weather [wind] [5] [information over pictures]

The next thing we have to consider is what happens when these air masses meet as in this animation.	A weather front is simply the boundary between two air masses.	1. Cold front This is the boundary between warm air and cold air and is where cold air is replacing warm air at a point on the Earth's surface.
2. Warm front This is the boundary between cold air and warm air and is when warm air replaces cold air at a point on the Earth's surface.	3. Occluded front The word occluded means 'hidden' and an occlusion occurs when the cold front 'catches up' with the warm front. The warm air is then lifted from the surface, and therefore hidden.	4. Stationary front Sometimes air masses exert similar strength forces on each other and no one wins. When one air mass is no stronger than the other, no movement occurs and you get a stationary front.

Look at all of the pictures in turn. As you hover over each one a blue screen appears with an explanation of what the picture shows. Important words are coloured. If you are certain that you know about the picture click on it and it will disappear. As you go through all of the pictures you will get rid of all those which you know about and be only left with those you may be unsure of. In some cases you may get rid of all of the pictures in which case you are ready to move on to other activities which will test your knowledge on that particular topic.