What is defined as a large expanse of air in which temperature, lapse rate, and humidity are uniform?

Airmass is defined as a large expanse of air that possesses uniform characteristics in terms of temperature, humidity, and lapse rate. These uniform properties arise because the airmass originates from a specific source region, such as an ocean or land area, where those characteristics are established.

Airmasses can be classified based on their origin and characteristics, such as maritime tropical (warm and humid) or continental polar (cold and dry), which further influences the weather patterns they bring to the regions they move into. Understanding airmasses is crucial for meteorology as they play a significant role in determining the weather and climate of an area.

The other options do not accurately describe a large body of air with uniform characteristics. For example, thermal cells refer to localized convection processes, weather fronts indicate the boundary between different airmasses, and pressure zones are areas of varying atmospheric pressure that do not necessarily indicate uniform temperature and humidity.