ANNUAL CLIMATOLOGICAL SUMMARY
FT. SNELLING MN
The 1846 Ft. Snelling climatological record consists of: four daily fixed time readings taken from the station's "detached" thermometer; daily precipitation values; four daily fixed time sky cover observations; summary entries indicating the total number of "cloudy" days (probably indicating a day with any significant cloud cover) and "fair" days (probably indicating days with little or no cloud cover); four daily fixed time observations of wind "force" and wind direction; two daily fixed time wet bulb readings; four daily barometric readings; and four daily readings from the station's "attached" thermometer (i.e. from an indoor thermometer used to adjust barometric readings for the effects of temperature on the station's mercury barometer) . Temperature, sky cover, wind direction/force and barometric observations were taken at or about local sunrise, at or about 0900 hours, at or about 1500 hours and at or about 2100 hours local solar time. Wet bulb readings were taken at local sunrise and at 1500 hours local time. Although extant records give no indication of the time at which 1846 precipitation observations were taken, contextual evidence suggests either a 2100-2100 schedule and/or an early morning (probably "sunrise") schedule (which, in at least in some instances, may have entailed "shifting" of precipitation values from the date on which the observation was actually made to the day immediately preceding).
So far as can be determined, all 1846 temperature observations were taken from a thermometer manufactured by George Tagliabue, New York City and all precipitation observations were taken from a DeWitt rain gauge (which was probably mounted on a pole or post on the fort's parade ground). All 1846 wind force values are expressed in terms of a quantitative wind force value selected by the observer after noting the effect of the wind on flags, trees, signs and other easily movable objects . The degree of cloudiness was similarly quantified, using a scale of zero (complete overcast) to ten (a totally cloudless sky) . As noted, 1846 Ft. Snelling records also include notations indicating the total number of "fair" and the total number of "cloudy" days observed during each month of the year. Unfortunately, the terms "fair" and "cloudy" are nowhere defined: presumably, however, they are terms referring to the general character of each day of the year, with "fair" indicating an essentially cloudless day and "cloudy" indicating an overcast or partially overcast day (thus probably including what, in modern terminology, would be considered a "partly cloudy" day). Although the 1846 sky cover record includes a complete set of daily entries, the force of wind record is often incomplete: air movement records include a significant number of wind direction entries UNACCOMPANIED by corresponding wind speed indicators. Extant records give no explanation for these lapses: perhaps, however, the missing entries indicate a force of wind value of more than zero but less than one or, alternatively, that fort personnel, for whatever reason, were unwilling to expend the time and effort required to take consistent wind speed observations.
Comparison of sunrise, 0900 and 1500 hour readings suggest that the Ft. Snelling thermometer(s) was exposed in a properly shaded location during 1846: so far as can be determined, fixed time readings taken during 1846 are consistent with "normal" diurnal temperature patterns (indicating, of course, that the station thermometer had been re-located and/or sheltered as necessary to protect it from the early morning, mid-morning and/or afternoon sun). Extant evidence similarly indicates that 1846 Ft. Snelling precipitation records are, on balance, more complete and reliable than their pre-1843 counterparts. When compared with modern records they do, nevertheless, appear to significantly understate the number of days with precipitation and/or measurable precipitation. This suggests that the fort observers -- following the example of their predecessors -- did not routinely/consistently measure and/or record small 1846 precipitation events, sometimes either ignoring small rainfall or snowfall events or, alternatively, using the term "inappreciable" to indicate small, but otherwise measurable, amounts of precipitation.
Although the 1846 Fort Snelling record includes daily liquid/melted precipitation values and a record of the TYPE of precipitation observed, it includes NO QUANTITATIVE snowfall values (whether of fresh snowfall or accumulated snow cover). Any snowfall values contained in the foregoing compilation, therefore, are estimates obtained by applying the National Weather Service meltwater-snowfall conversion matrix to the meltwater values recorded by Ft. Snelling observers on 1846 "snow days".
In addition to outdoor temperature, sky cover, precipitation and air movement data, the 1846 Ft. Snelling record includes four daily readings taken from the station's mercury barometer and from the "attached" thermometer (readings from which, as noted above, were probably used to correct barometric readings for the effects of temperature on the mercury in the station barometer). So far as can be determined, 1846 barometric values are station pressures (i.e. readings which have not been adjusted to compensate for elevation above mean sea level.
The foregoing 1846 climatological record includes both unadjusted (UNADJ) and adjusted (ADJ) mean temperature values. Unadjusted values are the averages of the four fixed time readings taken daily during 1846 . 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 which would have been recorded had the Ft. Snelling station been equipped with self-registering thermometers read and re-set at midnight . The foregoing 1846 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 by fort observers. All 1846 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.
All foregoing mean cloudiness values are the simple average of the station's four daily fixed time sky cover entries. The foregoing prevailing wind values are based on daily entries indicating the direction of the wind at sunrise/0900/1500/2100: prevailing/predominate winds are those most frequently observed/recorded during any given month.
The May 1846 precipitation record is incomplete/partially missing: a "heavy gale" from the east blew the station rain gauge "from its stand", thus precluding measurement of the rain produced by a heavy thunderstorm which occurred during the night of 16-17 May. Precipitation during September 1846 may have totaled 2.33 inches (rather than the 2.23 inches indicated in the foregoing compilation): the station's 10 September 1846 precipitation entry cannot be satisfactorily or definitively deciphered.
Record warm and apparently cloudy (twenty two cloudy days noted) January. Prairie fires noted on 18, 24 January. Very little snow during month: rain on 28, 29, 30 January. Readings of 59 F, 49 F and 50 F at 1500 hours on 24, 25, 28 January, respectively. Sunrise readings of 34 F, 33 F, 34 F, 44 F and 33 F recorded on 1, 25, 26, 29, 30 January, respectively. No subzero temperatures during January. Warm early February: afternoon readings of 48 F and 52 F on 4, 6 February, respectively. Very dry February with virtually no snowfall. Brief cold wave late February. Force of wind record, although incomplete, indicates that winds during January and February were generally very light. Warm, cloudy (twenty cloudy days recorded) March. Outbreak of prairie fires: "fires in every direction" noted on 1 March; smoky conditions, 1-10 March; and "dense smoke" noted on 17 March. Readings of 62 F, 60 F, 62 F, 61 F and 64 F at 1500 hours on 5, 7, 8, 9, 17 March, respectively. Sunrise reading of 49 F on 18 March. Hail on 5 March. Heavy rain, 10-12 March. Rainy, cloudy April. Reading of 29 F at 1500 on 7 April. Reading of 85 F at 1500 on 20 April. Smoky conditions on 20 April. Warm, cloudy May. Light frost with some damage to vegetation on 15 May. Smoky conditions on 8, 9, 10, 11, 20 May. Cool June: afternoon readings of 54 F and 57 F on 1, 2 June, respectively. Warm, rainy July: 2.87 inches of rain during a heavy thunderstorm on 3 July. Mississippi river "remarkably high" during the month. Heat wave with sunrise readings in the 70's F and afternoon readings in the 90's F, 7-9 July. Warm, humid and hazy August. Much fog during month. Twenty six "cloudy" days noted. Very warm conditions, 3-11 August. Rainfall of 1.7 inches in one hour on 25 August. Warm, foggy and hazy September. Hail on 10 September. Twenty three "cloudy" days recorded. Reading of 89 F at 1500 on 13 September. Smoky conditions and/or prairie fires noted on 4, 6, 22, 23, 25 October. Twenty two "cloudy" days recorded during October. Very warm but apparently cloudy (twenty six "cloudy" days) November. Prairie fires "raging furiously" on 4 November. Very little snow during November. Sunrise readings of 49 F, 52 F, 47 F, 48 F, 47 F on 9, 10, 12, 13, 17 November, respectively. Warm, dry December. Twenty nine "cloudy" days during December.