A cumulonimbus cloud with dry
lightning. A firefighter (usually a wildland firefighter) calls this a cumulus overtimus in
reference to the overtime that he / she will
make if there are down strikes that start a fire.
While standing at the station looking at the
clouds or when
listening to the weather report that predicts
dry lightening.
"It looks like there is a big build up of cumulus overtimus."
or
"Bring on the cumulus overtimus."