What happens at
the boundaries between warm and cold masses of air?
When warmer air replaces
cooler air, it is called a warm front. Similarly, a cold front describes
the situation when a mass of cooler air replaces warm air. These fronts
are shown on weather charts as lines with either red semi-circles (indicating
a warm front) or blue triangles (showing a cold front). These symbols
are placed on one side of the frontal line to show which way the front
As cold fronts move
more quickly than warm fronts, they can catch up with the warm front ahead.
This is called an occlusion.
How fronts change with altitude