Slides in Air masses and fronts
Huge masses of cold and warm air are always moving across the land and oceans. These air masses will often bump into each other but they do not easily mix. It's similar to how oil and water react when they're put in the same container.
The term "front" comes from military language - it means a battle area where opposing armies meet to fight.
Cold air is dense and tends to sink towards the ground, while warm is is less dense and tends to rise. When a cold air mass runs into a warm air mass, the cold air slides under the warm air.
Cold fronts move quickly, so they can cause abrupt weather changes. If there is a lot of moisture in the air, heavy rain or snow storms may occur.
Clouds, storms and rain can also occur along warm fronts. Warm fronts move much slower than cold fronts so the weather may be rainy or foggy for several days.
Sometimes cold and warm air masses meet, but neither one has enough force to move the other. When this happens, it is called a stationary front.
This is the most complex weather situation. Basically, in an occluded front a warm air mass is caught between two cooler masses. Take a look at the diagram at the right: