Rain-Free Base - A horizontal, dark cumulonimbus base that has no visible precipitation beneath it. This structure usually marks the location of the thunderstorm updraft.
RAIN-FREE BASE
A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft.
Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate. See HP storm, LP storm and supercell.
Rain gauge ...
A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm.
*Wall Cloud - A localized, persistent, often abrupt lowering from a rain-free base.
Updraft Base: Alternate term for a rain-free base.
Upper-Level Disturbance: A disturbance in the upper atmospheric flow pattern which is usually associated with clouds and precipitation.
Updraft Base - Alternate term for a rain-free base.
Upper Level System - A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.
A new wall cloud (extreme right) has formed from beneath the rain-free base, 3 or 4 miles east-southeast of the dying tornado. Within about 20 minutes this wall cloud fostered a violent, 1 mile wide tornado.
Most prominent beneath the rain-free base and on the leading edge of an approaching thunderstorm. Usually marked by gusty, cool winds.
Gust Front- the leading edge of a thunderstorm's downdraft air that is most prominent beneath the rain-free base and the leading edge of a thunderstorm; ...
WALL CLOUD -An isolated lowering of a cloud that is attached to the rain-free base of a thunderstorm, generally to the rear of the visible precipitation area. Wall clouds indicate the updraft of or the inflow to a thunderstorm.
The region may appear as a rain free clear slot below the rain-free base bounded by the RFD and FFD regions, if any. Not to be confused with the V NOTCH radar feature. Causes part of the BWER radar feature.
 See also: Storm, Precipitation, Updraft, Cloud, Rain
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