A complex sentence is a sentence consisting of an independent clause and a subordinating clause. Or, to put it more simply, complex sentences involve more than just a straightforward noun + verb. Complex sentences include extra phrases added to a base sentence, but which cannot stand alone as a sentence. For example:
Published 4:00 am, Thursday, January 9, 1997. 1997-01-09 04:00:00 PDT Yosemite National Park -- Yosemite Valley is slowly drying out after what park rangers are calling the worst flood here in recorded history, leaving behind hundreds of upturned tent-cabins and acres of wrecked camping sites.
Sept. 3 is Labor Day, and many hotels will be crowded. I assume you have accommodations in Yosemite booked, as they will be crowded even after Labor Day. Temps in Death Valley are more likely to be down to about 80F at night, not as a high during the day, and up to 110F during the day.