When my wife starts walking around the house wearing a heavy coat and gloves, and pretends to be scraping ice off her laptop screen...I get the message. Summer is over. It's getting cold. Time to turn on the furnace. Time to get the home all warm and toasty while imagining huge piles of money burning to supply that heat.
I personally can tolerate much lower temperatures. When I lived on my own, I usually waited until the cat's water dish froze over before firing up the furnace. But, alas, I must compromise. That's what marriage is all about.