Air Mass is defined as:

• A. A boundary between air masses
• B. An extensive body of air within which the conditions of temp. & moisture in a horizontal plane are uniform
• C. A region of high pressure
• D. A layer of atmosphere with uniform wind

Answer: (B)

An air mass is a large body of air with relatively uniform temperature and moisture over a broad horizontal area. This uniformity occurs because the air sits over a region long enough to take on the surface’s characteristics, so its temperature and humidity become similar across that expanse. It’s not about a boundary between air masses (that would be a front), not a region defined by high pressure, and not simply a layer with uniform wind. The defining idea here is that horizontal properties, specifically temperature and moisture, are uniform within a large air mass.