That's an absurd myth. Blood doesn't thin because you live in a warmer climate. There are valid biological reasons why people get acclimated to warmer weather, but it's not because of "thin blood."

Check out
the straight dope for more specifics.
Geez, forgive me, it was a just a term that we (people in my family and friends circle) always used as we had moved from the north to Florida. When this took place, I lost my tolerance for cold. Albeit, I have since moved back to the North, and I have once again gotten used to it.
I agree, not all people are alike when it comes to temperaturs and clothing, I was just suggesting to think about that, cuz if someone had asked my mom right now (who lives in Orlando) what to wear tomorrow, she would tell ya to pack a parka. So, I apologize, as I had based my suggestion on personal experience only. Forgive me for giving some personal experiences to try to help people out - gesh!