SHELF CLOUD: Though not a particularly rare weather event, shelf clouds are well known for their striking appearance that create quite a jarring reaction, when viewed from a distance. The size and scope of these clouds often extend across much of the horizon. This is because they form on the leading edge of a thunderstorm or area of heavy rain.
The ominous appearance of these clouds are formed with the interaction of rain cooled air moving out and away from a thunderstorm and smacking into warm and humid air. When this happens, warm air is forced to rise above the cool air mass. This mixing, creates the shelf cloud.
Interestingly, as you watch the shelf cloud approach you, you feel winds at your back, flowing into the storm. Then almost immediately, you will get the rush of cold air, blasting out of the storm and coming from that direction. A complete wind shift and air mass shift can be felt.