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."