"Winter rules" is not codified in any of the 34 rules that make up the Rules of Golf. The practice is only mentioned in an appendix to the rules, where there is also a sample local rule for use by courses, clubs or committees.
Generally, winter rules are put into place during, of course, the winter, when adverse weather conditions can impact the golf course itself. Under the most common winter rules, a player whose ball is in the fairway would be allowed to improve the lie if that ball was, for example, on a bare patch of ground.
Winter rules can differ from course to course; only a course, club or committee can declare winter rules in effect (a player may not declare, in absence of a ruling from the club or course, that he is playing under winter rules). If you carry a handicap, rounds played under winter rules must be reported for handicap purposes.
To learn more about winter rules, read our article in the Golf Rules FAQ: "What are winter rules (preferred lies)?"
