A Closer Examination of the Animation:
Initially, a warm air mass (in yellow) nudges against a colder air mass (in blue) ahead of it, (separated from each other by the warm front). The lighter warm moist air behind the front is lifted upward and "overrides" the colder air.

As the air rises, it cools, and if enough water vapor condenses, widespread clouds and precipitation develop.

A layer of thin clouds is occasionally observed more than a thousand kilometers in advance of a surface warm front. As the front gets closer, the clouds thicken and eventually light precipitation begins to fall. Because the frontal surface gently slopes up and over the cold air mass ahead of it, the upward motions associated with warm fronts are typically not as strong as the vigorous upward motions that occur ahead of a cold front.