Doc laughs because it would be just fitting that as they're heading to Katherine's house, the universe decides that an absolute downpour is on the menu.

It's Texas, in the summer, so the rain is warm and it gets in his eyes and it's not long before he's drenched and his hair is hanging in his eyes as they hurry along.

When a soft rumble of thunder rolls in the distance, it starts to rain harder, so Doc does what any proper gentleman would do in the situation:

Takes hold of her hand (so she doesn't slip, of course) and runs like hell.