What is the key characteristic of an air mass?

The key characteristic of an air mass is that it is a large body of air with uniform properties, which encompasses temperature, humidity, and pressure. Air masses form when air remains over a region for an extended period, allowing it to acquire the characteristics of the surface below it. This uniformity is essential because it affects the weather patterns in the areas the air mass moves into.

For example, if a maritime tropical air mass moves over land, it brings warm and moist conditions, while a continental polar air mass would bring cold and dry weather. Understanding the properties of air masses helps meteorologists predict weather changes and phenomena across different regions.

Other options describe different weather phenomena or features. A small volume of warm air does not represent an air mass, as air masses are defined by their larger scales. High-pressure systems are meteorological phenomena affected by air masses but are not characteristics of the air masses themselves. Localized weather systems typically refer to smaller-scale phenomena, such as thunderstorms, which do not capture the broader scope of air mass characteristics.