Is Arrowhead Stadium an indoor stadium?

No, it is an outdoor stadium. While the original plans called for a rolling roof that could cover Arrowhead and neighboring Kauffman Stadium, it was never built due to cost overruns. Even with the roof, Arrowhead would have still been an open air stadium because the roof was never intended to seal out the weather entirely. It was only a cover designed to keep the field and fans dry if a game was played during bad weather.