The occluded front

Let’s return to the four stages of a depression you explored in Step 1.9. Remember that the grey shading shows the region of continuous cloud cover. The red warm front and blue cold front denote the boundaries of the central wave of warm air – often called the warm sector. The occluded front (purple) occurs when the wave breaks, and the cold front catches up with the warm front.

Figure 1: Four stages in the lifecycle of a depression

Figure 2: A cross section through a depression at stage 3 © Dr Peter Inness

You can see the cold front has caught up with the warm front and has been pushed up the warm front. On the ground, the cold air from ahead of the system is directly in contact with the cold air behind. If you imagine you’re standing on the ground again you won’t notice much of a temperature change as the occluded front passes. However, the remaining warm air, squeezed higher up in the atmosphere, is still rising, so you’re going to still need your umbrella as there will still be cloud and rain.

There are several physical reasons why the cold front catches up with the warm front. Conceptually, remembering that all the air is moving from west to east, it’s easy to imagine how it is relatively easy for the more dense, heavier cold air to push the less dense, lighter air out of the way on the cold front. Conversely, it’s much harder for the less dense, lighter warm air to push the more dense, heavier air out of the way on the warm front. The net result of the processes in action is that the cold front moves faster than the warm front and the warm air in between the fronts is squeezed up and away from the surface.