ANNUAL CLIMATOLOGICAL SUMMARY
FT. SNELLING MN
Beginning 1 January 1841, Ft. Snelling temperature observations were taken daily at or about local sunrise, 1400 hours local solar time: at or about local sunset; and at or about 2100 hours local solar time. Wind and sky cover observations, however, continued to be taken twice daily, probably at or about sunrise and at or about local sunset (or, alternatively, at or about 1400 hours local solar time). Quantitative precipitation observations were taken daily throughout 1841, using a conical rain gauge of the type designed in the early 1830's by Simeon DeWitt, chancellor of the New York university system. Like records from previous years, the 1841 record also includes intermittent records of snowfall and/or snow cover; episodic records of phenological, hydrological, astronomical and/or other natural events (windstorms, prairie fires, etc.); descriptive entries indicating the general duration (and, in some instances, the intensity) of precipitation; precipitation type ; and special atmospheric phenomena (fog, smoke, etc.). So far as can be determined, all 1841 observations were taken within the Ft. Snelling enclosure (on the bluffs overlooking the confluence of the Mississippi and Minnesota rivers) .
The foregoing 1841 temperature record includes both unadjusted (UNADJ) and adjusted (ADJ) mean temperatures. Unadjusted values are the simple averages of fixed time readings taken daily at sunrise, 1400, sunset and 2100 hours . Adjusted averages are from Charles J. Fisk's 1984 "Reconstruction of Daily 1820-1872 Minneapolis-St. Paul Temperature Observations". These values were obtained by averaging statistically derived estimates of the daily maxima and minima that would have been recorded had the Ft. Snelling station been equipped with self-registering thermometers read and re-set at midnight . The foregoing 1841 record also includes both the monthly and annual extreme temperatures (highest daily minimum, lowest minimum, etc.) estimated by Fisk and the monthly extremes actually recorded/observed (OBSRV) by fort observers. All 1841 temperature distributions (e.g. days 90 F or higher, 32 F or lower, etc.) are based on Fisk's estimates of daily maxima and minima.
The foregoing precipitation frequency record (e.g. number of days with snow, etc.) is from quantitative precipitation records kept by Ft. Snelling observers during a 1841 (a record which, however, is supplemented by descriptive remarks in the station's monthly meteorological register) . Because the original 1841 record contains no quantitative snowfall data, the foregoing monthly snowfall values are estimates (obtained in most instances from National Weather Service meltwater-snowfall conversion tables). Prevailing monthly wind values are based on entries indicating the direction of the wind as recorded by station observers on the morning and afternoon/evening of each day during 1841. Prevailing monthly winds indicate the direction most frequently observed/recorded during any given month. Sky cover distributions are based on observations taken twice daily during 1841 : fair days are those with "fair" conditions noted at both observations; cloudy days are those with "cloudy" conditions noted at both observations; and "mixed" days are those with fair conditions noted at one observation and cloudy conditions noted at one observation.
Although the sunrise/1400/sunset/2100 pattern of observations was officially inaugurated on 1 January 1841, Ft. Snelling observers failed to take the prescribed 1400 hour readings at any time during January 1841. Extant records provide no explanation for this lapse: it is possible, however, that it may be attributable to instrument malfunction; loss of the station's record of readings taken at 1400 hours during that month ; or changes in post staffing/scheduling which made it temporarily impossible to take the requisite afternoon readings. In addition to the missing January temperature values, the 1841 Ft. Snelling record is compromised by October-December sunset readings which, in contrast to normal diurnal patterns, are generally higher than corresponding 1400 hour readings. This pattern suggests that station thermometers during this period may have been exposed to the direct rays of the setting sun; that observations during this period were consistently taken at times other than local sunset; or, more plausibly, that the 1400 hour and sunset readings were transposed by the clerks who transcribed the station's October-December 1841 quarterly meteorological report.
The 1841 Ft. Snelling precipitation record indicates that two extremely heavy rainstorms occurred during September, producing a total of more than nine inches of precipitation in an interval of less than ten days. Contextual evidence suggests, however, that one, or perhaps both of the values involved may be erroneous, the result of a misinterpreted entry and/or an egregious recording error. Specifically, although September 1841 daily precipitation values add to 9.30 inches, the station's July-September meteorological report to the Surgeon General's office shows a September total of only 6.10 inches. This suggests that the precipitation value (also 6.10 inches) entered in the station's weather diary for 25-27 September 1841 may have been a cumulative total for the month to date, not the amount of rain which actually fell during the three day period involved. Moreover, the 25-27 September entry is also exactly double the 3.05 inches of rain recorded on 19 September, suggesting, alternatively, that the larger, later value may, for whatever reason, have been "copied down" and then added to the previous total (resulting in a greatly exaggerated monthly precipitation record).
Dry cold January. Very cold mid-January with a sunrise reading of -32 F on 17 January. Rapid warm-up with readings approaching 50 F on 23 January. Dry, sunny and warm February. Readings of 54 F, 49 F and a record 60 F at 1400 hours on 20, 21, 22 February, respectively. Warm, sunny March. Heavy snowfall, 28-29 March. Cool, sunny April. Readings of 29 F at 1400 hours on 29 April: mixed rain and snow on that date. Very cold during the opening days of May: 23 F at sunrise on 2 May. Warm mid-May and again in late May. Record indicates that May was a very sunny month. Warm June. Sunrise reading of 74 F on 11 June. Afternoon reading of 92 F on 11 June followed by thunderstorms and a rapid drop in temperature (with an afternoon reading of 66 F on 12 June). Warm, sunny and dry July. Sunny, dry August. Post surgeon noted that the summer of 1841 was "particularly dry". Cool September. Afternoon readings in the low 90's F, 4-6 September followed by unseasonal cold on 7 September (with an afternoon reading of 51 F). Sunrise readings of 40 F, 32 F, 35 F, 36 F, 34 F, 34 F, 32 F on 8, 17, 18, 22, 23, 29, 30 September, respectively. Afternoon readings of 45 F, 45 F, 40 F on 15, 16, 18 September, respectively. Rain with high winds on 6 September and heavy rains (apparently) on 19, 25, 26, 27 September. Snow on 23 October. Afternoon readings of 30 F and 27 F on 23, 24 October, respectively. Warm during the closing days of October. Seasonable to mild November with much sunny weather. Reading of 62 F on 10 November. Warm, sunny December. Readings near 50 F on several days at the beginning of the month. Rain on 12 December. Heavy snow on 14 and 18-19 December. No readings of zero F or below recorded during December.