“It’s not the heat, it’s the humidity” is an old line about the weather. “It ain’t the heat, it’s the humility” is a popular variation. “‘It’s not the heat of his wooing,’ complained Winnie Winkle, ‘it’s the humility’” was cited in 1923. “It ain’t the heat. It’s the humility” was cited in 1929.
 
Yogi Berra (1925-2015), a baseball player and manager of the New York Yankees and a manager of the New York Mets, is frequently credited with the line that existed before he was born. In the book What Time Is It? You Mean Now?: Advice for Life from the Zennest Master of Them All (2002), Berra said that he never said it.
 
 
26 July 1923, Seattle (WA) Daily Times, “Hammer and Tongs” by Anvil Smith, pg. 1, col. 1:
“It’s not the heat of his wooing,” complained Winnie Winkle, “it’s the humility.”
 
26 April 1929, Tampa (FL) Morning Tribune, “The Gulf Stream,” pg. 6, col. 3:
It Isn’t the Heat,
It’s the Humility
 
3 September 1929, Evening World-herald (Omaha, NE), pg. 16, col. 2:
IT ISN’T THE HEAT.
(San Diego Union.)
The average man is afraid to wear the kind of hot weather garments he’d like to. It ain’t the heat. It’s the humility.
 
1 August 1934, Lexington (KY) Leader, pg. 4, col. 4 cartoon caption:
It’s Not The Heat It’s The Humility
 
20 July 1936, Hammond (IN) Times, “The Passing Show,” pg. 4, col. 3:
It isn’t the HEAT it’s the humility.
     
Google Books
Ronald Reagan, Book 1
United States. President (1981-1989 : Reagan)
Office of the Federal Register, National Archives and Records Service, General Services Administration
1986
Pg. 794:
Remarks at the Presentation Ceremony for the “C” Flag Awards
June 18, 1986
(...)
As one newcomer to Washington summers once remarked: “It’s not the heat, it’s the humility.” He obviously hadn’t been here long, because humility is not in great supply in this town. [Laughter]
   
Google Books
Love to Love You Baby
By Kasey Michaels
New York, NY: Zebra Books
2001
Pg. 243:
It ain ‘t the heat, it’s the humility. — Yogi Berra
 
Google Books
What Time Is It? You Mean Now?:
Advice for Life from the Zennest Master of Them All
By Yogi Berra with Dave Kaplan
New York, NY: Simon & Schuster
2002
Pg. 46:
It’s been said that I have pretty good humility, though I never did say, “It’s not the heat, it’s the humility,” which is another one of those sayings people say I said. Truth is, someone once told me that “humble” is a word that means “being close to the ground.” So being short and squatting all those years behind the plate, maybe that’s how I got a little humble.
 
16 December 2006, Rockford (IL) Register Star, “Swing and a hit; Ron Gayle deal a home run” (editorial), pg. 5A, col. 1:
Yogi Berra must have known Mike Broski: “It ain’t the heat, it’s the humility.”
 
CBS News
CBS/AP September 23, 2015, 6:47 AM
“Overwhelming underdogs”: Yogi Berra quotes through the years
NEW YORK—Some of the more widely quoted philosophy of Yogi Berra, the New York Yankees Hall of Fame catcher who died Tuesday at age 90:
(...)
On the weather: “It ain’t the heat; it’s the humility.”