In my chases I have encountered dozens of wall
clouds and a handful of funnel clouds. Above are two of my better photographs:
on the left is a massive wall cloud that appeared in southern Illinois
on April 15, 1998, on the right is a large funnel I witnessed on May 29, 2001.
Wall clouds take on many shapes, sizes and colors. The lowering denotes
a storm's updraft where rapidly rising air causes lower pressure just below
the main updraft. In lower pressure water can condense and form clouds
at a relative humidity below 100%, thus you may have a visible lowering
of the cloud base such as in these two examples below.
on the left is a large wall cloud that appeared south of Scottsbluff,
Nebraska on May 21, 1998. On the right is a picture of a wall cloud associated
with storms near Davenport, Iowa on May 18, 1997.
If a wall cloud has an intense, rotating updraft it should be considered rather dangerous since it is this type of wall cloud that can produce a tornado.
The rotating wall cloud on the left prompted a Tornado Warning but no
funnel was spotted. The Hayes Center, Nebraska storm of May 21, 1998 was obviously
associated with a mesocyclone. The funnel is very faint at the lowest point
of the wall cloud. About a minute later a weak tornado appeared, the tornado
was difficult to spot due to nighttime darkness.
Many wall clouds are harmless and do not pose a significant threat.
The wall cloud on the left showed some signs of slow rotation but failed
to produce a tornado because of weak low level winds. (West Texas May 25,
1999) On the right is a wall cloud that exhibited no visible signs
of rotation. (May 25, 1999).
Sometimes wall clouds can appear harmless at a distance but they can produce ground based circulations.
This wall cloud looks harmless enough but a few minutes later a significant ground based circulation appeared (photo on the right).
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Brian William Jalas.