At our university, at least when I studied there some 15 years ago, the whole cluster was occupied doing weather predictions each and every night.
No point in staying up waiting for a job, it'd get rescheduled in the early morning at best.
It wasn't the largest cluster around, IIRC 768 quad-core nodes, but I'm sure the meteorological department would find a way to utilize any extra capacity, so still requiring the whole thing all night.