One Hundred Years of Utah Climate

Utah Historical Quarterly

Vol. 46, 1978, No. 4

One Hundred Years of Utah Climate

BY DON R. MURPHY

AS A CHILD MANY A UTAHN has probably sat at grandfather's knee and heard him tell stories of years past when he was a lad and the winters were much colder and the snowfall was much deeper than at present There may or may not be a basis for grandfather's claims because within Utah, and other midlatitude settings where economic activities and human responses are adjusted to great seasonal variations, comparisons can and often are made between the various summer or winter seasons. Thus, Utah's octogenarian and nonagenarian citizens are prone to recall certain winters as having had especially heavy snowfalls, certain Januarys with extremely cold temperatures, or certain summers that were especially hot or dry, or both. In attempting to recall climatic conditions of childhood days the senior citizen is apt to remember most those years of extreme conditions and having not experienced such extremes lately may conclude that Utah's climate is undergoing change. If Utah's climate is indeed changing, this change should be verifiable by referring to past monthly and annual temperature and precipitation statistics. If the statistics indicate that no dramatic change has occurred, the misimpression of "rougher winters in the past" may be due to man's advancing technology that more and more helps us to overcome our physical environment. Such technological advances now allow us to travel in heated automobiles on well-maintained highways instead of open buggies along snow-drifted lanes or relax through hot summers in air-conditioned homes instead of spending such summers suffering through what seemed like endless hot days and sleepless nights.

In March 1874, monthly temperature and precipitation statistics began to be recorded for Salt Lake City with 1875 having the first full year of data. A one-hundred-year record is therefore available in which to note fluctuations, to observe which years represented extremes, or to detect changes in the climate of the Salt Lake City area, if indeed it is undergoing change. The site where these statistics were first recorded was within downtown Salt Lake City, but during 1928 the official weather station was transferred to its present site at the old Salt Lake airport. It may be suggested that such a shift in location could have resulted in an immediate, short-term variation in average statistics, but such a disruption would soon be absorbed when considering long-term averages. The Salt Lake City statistics are exclusively used for this investigation because of their long-term nature. However, past weather and climatic conditions for Salt Lake City are probably not representative for the entire state and may not even be representative for the entire Wasatch Front area. Thus, a reasonably dry winter in Salt Lake City may actually have been quite severe for some other community within the general region. Some regional trends, however, can be observed by reference to past Salt Lake City data.

The statistics used in this investigation are mean monthly temperature and monthly precipitation figures for every month since January 1875. Mean monthly temperatures are derived by obtaining an average of the daily mean temperatures for a particular month. Daily mean temperatures are obtained by adding together the lowest and highest temperatures recorded during the 24-hour period and dividing that total by two (the number of observations). Thus, if on a particular date the lowest recorded temperature were 40 °F and the highest were 60 °F the mean temperature for that date would be 50° F. The daily means for a particular month would then be averaged to obtain the monthly mean temperature. Thus, for a particular July in Salt Lake City the monthly mean could typically be 77°F. However, this figure of 77°F does not reflect an especially hot afternoon's absolute temperature of 105°F, but it does give a realistic idea of average July conditions, and it is average monthly conditions that must be investigated in order to determine if climatic change is indeed occurring. Monthly precipitation figures are obtained by merely recording the total precipitation in inches of rainfall for a particular month.

Proper use of the available statistics can reveal several factors concerning the local climate. Present climatic conditions can be compared with the past, changing trends can be recognized, and land-use capacities can be suggested. In attempting to compare the present climate of Salt Lake City with the climate of a century ago caution must be exercised so that single extreme years will not present an unreal picture. For this study eight 30-year intervals have been compiled beginning with the interval 1875-1904, continuing with 1885-1914, and eventually ending with the interval 1945-74. To determine the present climatic condition for Salt Lake City the latest 30-year interval would be utilized. If the data for all 100 years were averaged, earlier conditions could possibly alter these averages and give a false picture of present-day conditions.

Two 30-year climate charts are presented (see figure 1). The years 1875-1904 (figure la) compare favorably with the years 1945-74 (figure lb), suggesting that little climatic change has actually occurred along the Wasatch Front during the past century. The charts of the intervening six 30-year intervals (1885-1914, 1895-1924, 1905-34, 1915-44, 1925-54, 1935-64) appear to be very similar to those intervals shown in figure 1. However, these figures are for long-term averages, and a breakdown of the same 30-year statistics can reveal some interesting extremes and trends.

Figure 2 shows a progression of mean temperature conditions for the various months through the various 30-year intervals. It should be noted that most months have demonstrated only minor change during the past century. As exception is January which demonstrated a warming trend until early in the twentieth century, followed by a cooling trend. The January average for the 1895-1924 interval was 30.5°F, over 2°F higher than the 28.4°F for the 1875-1924 interval the January average has dropped, reaching a low during the 1935-64 interval of 27.2°F. The averages, however, merely indicate trends, and a look at figure 3 will indicate that certain Januarys within those "warmer" spells were actually quite cold, with 1898 and 1917 being good examples. Note, however, the extremely cold months experienced during the Januarys of 1937, 1949, and the three-year cold snap of 1962-64. These cold winters falling within the 1935 64 interval have no doubt been instrumental in lowering the average January temperature for that period. December temperatures have also demonstrated some fluctuations, with a significant drop during the latest 30- year period. However, the November and March curves (the beginning and end of winter) demonstrate a cooling trend through the years with a subsequent increase in temperature during the latest 30-year interval. July temperatures may also be significant. Note on figure 2 that the present July temperatures are nearly 2°F warmer for the latest 30-year interval than they were for the period 1885-1914 (75.3°F vs. 77.1°F).

Briefly summarizing the changing monthly temperature conditions it appears that January temperatures presently average 2°F to 3°F colder than they did earlier in the twentieth century; July temperatures during the present century are 1.5°F to 2°F warmer than they were during the nineteenth century; and March and November temperatures have been dropping through the years except for a slight rise during the latest 30- year interval. This suggests that present Salt Lake City summers are slightly warmer and winters slightly colder than in the past.

Precipitation through the continuous 30-year intervals has also undergone some fluctuations. Figure 4 reveals a significant drop in average precipitation beginning with the 1915-44 interval and ending with the 1935-64 interval. As suspected, this decrease is due to sparce precipitation during the 1930s—that period of the dust bowl in the Great Plains area. This is further supported by figure 5 which indicates actual annual precipitation for the various years during the entire 100-year period. The years 1931 to 1940 show a greater extended period of drought than any other 10-year period on record.

However, figure 6 reveals some interesting and perhaps surprising seasonal precipitation trends. Monthly fluctuations in precipitation through the continuous 30-year intervals are marked by a continuous decrease in precipitation during the cooler months (see the curves for September through March on figure 6a) with the earlier cooler months (September through December) showing a slight increase during the last 30-year interval. However, the warmer months (see figure 6b), specifically April, June, July, and August, have actually shown an increase in average monthly precipitation since before the decade of the dust bowl. The month of May, however, does not fit this pattern because average precipitation for that month has been steadily declining throughout much of the twentieth century.

Agriculturalists may tell us that for certain areas a continual drop in precipitation during the month of May, which is such an important time for crop growth, may be more significant than the slight increase in precipitation during the following warm months. The trend, however, seems to hold up. Through the years Salt Lake City has been experiencing slightly drier winters and slightly wetter summers. In total annual precipitation these two trends apparently balance one another.

Further consideration should be given to figure 5. Significant yearto-year fluctuations in total annual precipitation is here indicated, but precipitation cycles also may be suggested. The suggested pattern is for a cycle lasting from 11 to 13 years in which precipitation drops from a 1- to 4-year wet period to a 1-, 2-, or 3-year drought period. Note the following cyclical wet years: 1875-76, 1884-86, 1896-99, 1906-9, 1920- 23, 1944-47, 1957, 1968-71. These wet cycles average 11 to 13 years apart with the one exception being during the 1930s when an extended period of drought occurred. Significantly drier years always occurred between these wet peaks. The fluctuations as seen on figure 5 would anticipate a period of reduced precipitation during the latter half of the 1970s.

The present climatic trend in the Salt Lake City area seems to be toward slightly cooler and drier winters with slightly hotter and wetter summers and with an 11- to 13-year cyclical pattern in precipitation fluctuations. This would not appear to support grandfather's claim to colder and wetter winters in the past, but it does not deny the fact that certain past winters may indeed have been extremely cold or extremely wet. These conclusions, drawn from the Salt Lake City weather station data, seem to support the belief of many climatologists that world temperatures have been dropping during the present century but with a recent slight increase occurring. Whether the present trends continue only the future will reveal.

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