On a weather map, what do fronts indicate?

• A. Boundaries between air masses
• B. Areas of high rainfall
• C. Ocean currents
• D. Temperature isotherms

Answer: (D)

Fronts mark the boundary between different air masses. They form where two air masses with distinct properties—such as temperature and humidity—meet and interact, causing changes in weather as one mass moves against or over the other. For example, when a warm, moist air mass advances into cooler air, a warm front develops, often bringing gradual warming and prolonged rain. Conversely, a cold front occurs when a cooler air mass slides under a warmer one, typically bringing a sharper change in weather, like brief intense rain or thunderstorms. The other ideas aren’t what fronts represent: isotherms are simply lines of equal temperature, and ocean currents are large-scale water movements not depicted as fronts on weather maps. Rainfall can be associated with fronts, but the defining feature of a front is the boundary between contrasting air masses.