What outfits will keep Mario warm?
The Aviator Outfit, the Snow Outfit, and the Space Suit are the only clothing sets that will keep Mario warm in cold environments.
In addition, these outfits will keep Mario cool in hot environments: Caveman Outfit, Boxer Shorts, Swimwear, the Space Suit and the Resort Outfit.
After reading Pyrite's answer, I decided to do a bit more experimentation with the outfits. I visited each kingdom in the game to see how the weather affected Mario's well-being.
Mario has 3 different states depending on the weather and his outfit.
- Cold: Mario will shiver immediately when idling.
- Normal: Standard idle animations.
- Warm: Mario wipes sweat off his forehead and fans himself after idling for a few seconds.
Outfits that keep Mario warm
- Aviator Outfit: (Lost Kingdom - 10 purple coins)
- Snow Suit: (Snow Kingdom - 10 purple coins)
- Space Suit: (Moon Kingdom - 10 purple coins)
Outfits that keep Mario cool
- Caveman Outfit (Cascade Kingdom - 10 purple coins)
- Boxer Shorts (Any Kingdom - 1,000 gold coins)
- Poncho (Sand Kingdom - 10 purple coins)
- Swimwear (Lake Kingdom - 10 purple coins)
- Resort Outfit (Seaside Kingdom - 10 purple coins)
- Football Uniform (Moon Kingdom - 150 gold coins)
Cold Kingdoms (Mario shivers)
- Sand Kingdom (night/underground) - Tostarena
- Snow Kingdom - Shiveria
Warm Kingdoms (Mario fans himself)
- Sand Kingdom (day)* - Tostarena
- Lost Kingdom - Forgotten Isle
- Seaside Kingdom - Bubblaine
- Luncheon Kingdom - Mount Volbano
Notes
- Mario will fan himself in the Sand Kingdom regardless of what outfit he is wearing.
- Mario will fan himself when wearing a warm outfit in a non-cold kingdom.
- Hats do not seem to have any effect on Mario's reaction to the weather.
Thanks to IGN for the list of outfit locations and prices.