The soldiers stationed at Fort Yellowstone had a saying: “In Yellowstone there are only two seasons: winter and July.” While this may be a slight exaggeration, it is true that Yellowstone’s winter is by far its longest season–as many of the park’s permanent residents will attest to!
Climate is the long-term combination of atmospheric conditions produced by day-to-day weather. The physical geography of an area has an important influence on the type of climate a region experiences. Yellowstone is primarily a forested, volcanic plateau with an average elevation of about 8,000 feet. Because Yellowstone is located deep within the interior of the North American continent far from the climate-moderating influences of the oceans, one would ordinarily expect such an area at this latitude to have a severe (hot summers and cold winters) and dry climate. However, Yellowstone’s high elevation moderates the severity of its summers as well as the amount of precipitation it receives.
Weather observations were first taken at Mammoth Hot Springs by U.S. Army personnel in January 1887. By 1890, these observations were being taken on a regular and consistent basis, continuing to this day. Records taken include temperature, precipitation, barometric pressure, wind speed and direction, sunshine, and humidity. The weather station at Mammoth Hot Springs is the second oldest weather observation station in continuous existence in the state of Wyoming. Yellowstone has also participated for many years in a national program to collect snow accumulation data from various locations in the park. This data is used by the Natural Resources Conservation Service (formerly the Soil Conservation Service) to calculate water content amounts in the snow in order to predict spring runoff flows for the nation’s rivers.
The weather in Yellowstone is characterized by summer days that are usually mild and nights that are cool. Daytime temperatures range between 70 and 80 degrees F, and nighttime temperature often fall below 40 degrees F. July is the warmest month in the park, but temperatures rarely exceed 90 degrees F. Winters are cold, characterized by daily maximum temperatures that frequently do not rise above freezing. Spring and autumn are transitional seasons between the long, cold winter and the short, mild summer.
The highest recorded temperature in the park was 98 degrees F at Lamar Ranger Station in 1936, although 103 degrees F was recorded at Gardiner, Montana (on the park’s northern border), in 1960. The coldest temperature ever recorded in Yellowstone was -66 degrees F on February 9, 1933, at the old Riverside Ranger Station, which was about a mile east of the West Entrance. On that day, bitter cold temperatures were also recorded at Tower Fall (-52 degrees F), Lake Yellowstone (-56 degrees F), Lamar Ranger Station (-57 degrees F), and Mammoth (which recorded a mild -40 degrees F!).
There is a wide variety of elevations in Yellowstone, and the average annual precipitation (which includes rain and melted snow) ranges from as little as 11 inches at Gardiner, Montana, to about 38 inches at Bechler River (in the park’s southwest corner). Of course, greater amounts of precipitation occur high in the mountains. Park records indicate that annual precipitation may exceed 70 inches on the Pitchstone Plateau in the west central portion of the park, mainly a result of winter snowfall. Snow accumulation begins in mid- to late October, and snow stays on the ground until late March or early April. The average duration of snowcover is about 213 days for elevations up to 7,000 feet; this duration increases with elevation at the rate of 29 days for every 1,000 feet.
Climate (like weather) is constantly changing, and it does so on several scales. Departures from “normal” are the rule, not the exception. Analysis of pollen found in bogs and shallow ponds in the park show climatic changes on a scale of thousands of years and indicate that Yellowstone has had climatic conditions ranging from arctic to subtropical. Following the retreat of the glaciers (about 14,000 years ago), pollen studies indicate a warming trend followed by a cooling trend. More relevant to humans are the climatic changes that occur on the scale of decades, such as the drought of the 1930s and the high precipitation of the 1940s. These types of trends have occurred for at least the past 230 years, as indicated by tree-ring analysis. The period from about 1870-1900 was considerably wetter than present-day conditions, and in the 1840s and 1850s, dry conditions occurred that were similar to those of the 1930s. The global climate of the 1980s and 1990s has produced seven of the world’s warmest years since weather records have been kept. It is not yet clear if this represents a change in climate or is simple a departure from “the norm.”
The bottom line for visitors to Yellowstone is that they should expect any kind of weather at any time of year Snow and cold weather can occur in any month of the year here. In fact, one snowstorm on August 25th in the early part of the century stranded a number of visitors at Old Faithful. They decided to turn their “misfortune” into an adventure by celebrating “Christmas in August.” That one weather event has resulted in a tradition in Yellowstone, and every August 25th visitors find gaily decorated Christmas trees in hotel lobbies and employees singing Christmas carols. Weather and climate can affect more than just the great outdoors!