Old Fairfull

Predicting Old Faithful:

It is not possible to predict more than one eruption in advance. Old Faithful is currently bimodal. It has two eruption durations, either a long (over 4 minutes) or more rarely a short (about 2-½ minutes). Short eruptions lead to an interval of just over an hour and long eruptions lead to an interval of about 1-½ hours. In the past the interval was predicted using more precise measurements of the duration of the current eruption. The duration was timed from the first heavy surge which lifts water skyward at the start of the eruption until the last small splash above the cone at the very end. The longer the eruption lasted, the longer the interval until the next eruption. The following regression table was used.

Duration Interval 1.5min 51 min 2.0min 58 min 2.5min 65 min 3.0min 71 min 3.5min 76 min 4.0min 82 min 4.5min 89 min 5.0min 95 min The above table is shown for historic purposes. Predictions are currently based only on short or long durations. This simpler prediction model still results in a 90 percent accuracy of the predictions.

Common Misconceptions:

Old Faithful plays on the hour every hour This legend dates back to the early history of Yellowstone Park. Old Faithful has never played every hour on the hour. It is just as likely to play on the hour as it to play 17 minutes after the hour or 23 minutes after the hour or …… Old Faithful plays hourly Old Faithful's intervals vary from 35 minutes to 2 hours. You might see one interval of 60 minutes but it is very unlikely that you will see two in a row. This misconception seem to date back to the 1870 Washburn expedition where one of the members reported that Old Faithful played “nearly hourly”. No geyser, including Old Faithful, plays at set times and intervals. There is always some variation. Old Faithful is slowing down and becoming less regular Part of this statement is true; part is not. Because of changes in circulation that resulted from the 1959 Hebgen Lake and 1983 Borah Peak earthquakes, as well as other local and smaller earthquakes, the average interval between eruptions has been lengthening during the last several decades. Old Faithful has slowed from an average of about 65 minutes from 1870 through 1947 to an average of 90-92 minutes from 2001 through 2010. The range of intervals has also increased slightly, from about 60 minutes through the 1960s to about 80 minutes in more recent decades. However, regularity, as measured by the difference between actual intervals and average intervals or standard deviation, decreased during the most recent decade. Old Faithful is getting shorter Measurements show that Old Faithful is still as tall as it has ever been, 90-180 feet. There are a number of reasons people may feel that Old Faithful is getting shorter. They may have seen an exceptionally tall eruption last time and this time saw a more ordinary or even a short eruption. The first time they saw an eruption they may have been more excited and their excitement caused them to over estimate its height. Or they may have first seen it a number of years ago when it was possible to get closer to the geyser. To accommodate the summer crowds, the boardwalk has since been moved further back from Old Faithful. Many people don't realize just how far from the geyser the boardwalk is. With nothing to judge the distance by, most people severely underestimate the height of Old Faithful. Its not until they get back farther from the geyser and see the buildings around the geyser that they realize just how big the geyser is. Old Faithful erupted late or early It is only possible to make predictions of the time Old Faithful may erupt. The geyser will erupt when it is ready. Its the prediction that is early or late not the geyser.

What to look for: A crowd gathered around any geyser is often an indicator of an imminent eruption. After an eruption of Old Faithful everyone that was seated around the benches leaves. Especially in summer as the predicted time for the next eruption approaches, the benches begin to fill and eventually overflow. Thus, by looking at the crowd, you can get a rough idea as to when the next eruption is predicted.

If crowds are sparse or non-existent you'll have to look at the geyser itself. Old Faithful goes through a period of preplay prior to an eruption. This preplay can last anywhere up to twenty minutes. The preplay consist of splashing and small jetting which often occurs every few minutes reaching from 1 to (rarely)20 feet in height. It often lasts just long enough for some excited visitors to start taking pictures. The eruption usually starts from what initially looks like just another preplay jet. Short interval eruptions occasionally start with the first splash seen above the cone but most require a period of preplay.

Once the eruption starts, the jetting will grow, stop, grow again and in two or three of these steps, reach maximum height. It takes 10-20 seconds to reach full height. Unfortunately, the full height is maintained only for a short time. Within less than a minute, the eruption starts waning. The interval to the next eruption is based on the total duration of the current eruption. This includes the unimpressive play that ends the eruption. The main visible difference between a long and short eruption is the duration of the end play. In a short duration eruption the end play stops quickly. In a long duration eruption you may see up to 4 minutes of unimpressive, 5 feet tall, end play.