Above: Ranger Darlene Bos explains how we predict eruptions for this popular geyser. Produced by NPS/Neal Herbert, with additional filming by David Restivo and Jay Elhard.
The world's most famous geyser, Old Faithful in Yellowstone, currently erupts around 20 times a day. These eruptions are predicted with a 90 percent confidence rate, within a 10 minute variation, based on the duration and height of the previous eruption.
Old Faithful was once called “Eternity’s Timepiece” because of the regularity of its eruptions. However, this geyser has never erupted at exact hourly intervals as many believe. People also speak of the average time between eruptions. This is misleading. The mathematical average between eruptions of Old Faithful is currently 74 minutes, but it doesn’t like to act average! Intervals can range from 60-110 minutes.
During Old Faithful Visitor Center hours, geyser statistics and predictions are maintained by the park staff. This is done by good old-fashioned observation, timing with a stopwatch, and writing in a log book. Visitors can check for posted prediction times in most buildings in the Old Faithful area and on the webcam web page.