Generally, Major League Baseball teams will continue play in light to moderate rain but will suspend play if it is raining heavily or if there is standing water on the field. Games can also be delayed or canceled for other forms of inclement weather, or if the field is found to be unfit for play, and for other unusual causes such as bees. Bee delays and cancellations have occurred in games such as the spring training game in 2005 that was canceled, as well as the 2009 Astros-Padres game that was delayed in the ninth inning. However, rain is by far the most common cause for cancellations or stoppages of play.
Before a baseball game commences, unless it is the second game of a doubleheader, the manager of the home team is in charge of deciding whether or not the game should be delayed or canceled due to rain or other inclement weather (see Rule 3.10 of baseball's Official Rules). Once the home team manager hands his lineup card to the umpire shortly before the game is to begin, the umpire-in-chief has sole discretion to decide if a game should be delayed or canceled (see Rule 3.10 and Rule 4.01 of the Official Rules). This also applies to the second game of a doubleheader. Umpires are required by rule to wait at least 30 minutes to see if conditions improve; this is referred to as a rain delay and is not counted as part of the length of the game listed in the box score. In practice, umpires are encouraged to see that games are played if at all possible, and will sometimes wait as long as three hours before declaring a rainout.
If a game is rained out before play begins, it is rescheduled for a later date. If it has already begun and rain falls, several scenarios are used to determine the need to resume play:
If the game has completed the top half of the 5th inning and the home team is ahead, the game can be deemed an official game. The home team is declared the winner, and the game officially counts in standings.
If the game has completed the bottom half of the 5th inning and either team is ahead, the game can be deemed an official game. The leading team is declared the winner, and the game officially counts in standings. In some Minor League Baseball and college games, this scenario is possible only in the final day of a series.
If the game has completed the 5th inning, and the teams are tied, if the game is a playoff game (regardless of inning), or in college and some Minor League Baseball games, it is not the final game in the series (the first or second game in a three-game series, also regardless of inning), the game is considered suspended, and the resumption of the game is scheduled for a future date (usually the following day). The game picks up from where it left off.
If none of the previous three scenarios apply, the game can not be deemed official. The umpire declares "No Game," and a make-up of the game is scheduled for a future date unless it is not feasible (most likely end of season, half in some minor leagues, or in college). The statistics compiled during the rained out game are not counted.