Q. When does Old Faithful erupt?

A. Basic prediction of Old Faithful is dependent upon the duration of the previous eruption. During visitor center hours, geyser statistics and predictions are maintained by the naturalist staff. People 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 45-110 minutes. Visitors can check for posted prediction times in most buildings in the Old Faithful area.

Q. How high does Old Faithful erupt and how long will it last?

A. Old Faithful can vary in height from 100-180 feet with an average near 130-140 feet. This has been the historical range of its recorded height. Eruptions normally last between 1.5 to 5 minutes.

Q. I heard Old Faithful isn't as faithful as it used to be. Is it slowing down?

A. It depends on what you call faithful. The famous geyser currently erupts around 20 times a day and can be predicted with a 90 percent confidence rate within a 10 minute variation. Prior to the 1959 earthquake, Old Faithful erupted 21 times per day. That's a significant decrease in activity for geologists tracking each eruption, but to visitors seeing one or two eruptions . . . it looks just fine.

Q. How many gallons of water are expelled during an eruption?

A. It depends on the duration of the eruption. Scientists estimate that the amount ranges from 3,700 gallons (for a short duration of 1.5 minutes) to 8,400 gallons (for a longer duration of 4.5 minutes).

Q. How hot is the water in Old Faithful?

A. During an eruption, the water temperature at the vent has been measured at 204°F (95.6°C). The steam temperature has been measured above 350°F!