The following is a reproduction of the "You Too Can Predict Old Faithful" brochure written by the Old Faithful Staff, Yellowstone National Park, revised 5/95:

YOU TOO CAN PREDICT OLD FAITHFUL

In 1938, a ranger discovered a correlation between Old Faithful Geyser's duration and its subsequent interval. While the specific numbers have changed over time, we still use this principle to predict Old Faithful's eruptions today. You can too! Here's how:

1. Take note of the time that the water comes out and stays out of the cone (the beginning of the continuous water phase).

2. Time the length of the eruption until absolutely no more water is splashing out of the cone.

3. If the eruption lasts: To the starting time add:

1.5 minutes 51 minutes

2.0 minutes 58 minutes

2.5 minutes 65 minutes

3.0 minutes 71 minutes

3.5 minutes 76 minutes

4.0 minutes 82 minutes

4.5 minutes 89 minutes

5.0 minutes 95 minutes

As you can see, the time between eruptions is based on the length of the eruption.

Example: Old Faithful erupts at 10:07. The eruption lasts 4.0 minutes. So, to 10:07 you add 82 minutes (1 hour and 22 minutes), and the prediction will be for about 11:29, plus or minus 10 minutes.

The following is a reproduction of the "Facts and Theories about Old Faithful Geyser" brochure written by the Old Faithful staff, Yellowstone National Park, revised 3/91:

Old Faithful was undoubtedly known to Indians, but was not named or fully described until the Washington-Langford-Doane expedition in 1870.

It plays as high now as it did when it was first described. The height to which Old Faithful has been measured on a number of occasions with a transit. Eruption heights ranged from 32 to 56 meters (106 to 184 feet). Few eruptions of this geyser are less than 35 meters or more than 50 meters (155 and 164 feet, respectively). The average height of an eruption is about 40 meters, or 130 feet. The stronger the wind, the lower the height of the eruption because the wind blows the top off the ascending water column.

During 1988, the 6,900 timed intervals averaged 76.17 minutes. The shortest observed interval was 41 minutes, and the longest interval was 114 minutes. Of more than 137,000 observed and recorded eruptions in the past 120 years, Old Faithful's yearly average interval has always been between 60 and 79 minutes. In this same time span, the minimum interval recorded was 30 minutes, and the maximum 120 minutes. After each of the last three major regional earthquakes (Hebgen Lake, 1959, magnitude 7.5; Yellowstone Plateau, 1975, magnitude 6.1; Borah Peak, 1983, magnitude 7.3) Old Faithful has had progressively longer average intervals due to shifts in circulation of hot water away from the geyser.

The duration of Old Faithful's eruptions varies from 1.5 to 5.5 minutes. This is proportional to the amount of water and energy expended. Before the Borah Peak earthquake, a very simple equation had been derived from thousands of measured eruptions which could be used to predict the next play of Old Faithful, with about two-thirds of the predictions being correct within plus-or-minus five minutes. The equation was I=10D+30, where the next interval (I) is equal to 10 times the duration (D, in minutes) of continuous visible water, plus 30 minutes. Now, because of less regularity, Old Faithful's predictions are made with a leeway of ± 10 minutes with the following rule of thumb: if the eruption is a short one, lasting less than 2.5 minutes, predict 53 minutes to the next one; if it is a long one, greater than 3.75 minutes, predict 80 minutes. For a rare 3 minute eruption, predict 64 minutes.

The amount of water forced out of the geyser during an eruption was carefully measured as part of a study in 1974, by Dr. Eugene Robertson of the U.S. Geological Survey. It has been determined that from about 14,000 to 32,000 liters (3,700 to 8,400 gallons) of water is ejected during each eruption, most of which flows into the Firehole River.

The tremendous amounts of water which erupt from geysers such as Old Faithful, Giantess, Grand, Echinus, and other major geysers show that large volumes of water move rapidly through the geyser's plumbing during an eruption from one or more porous, permeable beds of rock, called aquifers.

It is believed that almost all of the water in geysers and hot springs has its origin as surface water from rain or snow. The water is returned to the surface by forces involving hydrostatic pressure of cold, dense water replacing water that is heated at depth, and by the less dense hot water boiling to steam, pushing out the water about it as it rises.

Research suggests that magma (partially melted rock) is the source of the heat for geysers and hot springs. The heat is transmitted through solid rock to water which has seeped to depths of perhaps 3 kilometers (10,000 feet) below the surface. The very hot water then rises the plumbing system of a spring or geyser, heating rocks and water that it meets along the way.

During an eruption of Old Faithful, water temperature at the vent is approximately 95.6°C (204°F). The boiling point of Old Faithful, at an elevation of 2,245 meters (7,366 feet), is 93°C (199°F).

Geyser tubes or plumbing systems are not uniform in shape. They are usually crooked or constricted in many places. Attempts to plumb the depth of Old Faithful indicate there is a constriction at about 22 meters (73 feet). It is not known how much deeper the joints or fissures which make up the geyser plumbing system go.

The existence of a geyser depends on:

a. An adequate source of heat.

b. A supply of water.

c. A plumbing system - a series of fissures and fractures that reaches deep into the earth but restricts the free convection of heat and water below a critical rate.

d. A rock formation that is sufficiently strong to maintain the plumbing system against the pressure of steam explosions.

No two geysers erupt alike because of a wide variety of combinations of the rates of heat flow, the rates of water movement, and intricacies in the plumbing systems.

If more detailed information is desired, a booklet entitled The Story of Old Faithful Geyser by George D. Marler is available and contains numerous colorful illustrations throughout its 48 pages. Orders for this publication can be made through the Yellowstone Library and Museum Association, P.O. Box 117, Yellowstone National Park, Wyoming 82190.

"GEYSERS ARE RARE AND BEAUTIFUL. LET US TREASURE AND PRESERVE THESE WONDERFUL DEMONSTRATIONS OF NATURE'S ENERGY!'