From Minecraft Wiki
Sets the weather.
- Java Edition
weather <clear|rain|thunder> [duration]
- Pocket Edition
weather <type: string> [duration: int]
- clear|rain|thunder (PE: type: string)
clear– Set the weather to clear.
rain– Set the weather to rain (or snow in cold biomes).
thunder– Set the weather to a thunderstorm (or a thunder snowstorm in cold biomes).
- duration (PE: duration: int) (optional)
- Specifies the time in seconds for the specified weather to last. Must be between 1 and 1,000,000 (inclusive, without the commas). If duration is omitted, the duration will be set between 6,000 and 18,000 ticks.
- Fails if the arguments are not specified correctly.
- On success, changes the weather for the specified duration (in seconds).
- To get clear weather for one Minecraft day:
weather clear 1200
- To make it rain, but let it end at its own pace: