Firefighters were called to three deliberately-started fires in three hours in the same area of Baildon.
The third blaze was substantially bigger than the previous two, covering an area of about 100 square metres, said a spokesman for Shipley fire station.
Crews from the station were called to Baildon Bank at 8pm, 10pm and 11pm last night.
The spokesman said crews believed youths were responsible for starting the fires, and added: “The first two were not that big, but the third one was about 100 square metres. It spread because it was quite dry.”