What is a warm front?

• A. The boundary area formed when a cold air mass contacts and flows under a warmer air mass.
• B. A boundary where warm air mass remains below the colder air mass and pushes it upward.
• C. The boundary area formed when a warm air mass contacts and flows over a colder air mass.
• D. A front that leads to clear skies.

Answer: (C)

A warm front occurs when a warm air mass moves into an area occupied by a colder air mass and slides over it. Because warm air is lighter than cold air, it rides up and over the cooler air, creating a gradual, overrunning boundary with a gentle slope. This lifting of the cold air leads to the formation of widespread stratified clouds (like stratus and nimbostratus) and steady, widespread precipitation that can last longer than the rain associated with a cold front. After the front passes, you typically see a rise in temperature and often improving visibility, though the sky remains overcast for a time as the clouds move through.

The other descriptions don’t fit this pattern: a cold front involves cold air pushing underneath and lifting the warm air rapidly, usually bringing heavier, more convective precipitation; a warm front described as the warm air staying below and pushing the cold air upward isn’t how the warm front behaves—it's the warm air moving over the cold air. And a front that leads to clear skies ignores the common cloud and rain associated with a warm front.