ANNUAL CLIMATOLOGICAL SUMMARY
FT. SNELLING MN
The 1840 Ft. Snelling climatological record consists of fixed time temperature readings taken daily at or about local sunrise (designated in the official record as the "AM" reading), 1400 hours local solar time (designated in the official record as the "PM" reading) and 2100 hours local solar time (designated in the official record as the "evening" reading); two daily entries indicating the prevailing direction of the wind and the general condition of the sky (expressed as "fair" or "cloudy"); a daily quantitative precipitation record (derived from measurements taken from the station's DeWitt rain gauge); 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 1840 observations were taken within the Ft. Snelling enclosure (on the bluffs overlooking the confluence of the Mississippi and Minnesota rivers) .
The foregoing 1840 temperature record includes both unadjusted (UNADJ) and adjusted (ADJ) mean temperatures. Unadjusted values are the averages of fixed time readings taken daily at sunrise and at or about 1400 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 1840 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 1840 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 days" record is derived from quantitative precipitation records kept by fort observers during 1840 and/or from notations entered in the remarks section of the 1840 record . The 1840 record contains few quantitative snowfall values: the foregoing monthly snowfall values, accordingly, are estimates (obtained for the most part from National Weather Service meltwater-snowfall conversion tables) . Prevailing monthly wind direction is based on the air movement observations taken by station observers twice daily during 1840: prevailing monthly winds indicate the direction most frequently observed/recorded during any given month. Sky cover distributions, similarly, are based on observations taken twice each day during 1840: 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.
Ft. Snelling precipitation records for March 1840 and June 1840 are ambiguous (due to possible transcriptional and/or recording errors). As a result, total March and June 1840 precipitation values could be 0.65 inches and 3.50 inches, respectively, not 0.25 inches and 3.20 inches, the values indicated in the foregoing compilation. The foregoing December 1840 precipitation value (0.13 inches) could be understated/incomplete: extant records suggest that a substantial but unrecorded amount of snow fell on 25 December of that month. The original December 1840 temperature record includes several implausibly warm afternoon ("PM") readings (e.g. 65 F on 9 December; 72 F on 10 December; 82 F on 13 December; 64 F on 22 December; 56 F on 27 December). These readings may have been the result of improper instrument exposure; instrument malfunction (e.g. column separations); transcriptional and/or observational errors; or deliberate falsifications. The values involved were replaced with statistically estimated values derived by Charles J. Fisk (op. cit.) from the December 1840 Ft. Howard (WI) record.
Cold, cloudy January. Early January warmth followed by extreme cold at mid-month and again late in the month. Sunrise readings of 32 F, -26 F, -25 F, -37 F and -32 F on 10, 15, 23, 24, 25 January, respectively. January snowfall was probably light to moderate. Warm, cloudy and dry February. Sunrise readings of 33 F and 34 F on 5, 18 February, respectively. Rain on 9, 18 February: little snow during the month. Brief cold wave with probable overnight minimum near -30 F on 24 February. Afternoon reading of -12 F on 23 February. Very warm at end of month: afternoon readings of 50 F and 54 F on 28, 29 February, respectively. Sunrise reading of 40 F on 29 February. Warm early March: afternoon readings of 55 F, 55 F, 59 F, 53 F on 1, 2, 3, 4 March, respectively. Sunrise reading of 0 F on 20 March. Sunny and dry early April. Thunderstorm with high winds noted on 22 April: 1.51 inches of precipitation recorded, 22-23 April. Warm May with summerlike temperatures at mid-month. Very dry conditions, 1-16 May. Heavy rain, 17-19 May: 1.7 inches recorded. Afternoon temperatures in the low 90's F recorded each day, 29-31 May. Warm but relatively cloudy June. Sunny but temperate July. Cool, relatively cloudy August. Afternoon temperatures of 66 F, 64 F, 67 F, 64 F and 62 F recorded on 7, 11, 17, 21, 30 August, respectively. Heavy rain on 17 August. Cool, sunny September. Sunrise readings of 40 F and 41 F, 36 F on 10, 11, 17 September, respectively. Cold, cloudy October. Early autumn snowfall (with probable accumulation of about one inch) noted on 1-2 October. Reading of 31 F at 1400 on 2 October. Sunrise reading of 28 F on 3 October followed by rapid warm-up to 64 F at 1400 on the same date. Sunrise reading of 12 F on 26 October. Afternoon (1400 hour) readings of 30 F, 28 F, 24 F and 28 F on 23, 24, 25, 26 October, respectively. Warm early November. Heavy snow, 19-21 November: 1.42 inches of precipitation recorded on those dates. Readings several degrees F below freezing during snowstorm. Sunrise and afternoon readings of -1 F and 7 F, respectively, on 25 November. Warm, dry and very sunny December.