人工芝
An important type of ballpark is the indoor park. These parks were covered with a fixed roof, usually a hard concrete dome. Reasons to build indoor parks were varied. The Astrodome, the first indoor sports stadium ever built, was built to escape the hot and very humid climate of Houston and the Kingdome was built to escape Seattle's constant rains. In Japan, domed stadiums were built to escape frequently rainy climates, as well as extreme snowfall in Sapporo. There is little to no natural light in these parks, necessitating the use of one of the most distinguishing aspects of an indoor park: artificial turf. While roofing technology now allows grass to be grown in indoor venues (see Forsyth Barr Stadium, a rugby venue in New Zealand with an ETFE roof), the first generation of indoor parks predated such roofs. Since there was not enough light to grow grass, artificial turf was installed, and this affected the game. Artificial turf was harder, and thus a ball hit on the ground moved faster and bounced high. This, coupled with the usually dull white or gray roofs that could camouflage a fly ball, caused what Twins fans called a "dome-field advantage".