pocketmine\level\weather\WeatherManager::registerLevel PHP Method

    public static function registerLevel(Level $level)
    {
        self::$registeredLevel[$level->getName()] = $level;
        return true;
    }

 public function __construct(Level $level, $weatherTime = 12000, $weatherNow = 0)
 {
     $this->level = $level;
     $this->weatherTime = $weatherTime;
     $this->weatherNow = $weatherNow;
     $this->weatherLast = 0;
     WeatherManager::registerLevel($level);
 }