Air + Water Temperature "Rules"
A Dangerously Misleading Message
These formulas advise you to add air temperature + water temperature to determine whether you should wear thermal protection. In other words, they say that if the air temperature is high enough, you can safely paddle on cold water without the protection of a wetsuit or drysuit. There is absolutely no scientific evidence to support these formulas and they're dangerously misleading.
A common example says that if the combined air and water temperatures is above 120 degrees Fahrenheit, you don't need to wear any thermal protection.
Do The Math
Using this formula, a person could mistakenly conclude that if the air temperature was 75F (24C) and the water temperature was 53F (11.6C), no protection would be needed because the total is above 120. That’s dangerously misleading because without thermal protection, immersion in 53F (11.6C) water is immediately life-threatening.
The same is true if you add 80F (27C) air + 45F (7C) water.
An Example From A Boat Ramp In Ohio
In addition to promoting the 120 "Rule", this misleading infographic says nothing about cold shock or incapacitation, and says you should "consider" a wetsuit or drysuit. Extra layers of clothing in a drybag are no substitute for thermal protection and will do nothing to protect you when you're struggling in the water.
The Cold Facts
Air temperature has nothing to do with cold water safety.
The only thing that matters when you’re in the water is the water temperature. The air temperature can be 80 or 90F (27C or 32C), but if the water is cold, you'd better be dressed for immersion in either a wetsuit or drysuit.
Bonus Article: Hot weather is no excuse for skipping thermal protection: