Lightning from a thunderstorm that moved across the northern portion of the Yellowstone National Park Sunday evening started a fire on Sepulcher Mountain, near the park's northern boundary and about three miles west-northwest of Mammoth Hot Springs.

The Rainbow fire is less than a half acre in size. As a precaution, rangers have temporarily closed the Sepulcher Mountain Trail. The closure does not affect any popular fishing areas or backcountry campsites.

The fire is well away from Mammoth Hot Springs, Gardiner and all park roads and visitor services. All park entrances and seasonal visitor services are open.

Most fires occurring in the Greater Yellowstone Area are caused by lightning. This is the 17th fire reported in Yellowstone National Park this summer. Only two were caused by people.

The only fire in the park to grow more than a half-acre in size this summer is the Butte fire in the Lamar Valley. It has exhibited little activity in the past few days and remains estimated at 200 acres in size.