Awarm front forms when
a. cold air moves up over warmer air.
b. a warm air mass meets another air mass of the same temperature.
c. warm air glides up over colder air.
d. the temperature in a warm air mass increases. reset selection
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C. warm air glides up over colder air.  

Warm front is a transition zone between warm air mass and cold air mass. A typical warm front moves from a southwest to northeast direction. The air behind the warm front is warmer and moist whereas the air in front is cooler. A warm front bring change to the atmosphere of a region. It makes the atmosphere warmer and humid.

Warm air glides up over colder air results in a formation of warm front as colder air is replaced by warm air.