Old Faithful is the most popular attraction in Yellowstone National Park, erupting on a predictable schedule during the day, making it simple to come and enjoy the spectacular display.

Geysers are dynamic and constantly evolving--and Old Faithful is no exception. They evolve in response to small, natural changes in their plumbing systems, water temperature, dissolved gas and mineral content of the thermal water, amount of water, amount of heat, changes in pressure, and other factors. Geysers are also affected by natural events in Yellowstone such as frequent earthquakes (over 1,000 a year).

Predicting any geyser's eruption can be difficult because of the complex interactions of these constantly changing factors. To predict a geyser's next eruption, observers analyze past information such as intervals between eruptions, length of eruption, and the character of an eruption.

Old Faithful is perhaps the most studied and predicted geyser. Mathematicians, statisticians, and dedicated observers have analyzed it for many years. For example, a direct relationship exists between the duration of Old Faithful's eruption and the length of the following interval. Short eruptions (around 2 minutes) lead to short intervals (about 65 minutes); long eruptions (4 minutes or so) lead to long intervals (about 94 minutes). During a short eruption, less water and heat are discharged; thus, they rebuild again in a short time. Longer eruptions mean more water and heat are discharged and they require more time to rebuild. As of August 2002, the average interval was 94 minutes.

Over time, the average interval between Old Faithful's eruptions increases, in part due to ongoing processes within its plumbing. Changes also result from earthquakes. Prior to the 1959 Hebgen Lake Earthquake, centered 12 miles northwest of the park's west entrance, the interval between Old Faithful's eruptions averaged slightly more than one hour. Its intervals increased after that earthquake and again after the 1983 Borah Peak Earthquake, centered in Idaho. In 1998, an earthquake near Old Faithful lengthened the interval again; later, another swarm of earthquakes further increased intervals.

Between long intervals and other variable, waiting for Old Faithful's eruptions can stretch beyond the predicted time. Think of it this way: you've got time now to meet other visitors, enjoy many other Upper Geyser Basin's geysers and thermal features, or read about the park, or take a much needed rest. So relax, be flexible, and enjoy the time you spend with the world's most famous geyser.

Myths:
• It erupts every hour on the hour
• It is so predictable, you can set your watch by its eruption
• It is the only geyser that can be predicted
• It is the most predictable geyser
• Its eruptions is not as high as it used to be
• Its eruption lasts less time that it used to
• Park rangers can control the eruption of Old Faithful
• No one can predict Old Faithful anymore.

Facts:
• Its eruption length and height, and time between eruptions varies from day to day and year to year
• As of March 2003, the eruption length ranges from 1.5 to 5 minutes; the average interval between eruptions is 94 minutes
• Old Faithful's height ranges from 106 feet to more than 180 feet, averaging 130 feet
• Its average eruption length, height and interval will change again--often as a result of an earthquake
• 3,700 to 8,400 gallons of water are expelled per eruption, depending on the length of eruption
• Just prior to eruption, water temperature at the vent is 204° F / 95.6° C
• It's on the more than 300 geysers in Yellowstone
• Old Faithful is a cone geyser, which erupts in a narrow jet of water, usually from a cone. Fountain geysers, such as Grand (also in the Upper Geyser Basin), generally shoot water in various directions, most often from a pool.

On a more curious note, one of the first explorers to see Old Faithful described the encounter as "magical" and was sure he'd just entered hell, in fact, he wrote that much in his diary noting how it wasn't "as bad as described in the Good Book".