Which meteorological term describes a boundary between two air masses with different temperatures and humidity?

The term that describes a boundary between two air masses with different temperatures and humidity is a front. Fronts are essential features in meteorology, as they are pivotal in determining weather patterns. There are different types of fronts, including cold fronts, warm fronts, stationary fronts, and occluded fronts, each characterized by the movement and interaction of the air masses involved.

When two air masses meet, they do not mix immediately due to their differing densities—typically a warmer air mass might rise over a denser, colder air mass. This interaction can lead to various weather phenomena, such as precipitation, clouds, and changes in temperature. Understanding fronts is crucial for meteorologists in predicting weather changes and systems.

The other options are distinct meteorological concepts. A pressure system refers to areas of high or low atmospheric pressure and can influence weather but does not specifically describe the boundary between air masses. An air mass is a large body of air that has uniform characteristics of temperature and humidity, but it is not a boundary itself. A thermal layer refers to a portion of the atmosphere where temperature changes occur rapidly, but again, it does not specifically denote a boundary between differing air masses.