What boundary forms when a warm air mass slides over a colder air mass?

A warm front forms when a warm air mass advances and slides over a colder air mass.

Because warm air is less dense, it generally rises gradually over the denser cold air rather than lifting it abruptly. This gentle slope can produce broad areas of layered cloud, often beginning with high cirrus and progressing toward thicker altostratus and nimbostratus.

Rain or snow ahead of a warm front can be steady and widespread. As the boundary passes, temperatures usually rise and winds may change direction. The exact sequence depends on moisture, terrain and the strength of the air masses.

A warm front differs from a cold front, where advancing cold air wedges beneath warmer air and forces a steeper, faster ascent. Weather maps commonly mark a warm front with a red line decorated with semicircles pointing in the direction of movement.