If you live in the Midwestern United States, you've probably seen them: towering cumulonimbus anvil clouds rising into a sky colored with the palette of bruised skin- sickly green, purple, or orange-brown. One glance at these phenomena is enough to tell any observer: tornado weather. In addition to the eerie colors, most tornados are preceded by signature types of cloud formations. Here is what to watch for in the upper, middle, and lower levels of a storm front.
While a storm is still some distance away (thirty or forty miles), you may be able to spot the "anvil," a well-defined wedge of cloud jutting from the upper level of a heaped cumulonimbus formation. Storm systems that include a sharp anvil are most likely to produce severe weather, such as tornados.
When the storm approaches to within ten to twenty miles of your position, you will be able to make out details of the middle level of the formation. The storm tower will be well-defined, similar in appearance to a head of cauliflower. Well-organized (and hence potentially dangerous) storms will also have a flanking line, a stair-step series of smaller cloud towers that mount up to the main tower.
At a distance of 10 miles or less, the large-scale features of a tornado-producing storm will no longer be visible. Instead, you will see the low-level features of the tower. One key feature is the rain-free base, a sure sign that powerful up-drafts are operating in the middle of the storm system. The rain-free base is a low-hanging plateau of cloud from which no visible sheets of precipitation are falling. (Often large hail actually is dropping from this area, but it's not as visible as the curtains of rain around the periphery.) Most tornadoes form close to this base, often to the north or east.
Another clue is the wall cloud, a lowered section of the rain-free base. This may be flat, or may have tendrils reaching down on either side. The wall cloud is generally the source of tornados, and any wall cloud that persists for 10 minutes or more, and shows rotational movement, is very dangerous indeed.
It's tempting to stand, awestruck, and stare upward at the beauty of a menacing sky. Once the first small fingers or crowns of a wall cloud begin to reach down toward the ground from the rear of the formation's base, though, it's time to head for shelter.