You see the fan come on a lot on turbo diesels because of the extra 300+ degrees of the intercooler under full boost cranking up a large hill. The fan locks on when the air temp of about 205F (on my 7.3 Ford) hits the bimetal curled sensing clock spring which turns and allows the fluid to lock the fan.
What is annoying is when you are cranking up a hill at 70 mph or more (plenty of airflow) and the stupid fan comes on anyway dragging 25-30 hp, even though it shuts right off after 20 seconds of cresting the hill.
Im actually thinking of adding a windshield wiper squirter that is aimed right at the center of the fan clutch. When it locks on a hill, I can just push the button for 5 seconds of water and the fan should instantly unlock for a bit.