Explanation:
Perhaps it's time to go inside.
Such thoughts might occur to people witnessing the approach of an impressive
shelf cloud.
Shelf clouds are typically seen leading
thunderstorms,
although they may
precede
any well defined front of relatively cold air.
Shelf clouds
differ from
roll clouds
because shelf clouds are attached to a larger cloud system lurking above.
Similarly, shelf clouds
differ from wall clouds because wall clouds typically trail
storm systems.
The above pictured
shelf cloud
was photographed toward the southwest during a trip crossing the prairies of
Saskatchewan,
Canada on the
Trans-Canada Highway
in 2001 August.
A rising Sun illuminated the
impressive cloud from the east as it advanced from the west.
