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Originally Posted by towerguy3
There are apparently two types of funnel clouds, cold funnels and warm funnels. What is the difference in how they form? Do they appear different? And what exactly is a wall cloud?
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A cold air funnel is just a weak and tight circulation which can occur out of any thunderstorm or rain cloud, often from a high cloud base. Condensation from low air pressure from the rapidly spinning column of air makes a funnel cloud visible. Cold air funnel clouds are so weak that they very rarely ever touch ground and become tornadoes. However, the tornado which hit Grande Prairie in July, 2004 in fact started out as an unusually large and strong cold air funnel.
If they occur over any body of water and touch water, they become waterspouts. Waterspouts are generally most common over subtropical/tropical waters, such as off Florida, but also occur in colder waters off BC and Atlantic Canada and lakes almost anywhere in Canada when conditions are right. There is really no such thing as a "warm" funnel.
A wall cloud is a significant, roughly round lowering attached to a rain-free base (also known as the updraft base) of a thunderstorm. They're most common in severe thunderstorms, particularly supercells, which are severe thunderstorms with rotating updrafts. Wall clouds in supercells will often be seen rotating persistently. Most strong/violent tornadoes come from supercells and a wall cloud is generally where a large funnel/tornado will drop down from. The rain-free base and wall cloud is usually located in the southwest side of a supercell.
Wall cloud with a tornado in Kansas, as taken by Mike Hollingshead, a storm chaser from Nebraska.
The 1987 tornado came out of a huge wall cloud, as can be seen below.