new to watching the tour but why do they take their helmets off while climbing? what are the rules about it?

new to watching the tour but why do they take their helmets off while climbing? what are the rules about it?
They are hot, and extra weight. They are allowed to take their helmets off if the last climb is longer than 5K. only on the last climb.

If the last climb leads to an uphill or summit finish.

Heat is the reason, 200 g (1/2 pound) is not reason enough to toss them - it's heat.

It's almost 100 degrees F. on the tarmac these days in Southern France.