What defines an air mass?

• A. A large body of air with relatively uniform temperature and humidity
• B. A weather system over a region
• C. A type of cloud
• D. A rainfall event

Answer: (A)

Air masses are defined by their large size and relatively uniform temperature and humidity. They form when air sits over a surface long enough to take on those surface characteristics, so the properties remain similar across a wide area. The idea is that the mass has consistent conditions, which is why they’re labeled by their origin (like where they formed) and by whether they’re dry or moist and warm or cold. Other options point to weather systems, clouds, or precipitation, which are different phenomena and not the single, uniform body of air described by an air mass.