“As the days get longer, the cold gets stronger” is an old weather proverb. The days get longer and the cold gets stronger after the winter solstice (December in the Northern Hemisphere).
 
“The New Year now begun, as the Days began to lengthen, so the cold began to strengthen” was cited in print in 1631. “As the days begin to lengthen, the cold begins to strengthen” was cited in a Vermont newspaper in 1818. “Poor Richard used to say: ‘As the days grow longer the cold grows stronger’” was cited in a Kansas newspaper in 1870. “As the days get longer, the cold gets stronger” was cited in a Missouri newspaper in 1899.
 
 
Wikipedia: Winter solstice
Winter solstice is an astronomical phenomenon which marks the shortest day and the longest night of the year. Winter solstice occurs for the Northern Hemisphere in December and for the Southern Hemisphere in June.
 
Google Books
Elements of Elocution:
Being the substance of a course of lectures on the art of reading, delivered at several colleges in the University of Oxford, Volume 1
By John Walker
London: Printed for the author, and sold by T. Cadell, T. Becket, G. Robinson, J. Dodsley
1781
Pg. 32:
... in the proverbial sentence : As the day lengthens the cold strengthens : we may place a comma, and even a semicolon at lengthens, without appearing to injure the sense ; ...
 
29 June 1818, Vermont Intelligencer (Bellow Falls, VT), pg. 3, col. 2:
This, no doubt, gave origin to the common remark, that “as the days begin to lengthen, the cold begins to strengthen.”
 
3 January 1870, Leavenworth (KS) Bulletin, “the Open Winter,” pg. 1, col. 2:
Poor Richard used to say: “As the days grow longer the cold grows stronger.”
 
Chronicling America
14 January 1876, Wheeling (WV) Daily Intelligencer, pg. 4, col. 2:
As the days grow longer the cold grows stronger, saith the proverb.
 
29 December 1899, Kansas City (MO) Star, pg. 7, col. 2 ad:
“As the days get longer, the cold gets stronger.”
(Jones Dry Goods Co.—ed.)
 
Google Books
The Country Gentlemen’s Estate Book
1909
Pg. 160:
... conditions are more variable, and as ‘the days get longer,’ as the old proverb says, ‘the cold gets stronger.’
 
Chronicling America
25 December 1912, Omaha (NE) Daily Bee, pg. 4, col. 3 ad:
Wool Dresses here also—Serges, Henriettas and Mixtures—warm and sensible garments—worth remembering here the old proverb, “As the days get longer, the cold gets stronger”—ages 6 to 14 years.
(Thomas Kilpatrick & Co.—ed.)
 
Google Books
The Very Thought of Thee;
Adventures of an Arctic Missionary
By Bernice Bangs Morgan
Grand Rapids, MI: Zondervan Pub. House
1952
Pg. 123:
When the days become longer in the early spring, the Alaskans have another saying, “When the days get longer, the cold gets stronger.” The sun rises higher every day and gives more light. For some reason it does seem colder.
 
Google Books
Popular Beliefs and Superstitions from Utah
By Anthon S, Cannon, Wayland D. Hand and Jeannine Talley
Salt Lake City, UT: University of Utah Press
1984
Pg. 343:
COLD WEATHER
10894 When the days get longer, the cold gets stronger (M, 62, Swiss, Heber City, 1912)
 
Google Books
A Dictionary of American Proverbs
Edited by Wolfgang Mieder, Stewart A. Kingsbury and Kelsie B. Harder
New York, NY: Oxford University Press
1991
Pg. 119:
When the day is lengthened, cold strengthens. Vars. (a) As the day begins to lengthen, cold begins to strengthen. (b) As the days grow longer, the cold gets stronger. (c) When thedays begin to lengthen, the cold begins to strengthen. Rec. dist.: U.S., Can. 1st cit.: 1631 Pellham, God’s Power; US1850Judd, Richard Edney and Governer’s Family.
 
Google Books
The Wordsworth dictionary of proverbs
By George Latimer Apperson
Ware, Herts: Wordsworth Reference
2006
Pg. 131:
As the day lengthens, so the cold strengthens. 1631: E. Pellham, God’s Power, 27, The New year now begun, as the Days began to lengthen, so the cold began to strengthen.
(...)
As the days grow longer, The storms grow stronger. 1827: Hone, Table-Book, 667.
   
Washington (DC) Post—Capital Weather Gang
Posted at 10:03 AM ET, 12/21/2011
Winter solstice – and other interesting sun facts – explained
By Justin Grieser
If you’ve grown weary of short days and are looking forward to more sunlight, you won’t have to wait much longer. Tomorrow is the winter solstice in the Northern Hemisphere, bringing us the shortest daylight period and longest night of the year. Going forward, the days will gradually lengthen as the sun climbs higher in the sky over the next six months.
(...)
As the days lengthen, will the cold strengthen?
Despite the gradually increasing daylight, average temperatures in the D.C. area will continue to drop, with a minimum average high/low temperature of 43/28F occurring January 12-22. Given the above average conditions lately, the old adage, “as the days lengthen, the cold strengthens” may have questionable merit this year.
 
Twitter
Libby Rule
‏@Libby_Rule
@DanielleJStorey Every Winter Solstice my Dad always warns ‘As the days get longer, the cold gets stronger’. So True!
5:57 PM - 23 Jun 2014