Are players allowed to interrupt the game because it starts to rain?

This can be read very clearly in the Rules of Golf:

5.7a When players may or must suspend play
A player may not interrupt play during a round except in the following cases:

-Interruption by the match director. All players must interrupt the game if the match director suspends the game (see Law 5.7b).
-Interruption of play by agreement in hole play. Players in hole play may interrupt play by mutual agreement for any reason, except to delay the tournament. If they agree to interrupt the game and one of the players wishes to continue the game afterwards, the agreement is terminated and the other player must also continue the game.
-An individual player interrupts the game due to the risk of lightning. A player may interrupt the game if he considers there to be a justified risk of lightning, but he must inform the match management as soon as possible.
Leaving the court is not a stoppage of play as such. A delay in play by the player falls under Rule 5.6a, not under this rule.
If a player interrupts the game for a reason that is not permitted under this rule or fails to inform the match management as required, the player is disqualified.

He may not interrupt the game for rain only.

In the meantime, if you would like to know more about the Rules of Golf, please contact me directly at hans.kienesberger@golf.at.