Front is defined as:

• A. A boundary between two different air masses
• B. A boundary between high and low pressure
• C. A line of thunderstorm development
• D. A boundary between warm and cool air only

Answer: (A)

A front is a boundary where two different air masses meet. The defining feature is the contrast between the properties of those air masses—typically temperature and moisture—which creates a dynamic interface as one air mass slides against or over the other. This interaction often lifts air and leads to cloud formation and precipitation, with the exact weather depending on the characteristics of the two air masses involved.

This definition isn’t about pressure boundaries. Fronts can be associated with various pressure patterns, but the essential idea is the change from one air mass to another. It isn’t merely a line where thunderstorms occur, either; a line of storms can form along a front, but the front itself is about the boundary between contrasting air masses. And it isn’t limited to warm and cool differences only—the boundary exists because the two air masses have different properties, which can include humidity and density, not just temperature.