The short answer
Many of Yellowstone's hot springs sit near the boiling point, and some measure above it. Because the park is high, water boils at a lower temperature than at sea level. The National Park Service (NPS) gives the surface boiling point of water in Yellowstone as 199 degrees F (93 degrees C), compared with 212 degrees F at sea level. At the Old Faithful area, NPS gives 198 degrees F (92 degrees C) for that elevation and notes that surface water at Ear Spring is often above it.
Measured temperatures of named springs
NPS publishes average temperatures for individual features from its monitoring program. These are averages, and readings at a given visit can be higher or lower.
| Feature | Area | Average temperature (NPS) | Average pH (NPS) |
|---|---|---|---|
| Ear Spring | Old Faithful area | 197.1 F (91.7 C) | 8.4 |
| Avoca Spring | Old Faithful area | 193.7 F (89.8 C) | 8.3 |
| Celestine Pool | Lower Geyser Basin | 183.4 F (84.1 C) | 8.3 |
| Orange Spring | Old Faithful area | 181.2 F (82.9 C) | 8.7 |
| Silex Spring | Lower Geyser Basin | 174.7 F (79.3 C) | 8.4 |
| Oval Spring | Upper Geyser Basin | 162.9 F (72.7 C) | 8.2 |
| Morning Glory Pool | Upper Geyser Basin | 159.3 F (70.7 C) | 7.6 |
| Chromatic Pool | Upper Geyser Basin | 133.1 F (56.2 C) | 8.5 |
Most of these springs are alkaline, with pH above 7. Yellowstone also has strongly acidic features. NPS describes Sulphur Caldron near Mud Volcano as having a pH of about 1 to 2, similar to battery acid.
Superheated water below the surface
The water at the surface is the cooler end of the system. NPS explains that water deep in the plumbing can exceed the surface boiling point because the weight of water and rock above keeps it under pressure, which stops it from turning to steam. This is called superheated water. When it rises and the pressure drops, some of it flashes to steam.
Drilling has measured how hot it gets. NPS lists these readings from Norris Geyser Basin, the hottest area in the park:
- 1926: researchers Allen and Day measured 284 degrees F (140 degrees C) at Black Growler Steam Vent.
- 1929: Carnegie Institution investigators recorded 401 degrees F (205 degrees C) at 265 feet in a test well and stopped drilling because of the steam pressure.
- 1967: the U.S. Geological Survey (USGS) recorded 460 degrees F (238 degrees C) at 1,089 feet in a research drill hole, the highest subsurface temperature NPS reports.
Steam vents at the surface can also be superheated. NPS reports fumarole temperatures as high as 280 degrees F.
Why Morning Glory Pool cooled
Temperatures can change over time. NPS states that over the years people threw thousands of pounds of coins, trash, rocks and logs into Morning Glory Pool. The debris lodged in the vent, reduced circulation and lowered the temperature, and natural changes may also be cooling it. The pool now averages 159.3 degrees F. NPS explains that cooler temperatures allow orange- and yellow-colored microbes to thrive, which changes the pool's colors.
What the colors say about temperature
The bright rings around many springs are living mats of heat-loving microorganisms called thermophiles. Different species live in different temperature zones, so the colors work like a rough thermometer. NPS gives these points:
- Hottest water: colorless and yellow thermophiles grow in the hottest water. Hyperthermophiles live in water above 176 degrees F (80 degrees C).
- Up to about 167 F: cyanobacteria can live in water as hot as 167 degrees F (73 degrees C), where they are usually yellow-green.
- Cooler runoff: as the water cools, the cyanobacteria mats turn orange, rust or brown, and other thermophiles in cooler water change the colors further. Orange, brown and green mats grow in the cooler zones.
- Deep blue centers: NPS explains that blue appears where sunlight passes through deep, clear water.
Grand Prismatic Spring shows the full sequence. USGS gives its widest diameter as 370 feet and describes its brilliant blue center as near boiling temperature, with bands of thermophiles that thrive at different temperatures moving outward. NPS notes that its colors range from blue to orange. Color can also shift with the seasons and with strong summer sunlight, which stresses the mats.
NPS rules in thermal areas
Because of these temperatures, Yellowstone regulations control where people can walk. NPS states that visitors must walk on boardwalks and designated trails in thermal areas. The park explains that ground that looks solid can be a thin crust over super-heated water, and that even thermal runoff can be hot enough to cause burns. Pets are not allowed in thermal areas or on boardwalks, even when leashed or carried. NPS attributes part of the cooling of Morning Glory Pool to objects people threw into it.