The mountains aren't the only factor that shape the climate of the western San Juans. Like most other places in the western United States, the San Juans are at the mercy of storms (called "cyclones" because of their circular wind patterns, or "lows" because of their low barometric pressure) sweeping off the Pacific Ocean. From November through April, one or two cyclones a week (on the average) strike the coast and head for the Rockies. Most cyclones veer north of Colorado, but some slip south into Arizona and New Mexico.

Wispy cirrus clouds in the western sky are often the first sign of an approaching Pacific storm. Over the next twelve to twenty-four hours the clouds thicken and lower, and as the cloud deck envelops the highest peaks snow begins to fall along the ridges. Within a few hours the snow spreads to the valleys, though it may melt into rain before it reaches the surrounding low country.

From space a cyclone looks like a draining bathtub, with air streaming in toward the center. Water in a bathtub goes down the drain, but air in a storm rises as it nears the center. In the Northern Hemisphere the cyclone spins counterclockwise. The rising air expands and cools, and its load of moisture (humidity) condenses into clouds and snowflakes. The air gets an added upward shove where the wind blows up and over mountain ridges, complicating the pattern of snowfall. Though snow may fall over the entire San Juan area, the heaviest falls normally occur along the higher ridges and passes. Because of this "upslope" effect during storms, Red Mountain, Coal Bank, and Lizard Head Passes receive five to ten times as much snow each winter as do Cortez and Durango. Snow accumulations at lower elevations often melt before the next storm, but above 7,000 feet (2,130) the flakes continue piling up through the winter, reaching peak depths in early spring.

Frequent moderate snows from numerous storms tracking north of the area provide most of the winter's accumulation, but the heaviest individual falls come from storms that pass south of Colorado. It may seem strange that storms crossing the Arizona desert should be the heaviest snow producers for the mountains, but on this southerly track cyclones are better able to sweep moisture-laden air from the subtropical Pacific Ocean west of Mexico directly into the San Juans. Cyclones that have followed this path include the great storms of December 19-26, 1883; February 3-8, 1884, February 24-27, 1987; and February 15-26, 1993, each of which dumped 5 feet (or more) of snow in the mountains. Snowslides following the February 1884 storm blocked the rail line into Silverton, isolating the community until mid-April. By far the worst winter for avalanches, though, was 1905-1906, with twenty-four avalanche fatalities in San Juan County. Numerous slides killed sixteen on Saturday evening, March 17, at the end of a week-long snowstorm, with twelve miners dying as they sat for dinner at their boardinghouse east of Silverton. Snow can plague the lowlands, too. The massive, slow-moving system of December 13-21, 1967, buried Cortez, Mesa Verde, and Durango under 3 to 4 feet of snow and forced emergency aerial hay drops to livestock stranded in the Four Corners area.

As the storm continues east onto the Plains, clearing skies move in from the west. Snow showers and flurries may linger in the mountains for part of a day after the clouds have parted over the lowlands. Northerly winds on the heels of the storm bring lower temperatures, and nighttime readings may dip to zero or lower. However, these cold air masses usually come off the Pacific Ocean. Bitter Arctic air from Siberia and Alaska usually stays east of the Continental Divide, and extremely low temperatures are actually quite rare in the San Juans. Whereas other mountain areas of Colorado have recorded temperatures in the range of 40 to 60 degrees below zero, the coldest ever seen in the San Juans is a comparatively mild 39 below, at Silverton. See the table below for a list of climatic extremes in San Juan Mountains.

With sunny skies and light winds, daytime temperatures may rise 30 to 40 degrees above the morning low. Pleasant winter weather may last less than a day or continue for a week or more before cirrus clouds announce the approach of the next storm.

By March, when most of the Northern Hemisphere is feeling the first touches of spring, winter still reigns in the San Juans (and all across the Rockies). March is, on average, the snowiest month of the year in the high country, and April is often not far behind. Silverton, Telluride, and Ouray rarely see bare ground before mid-April, and above 10,000 feet the snow doesn't clear out until June.

Spring arrives with a bang. One or two winterlike storms may cross the area in early May, bringing rain to the lowlands and wet snow in the mountains, but then the storm track shifts toward the northern United States and Canada. As a warm high-pressure system settles in over the Southwest, the winter's accumulation of mountain snow starts melting in earnest. During the most rapid melt, from mid- to late May, the volume of snow turned into water each day is equivalent to a widespread rainfall of nearly 1 inch, and by late May the Animas, Dolores, and Gunnison Rivers are running high. River flows typically peak during the first ten days of June, then decrease considerably by the end of the month as the mountains run out of snow.

June is the sunniest and driest month of the year. The sun shines on Durango an average of 12.4 hours per day, or 84 percent of the total daylight hours (see chart below), and rain falls on but four of June's thirty days. Pleasantly warm days follow cool, even frosty nights, and the day-to-night temperature swing often exceeds 40 degrees. Only occasionally do the cumulus clouds that dot the afternoon sky develop into showers or thunderstorms. All in all, June has the best weather of the year for outdoor activities.

July brings the southwest monsoon, a flow of moisture from the tropical Pacific into the southwestern United States. The air becomes more humid, and afternoon thunderstorms become common. Cortez averages about forty thunderstorms per year, and the higher mountains may see eighty or more. More than half of these storms occur in July and August, when a hiker in the high country has a better than even chance of encountering thunder, lightning, and rain during the afternoon. Some of these storms (about three or four per year at any location) may drop small hail, but large, damaging hail is rare.

As the monsoon weather tapers off in September and ceases in October, the air becomes cooler and drier. Rainy days become less frequent, cloudiness decreases, and the sun shines nearly as much as it did in June. Once in a while the flow of air from the southwest picks up a dying hurricane off the coast of Mexico and swings its soggy remnants into the San Juans. It doesn't happen often, but when it does the ensuing torrents of rain can lead to disastrous flooding - as in October 1911, October 1972, and July 1981. The remnants of Tropical Storm Norma spawned a tornado 12 miles west of Cortez on September 5, 1970. (Only two other tornadoes have ever been reported in the area. One appeared near Cortez, on April 25, 1985, in a developing snowstorm; the other touched down south of Durango on May 30, 1992. Neither caused injuries or serious damage.) For the most part, however, September and October are pleasant months, weatherwise.

Late October and early November bring the return of winter-like storms. The first few snowfalls may melt quickly, but by mid-November snow once again covers the ground at higher elevations. The chances of a white Christmas (1 inch or more of snow on the ground) range from 40 percent at Cortez and 70 percent at Durango to more than 95 percent at Ouray, Silverton, and Telluride. December snows cover the lower elevations to close out a typical year of weather in the San Juans.

Not all years are typical, though. Average temperatures in some winters can be 20 degrees warmer than in others, and average temperatures for entire years may vary by 5 or 10 degrees in effect, Durango may have Telluride's weather for a year or vice versa. During the past century the warmest year in the San Juans was 1934, followed by 1954 and 1981, and the coldest years were 1912, 1924, 1973, and 1979. The warmest decades were the 1930s and 1950s, whereas the 1910s, 1920s, and 1970s were generally cool across southwestern Colorado. There have been plenty of ups and downs on San Juan thermometers, but over the past century there has been no evidence that global warming (or cooling) has taken hold of the region. Temperatures for the most recent full decade, the 1980s, averaged about the same as those in the 1890s.

For the most part, the San Juans lie in a semiarid area, and the amount of precipitation is more critical than the temperature to human endeavors. Snow and rainfall can vary tremendously from one year to the next. Most mountain locations had only 3 to 6 feet of snow during the drought winter of 1976-1977, but ten times as much snow fell on the same places just two winters later. Early weather records show that the winters of the 1870s and 1880s were quite a bit snowier (perhaps twice as snowy, on the average) than they are now. The decades of the 1890s and 1950s were on the dry side, but over the past century there has been no long-term trend toward greater or lesser precipitation. There is, of course, no guarantee that the climate will remain so stable in the future Tree-ring records indicate that the present century has been the wettest in 800 years. The sudden end of an earlier wet spell around A.D. 1170 may have driven the Anasazi from the San Juans - a historical lesson we should not ignore.

CLIMATE DATA

The range of climates found in the southwestern corner of Colorado is greater than that in many entire states. The table below summarizes climate data taken at weather stations along (and near) the San Juan Skyway, listed in order heading north from Durango. Among the places included are towns, mines, train stops, snow survey sites, ski areas, parks, and research stations - all places with an interest in reporting and recording the weather. A glance at these statistics reveals the incredible variety of climates: Cortez receives only 12 inches of precipitation per year, whereas Red Mountain Pass may expect an annual burial under 43 feet of snow. If you're planning a trip to the area, check the average temperatures and number of rainy (or snowy) days per year and dress accordingly. The data presented in this table.

Elevation (feet) - One of the most important factors in determining local climate.

Years of Record - The longer weather records have been taken, the more reliable the averages will be. It takes about ten or twenty years to come up with a truly representative average.

Average Temperature (Fahrenheit) - The average daily high and low temperatures for July and January give a good picture of the daily and seasonal ranges of temperature.

Temperature Extremes - The highest and lowest temperatures since records began. The longer the period of record, the more extreme these numbers are likely to be.

Average Annual Precipitation (inches) - Precipitation includes the water contained in snow, hail, sleet, etc., as well as rain. As a rule, 10 to 15 inches of snow melts down to 1 inch of precipitation.

Average Annual Snowfall (inches) - The total of all the individual storms in a year. For some locations this is estimated from U.S. Department of Agriculture/Soil Conservation Service snowpack data.

Deepest Snow on Ground (inches) - The greatest accumulated depth of snow ever measured.

Days with Precipitation - Average number of days per year with measurable rain (0.01 inch or more) or snow (0.1 inch or more).

REFERENCES

Armstrong, B. R., 1976. Century of struggle against snow: A history of avalanche hazard in San Juan County, Colorado. Boulder, CO: Institute of Arctic and Alpine Research, University of Colorado, Occasional Paper 18, 98 pp.

Armstrong, R. L., and J. D. Ives, 1976. Avalanche release and snow characteristics. Boulder, CO: Institute of Arctic and Alpine Research, University of Colorado, Occasional Paper 19, 256 pp.

Berry, J. W., 1968. Climates of the states - Colorado, in Climatography of the United States 60-65, U.S. Department of Commerce, Weather Bureau, 20 pp.

Bradley, R. S., R. G. Barry, and G. Kiladis 1982. Climatic fluctuations of the western United States during the period of instrumental records. Amherst, MA: Department of Geology and Geography, University of Massachusetts, Contribution 42, 169 pp.

Keen, R. A., 1987. Skywatch: The western weather guide. Golden, 0: Fulcrum Press, 158 pp.

Petersen, K. L., 1985. Climatic reconstruction for the Dolores project area (Chapter 20), in Dolores archaeological program - studies in environmental archaeology, Denver: U.S. Bureau of Reclamation.

Sherier, J. M., 1933. Climatic summary of the United States - Western Colorado, in Climatography of the United States 10-22, U.S. Department of Commerce, Weather Bureau, 33 pp.