I believe you cannot smoke in the airport (hardly anywhere in the US you can smoke nowadays except outside). Once you clear customs (if you are from outside the US) and get your luggage - you can exit the doors and smoke outside as long as you like.
Rain here is common, although it's not as common as in, say, Seattle. This time of year, the weather can be very hot. So, the ocassional rain can also help alleviate the heat. Don't listen to long-term weather forecasts. Anything beyond 2-4 days into the future is rarely accurate.
It's rare to have a day in New York when it would rain all day long. Usually, it's ocassional showers at worst.