void setup() { 
  Serial.begin(115200);
  char weather[] = "partly cloudy";
  const char main[] = "broken clouds";
  weather[0] = toupper(weather[0]); // does not work with const char in previous line
  String out = String(main);
  out[0] = toupper(out[0]);
  Serial.println(weather);
  Serial.println(out);
}

void loop() {
}