Rain stopping play in cricket is primarily a safety issue. In principle umpires stop play when it is dangerous and resume play when it is safe. In practice, it can feel confusing as a fan knowing when a game will stop or restart.
The Laws of Cricket don’t actually mention rain. Instead, they simply say:
2.8.2 The Umpires shall immediately suspend play, or not allow play to start or to recommence, if either umpire considers that the conditions of ground, weather or light, or any other circumstances are either dangerous or unreasonable.
2.8.3 When there is a suspension of play it is the responsibility of the umpires to monitor conditions. They shall make inspections as often as appropriate, unaccompanied by any players or officials. Immediately the umpires together agree that the conditions are no longer dangerous or unreasonable they shall call upon the players to resume play.
Rain is certainly the most common reason for such a suspension of play, but it’s important to remember a game does not stop automatically because it is raining. Instead, it must be dangerous or unreasonable. Dangerous usually means there’s a genuine risk of injury, for example for players running between the wickets. (Lightning usually stops a game immediately!)
Unreasonable usually means the conditions significantly affect somebody’s ability to play, for example if extreme wind made it impossible to bowl, or if the ground was too wet for a bowler to get a steady run-up. This can be a grey area as some effects of variations in weather and conditions, such as the ball swinging more in humid conditions or the wicket breaking up quicker on a hot dry day are considered part of the game and affect strategy.
Once the umpire’s suspend play, there’s usually a degree of interpretation of the wording that they should resume when “the conditions are no longer dangerous or unreasonable”. Usually, umpires will wait until conditions improve (such as when it stops raining), schedule an inspection and then, if the pitch looks acceptable, schedule a restart. Both a planned inspection and a planned restart will usually be delayed if it starts raining again in the meantime.
Spectators sometimes get confused about why the umpires carry out a pitch inspection (and it’s stopped raining) but don’t start play again immediately. It’s mainly because the inspection shows the pitch isn’t completely dry yet, but the conditions (eg sunshine) mean it should be dried out by the scheduled restart. However, it’s sometimes because players may have taken the opportunity to start eating or doing something else that means they need a short notice before they can restart!
The same suspend/restart rules apply in principle to bad light, though in practice the way umpires interpret it can depend on the umpire, the location and the competition. For example, umpires will sometimes tell the fielding captain that the game can continue if they agree to only use medium pace or spin bowlers. If the captain exercises their right to ignore this request, the umpires will suspend play immediately.
Some umpires, particularly in Test matches will use a light meter. The first time they suspend play for poor light, they will measure the light levels. This level will then become a benchmark for the rest of the game, meaning the umpires no longer make subjective calls.
The way poor light is handled may vary depending on the location. For example, in games in countries that are far from the Equator, such as England or Australia, daytime games are always finished before dusk. In countries nearer the Equator such as India and Pakistan, it can start getting dark quickly in the early evening, meaning poor light can be a bigger factor at the end of the playing hours. How the players respond to these conditions and whether they consider it acceptable to play as dusk approaches may depend on whether they would benefit from play ending, for example to get a draw or to take the last few wickets for a win!