moving molecules must occupy a smaller space. The pressure, volume, and temperature of a gas sample are related by the combined gas law.

Meteorologists track the variables that affect the gases in the atmosphere. For example, they record water vapor density, or humidity, and areas of differing air pressure. They study air masses and fronts where warm and cold air masses collide. They combine all the information and use it to make predictions about the weather.