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Donald G. Baker

Abstract

The increased interest and application of heating degree days (HDD) and growing degree days (GDD) prompted this study into the effect of different observation times upon the mean daily temperature. The study was based upon three years of hourly air temperatures measured at St. Paul. These data were used to calculate 1) a true daily mean, 2) a mean of the maximum and minimum between successive midnights as observed at first order stations, and 3) a mean of the maximum and minimum observed at all other hours of the day to simulate cooperative station means. Comparisons of the annual and monthly mean temperatures showed deviations can be of such magnitude as to discourage comparison of station temperatures and temperature-derived quantities such as HDD and GDD unless observation times are the same or corrections are applied.

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Donald G. Baker

The change in average summer, winter, and annual temperatures of 19 Minnesota stations was investigated. Records were studied for a uniform number of years for comparative purposes as well as for the total period of record. Seven of the stations were believed to qualify as climatic reference stations, 11 had records of intermediate reliability, and one was included solely for its long record dating from 1819.

Based on the linear-regression equation, the climatic reference stations showed an average increase in summer (1900 to 1958), winter (1900 to 1958), and annual (1900 to 1957) temperatures of 1.4, 3.7, and 1.3F, respectively. These rises were equivalent to 2.4, 6.3, and 2.2F per century. Stations of intermediate value generally did not show such large increases. Slopes of the temperature regression lines were generally significantly greater than zero for the reference stations and for a few of the others. Records for longer periods showed similar results.

The St. Paul record, originating in October 1819, indicated that the central date of the smoothing period in which summer, winter, and annual temperatures reached a minimum was in 1863, 1854, and 1865, respectively. The latter date appeared to be anomalous compared to east coast records. Recent summer, winter, and annual maxima were obtained in 1934, 1943, and 1933–34, respectively. Winter maximum temperatures at some other Minnesota stations occurred in the early 1930's.

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DONALD G. BAKER

Abstract

Seasonal and annual temperature and precipitation changes at five Minnesota stations considered as climatological reference stations were calculated during several extended time periods. For 1900–58 winter temperatures increased most and a decreasing fall precipitation trend was found. With a longer period, initial date earlier than 1900, fall temperature increases nearly equalled those of winter. A declining, though nonsignificant, seasonal and annual precipitation trend was noted in a majority of the station records.

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Donald G. Baker
and
David L. Ruschy

Abstract

Mean and median daily albedos of the November-April period are presented for a nonforested station in the North Central region of the United States where frozen soil and persistent snow cover are common winter features. Three distinct albedo periods were found, the occurrence of which can be explained by comparison with associated daily records of air temperature and snow depth. These periods are: I) Introduction to Winter, 9–22 November, a transitional period in which snowfalls begin to occur but with insufficient frequency or duration to greatly alter the mean albedo from growing season values; II) the High Albedo Season, 23 November–17 March, that is characterized by mean and median albedos of 50% or higher and by a negatively skewed distribution of albedo values in contrast to periods I and III; and III) the transition period, Introduction to Spring, 18 March–12 April, where late season snowfalls of brief duration occur, but the mean albedo is lower than in period I because of the more common occurrence of moist surfaces due to snowmelt and rains.

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Donald G. Baker
and
David L. Ruschy

Abstract

Incoming and reflected hemispheric radiation were measured at St. Paul over four different surfaces (sod, alfalfa, soybeans, and green peas) for a combined total of 5778 days between 21 November 1969–31 December 1985. Statistical summaries of the calculated mean daily albedos of all surfaces are shown for mouths, seasons and years. There are, in effect, three albedo seasons: the high albedo mason with snow cover (December-February), the low albedo season (April-October), and transitions between the two that occur in March and November. The 1east variation was associated with the low albedo season, increasing from a median value of 18% in April to 24% in October. The median monthly values for the high albedo season ranged between 74%-77%. The greatest variation in albedo values occurred in March and was due to the surface varying between extremes of a fresh snow cover and a bare soil with standing water.

Of the four climate seasons, only winter (December-February) was markedly different from the others. The winter median was 77% compared to a 20%–24% median albedo of the other seasons. The summer standard deviation was about one-fifth that of the other three seasons.

The distribution of albedo values was such that caution must be exercised in the application of the usual statistical measures. Only summer (June-August) exhibited a distribution close to that of a normal one; the mean and median annual values 36% and 24%, respectively, emphasize the degree of nonnormality.

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Blaine L. Blad
and
Donald G. Baker

Abstract

This study is based upon net radiation measurements made with a Suomi-type ventilated net radiometer. The average annual net radiation was 36,247 ly for the study period 1 April 1964–31 March 1967. April–October showed positive net radiation totals, November–February negative net radiation totals, and March had positive net radiation totals in two of the three years studied.

Only fourteen days with all negative net radiation and two days with all positive net radiation occurred in the three-year period. Average daily time of positive net radiation ranged from 244 min in January to 749 min in June. Although positive net radiation exceeded negative net radiation an average of 36,247 ly per year, negative or null net radiation was recorded 63% of the time.

The ratio of net radiation to incoming solar radiation varied widely during the year. However, from May–August the ratio ranged only from 0.45–0.54 for any one month during the three years and averaged 0.50.

Monthly net radiation values calculated by the method described by Sellers were within 10% or less of the measured values for all but two months. Budyko's calculated annual value of ∼35,000 ly also agreed favorably with the average measured value.

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Allen B. Johnson
and
Donald G. Baker

Abstract

This climatological study reports on the potential for atmospheric diffusion at Minneapolis–St. Paul, Minnesota, cities located in the heart of the North American continent. As such, the results can be considered typical of an urban setting within a continental climate.

Data were obtained from a nearly continuous 8-yr record of vertical temperature and wind measurements made on a 152.4-m tower. Temperature lapse rates between 21.3 and 152.4 m were grouped into three stability categories: 1) isothermal–inversion, 2) subadiabatic, and 3) superadiabatic. A subdivision of each was based upon wind speeds of less than 4 m s−1 and greater than or equal to 4 m s−1, resulting in six classes that were examined according to wind direction, time of day, time of year and, most importantly, the associated synoptic conditions.

The isothermal–inversion condition was limited to nighttime periods, especially when high pressure centers were dominant and winds were less than 4 m s−1. The highest frequency of occurrence was during midsummer, while the lowest was during late fall and early winter. The subadiabatic condition was primarily a nighttime phenomenon, except for the winter season when it was also common during the day. An interesting feature of the diurnal frequencies was that a morning and evening subadiabatic peak occurred due to the transition between nighttime stable and the daytime unstable conditions. The superadiabatic condition was mainly a daytime phenomenon and dominated the early afternoon period throughout the year.

The lowest diffusion potential, a result of very stable air and light winds, occurred during the nighttime period, particularly when under the influence of a high pressure center. Weak to moderate diffusion potential, found to occur with weakly stable air and light to moderate winds, was associated with the perimeter of the high pressure center and also with overcast skies near a low pressure center. This condition normally occurred during the night as well as during windy days. Moderate to high diffusion potential, resulting from superadiabatic conditions and light to moderate winds, dominated the early afternoon period. Strong nighttime ventilation was restricted to the winter season when northwesterly winds dominated the region immediately behind a cold front.

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Donald N. Baker
and
Sidney G. Williams

Abstract

A system with the following features has been designed and constructed for logging meteorological data. Wind direction is read once each 15 min. The number of revolutions of each anemometer on a six-anemometer wind system is recorded each period. Integral values of the outputs of 17 millivolt transducers are printed each 15 min in units appropriate to each transducer. The system has a low per-channel cost, and may be built up one channel at a time. The maintenance requirements have been minimal and of a routine nature.

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Richard H. Skaggs
and
Donald G. Baker

Abstract

No abstract available.

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Donald G. Baker
and
David L. Ruschy

Abstract

Freeze-thaw frequencies calculated by eight different counting methods were compared using daily maximum and minimum temperatures from eight north-central United States National Weather Service (NWS) stations. These frequencies were also compared to those obtained using hourly air temperature data from six of the same NWS stations. In addition, the calculated frequencies were compared to measured freeze-thaw frequencies at several depths in a bare soil and a sod-covered soil at the University of Minnesota St. Paul campus climatological observatory.

The necessary acceptance of the idealized daily heating cycle when using daily maximum and minimum air temperature data resulted in a higher occurrence of calculated freeze-thaw events than those obtained with hourly data; one method gave 23% more freeze-thaw events with the daily maximum and minimum temperatures.

With the freeze-thaw phenomenon centered upon those months in which the mean temperature hovers near O°C, a bimodal frequency occurs at the northern stations (October and April, as at International Falls, Minnesota, and November and March at Fargo, North Dakota), while in warmer climates the bimodal characteristic is replaced by a single-peak frequency in January as at Sedalia and West Plains, Missouri.

In the comparison between the calculated freeze-thaw frequencies based on daily maximum and minimum values and the hourly temperature measurements at several heights between the surface and the temperature shelter at the climatological observatory, it was found that the annual total frequencies increased as the height above the surface decreased. For the shallowest height above the surface there was an approximate 13% increase over those measured in the shelter with hourly temperature data.

The annual total frequencies of the calculated freeze-thaw events obtained with the daily maximum and minimum temperature measurements in the shelter approximated those actually occurring at the 1-cm depth in a bare soil at the climatological observatory. Results also indicated that on the same day as a thaw at 1 cm the average shelter maximum temperature was 6.3°C, while for a freeze at the same depth the average shelter minimum was −3.5°C. These values are explained, in part at least, by the frequent lack of direct relationship between the timing of a soil freeze-thaw event in a day and the occurrence of the daily maximum or minimum air temperature.

The freeze-thaw phenomenon extended below 10 cm in a soil bare of vegetation at St. Paul but averaged only about eight events per year at both 10 and 20 cm, compared to about 62 occurrences at 1 cm; even at 5 cm the average was less than 17 per year. Once frozen to a depth of about 20 cm, the soil remained frozen throughout the winter.

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