Final answer:
A blue line with triangles on a weather map indicates a cold front, where cooler air pushes and lifts warm air leading to various weather changes.
Step-by-step explanation:
A blue line with triangles facing warm air represents a cold front on weather maps. This type of front occurs when a cooler air mass moves towards and under a warmer air mass, forcing the warm air to rise. As the warm air cools and condenses, it can lead to cloud formation, precipitation, and sometimes storms. Weather maps depicting these fronts help meteorologists describe and predict weather patterns, including changes in temperature, wind, and precipitation.