What best defines an airmass?

The concept of an airmass is accurately captured by the definition of a large parcel of air with similar temperature and humidity. An airmass is essentially a body of air that has uniform characteristics in terms of temperature and moisture content. These properties are influenced by the geographical area over which the airmass forms. For example, an airmass that forms over a cold ocean will typically be cool and moist, while one that forms over a hot desert will be hot and dry.

Understanding airmasses is critical in meteorology because they are the building blocks of weather systems and help in predicting weather patterns. When different airmasses interact, it can lead to various weather phenomena, including storms and changes in temperature.

The other options describe weather phenomena or conditions but do not encapsulate the essence of an airmass. A massive storm system refers to a specific weather event rather than a defined parcel of air. A localized weather phenomenon signifies events that occur in particular areas without the focus on the uniformity of air characteristics. A region experiencing high pressure is a specific atmospheric condition but does not define an airmass, which can exist under varying pressure systems.