Which description defines an air mass?

• A. A small patch of air near the aircraft.
• B. An extensive body of air having fairly uniform properties of temperature and moisture.
• C. A specific humidity measurement.
• D. A type of atmospheric pressure region.

Answer: (B)

An air mass is an extensive body of air with fairly uniform properties of temperature and moisture. This happens because air sits over a large region long enough to take on the surface characteristics beneath it, leading to relatively uniform conditions throughout the mass. Air masses are categorized by moisture (maritime or continental) and by temperature (tropical, polar, arctic). As they move, they bring their temperature and humidity with them and can interact with other air masses to create fronts, changing weather patterns.

The other descriptions don’t define an air mass: a small patch near the aircraft isn’t representative of a large-scale body; a single humidity measurement is just a property, not the entire mass; and a region of atmospheric pressure is a pressure feature, not an air mass.