ANNUAL CLIMATOLOGICAL SUMMARY
FT. SNELLING MN
The 1847 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; 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 1847 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, 1847 Ft. Snelling temperature readings were taken from an instrument 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). With the exception of many of the station's 0900 hour readings , fixed time temperature readings taken during 1847 are consistent with "normal" diurnal patterns.
All 1847 wind force values are expressed in terms of a quantified 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, 1847 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 refer to the general character of each day of the year, with "fair" probably indicating an essentially cloudless day and "cloudy" indicating an overcast or partially overcast day (thus probably including what, in modern terminology, would be recorded as "partly cloudy" days). Although the 1847 sky cover record includes a complete set of daily entries, the force of wind record is sometimes incomplete: air movement records include a significant number of wind direction entries UNACCOMPANIED by corresponding wind force values. Extant records give no explanation for these lapses: perhaps 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.
Contextual evidence indicates that 1847 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 understate the number of days with precipitation and/or measurable precipitation. This suggests that -- following the example of their predecessors -- Fort observers did not routinely/consistently measure and/or record small 1847 precipitation events, sometimes either ignoring insignificant rainfall or snowfall events or, alternatively, using "inappreciable" or "slight" to denote small, but perhaps otherwise measurable, amounts of precipitation.
Although the 1847 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 1847 "snow days".
In addition to outdoor temperature, sky cover, precipitation and air movement data, the 1847 Ft. Snelling record -- as noted previously -- includes four daily readings taken from the station's mercury barometer and from the station's "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, 1847 barometric values are station pressures (i.e. readings which have not been adjusted to compensate for elevation above mean sea level.
The foregoing 1847 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 1847 . 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 1847 record also includes both the monthly and annual extremes (highest daily minimum, lowest minimum, etc.) estimated by Fisk and the monthly extremes actually recorded by fort observers. All 1847 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 numeric 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 amount of precipitation (2.66 inches) officially recorded during June 1847 may significantly understate the amount of rain which actually fell during the month: "showers" noted on 30 June with no accompanying quantified precipitation value.
Cold, dry January: sunrise readings below zero each day, 6-11 and 18-24 January. Readings of -24 F and -14 F at 0900 and 1500 hours, respectively, on 18 January. Observer recorded twenty three "cloudy" days during January. Dry, cloudy (twenty six "cloudy" days recorded) and warm February. Cold, dry March: sunrise readings of -10 F and -12 F on 10, 11 March, respectively. Outbreak of prairie fires during the closing days of March: "fires in every direction" on 26 March; smoky atmosphere on 27 March. Reading of 74 F at 1500 on 28 March. Twenty seven "cloudy" days recorded during March. Dry April with prairie fires noted on 11 April and smoky atmosphere noted on 20, 25, 26, 27, 28, 29 April. Cold at mid-month: afternoon readings of 31 F and 30 F at 1500 hours on 17, 18 April, respectively. Sunrise reading of 19 F on 18 April. Readings of 80 F, 81 F and 82 F at 1500 on 25, 26, 27 April, respectively. Twenty seven "cloudy" days during April. Force seven winds on 16 April. Cool, wet and cloudy (twenty nine "cloudy" days) May. Readings of 46 F at 1500 hours on 21, 24 May. Heavy rain on 6, 30-31 May. Sunrise readings of 32 F, 34 F, 35 F, 38 F, 38 F and 37 F on 3, 8, 12, 17, 21, 25 May, respectively. Cool June: readings of 56 F at 1500 on 13 June and 43 F at sunrise on 14 June. Reading of 74 F at sunrise on 26 June. Very cloudy early June. Warm, sunny July: sunrise readings of 80 F and 77 F on 15, 16 July, respectively. Afternoon readings of 66 F, 66 F, 67 F and 63 F on 17, 25, 26 and 29 July, respectively. Heavy rain on 6 July. Cool, relatively dry August: afternoon readings in the 60's F each day, 28-31 August. Sunrise readings of 48 F and 47 F on 19, 31 August, respectively. Cool early September: light frost noted on 8 September with sunrise reading of 42 F on that date. Sunrise readings of 40 F, 39 F and 39 F on 9, 12, 13 September, respectively: "heavy" frost on 12 September. Smoky conditions noted on 1, 28 September. Dry, sunny October: reading of 91 F at 1520 hours on 3 October. Sunrise readings of 21 F and 14 F on 14, 26 October, respectively. Afternoon reading of 40 F on 13 October. Smoky conditions noted on 17, 28, 29, 30 October: "remote light from prairie fires in every direction" on 31 October. Very warm early November: 74 F recorded at 1500 hours on 2 November. Heavy rain with hail and slushy snow on 6-8 November: 1.5 inches of precipitation recorded. Smoky conditions on 1, 2 November: light from prairie fires noted on 2, 3, 4, 14 November. Moderate December. Warm and misty conditions during the closing days of the month. Reading of -19 F at sunrise on 26 December.