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Climate of North Dakota

Temperature


Maps of normal temperature and precipitation were based on the 1931-60 time period. Analysis of the maps was based on data obtained from 60 climatological stations in North Dakota and 80 climatological stations in surrounding states and Canada. This network of 140 stations permitted detailed analysis of precipitation and temperature patterns in the state so that some of the more prominent topographical effects were revealed.

General Summary

The normal average annual temperature in North Dakota ranges from 37° F in the northeast to 43° F along the southern border. January is the coldest month with average temperatures ranging from 2° F in the northeast to 17° F in the southwest. July is the warmest month with temperatures averaging 67° F in the northeast to 73° F in parts of the south. The range of normal average monthly temperatures between the coldest and warmest months is 54° F in the southwest and 65° F in the northeast. These large annual ranges attest to the continental nature of North Dakota's climate.

The highest temperature ever recorded in North Dakota was 121° F at Steele on July 6, 1936, and the lowest temperature measured was -60° F at Parshall on February 15, 1936.

Temperatures of 100° or higher occur nearly every year somewhere in North Dakota. Chances of this occurring are greatest in the south central area where in about 85 percent of the years maximum temperature will equal or exceed 100° F. These temperatures of 100° F or more last only for a day or two. In the northeast, temperatures reach 100° F or higher in only three years out of 10.

Annual

The normal temperature for the year in North Dakota ranges from less than 37° F in a small area in northern Cavalier and Towner counties in the northeast to about 43° F in the extreme southeast and southwest corners of the state (Figure 2).

The annual temperature pattern reflects several features which are found in most maps of normal monthly temperature. Three warm air ridges oriented north-south are evident. Two of the warm air ridges are located near the eastern and western borders of the state, while the other is positioned through the central part of the state extending sharply northward from Sioux county into Renville and Bottineau counties. The ridge of warm air along the eastern border results in temperatures on the North Dakota side of the Red River Valley being warmer than on the Minnesota side.

Two troughs of colder temperatures are sandwiched between the three ridges. The western trough extends southeastward from Burke county into McLean county before curving southwestward into Slope county. The eastern trough extends southward from Cavalier county into eastern Eddy and Foster counties before curving southwestward into Logan and McIntosh counties through Stutsman county. The annual map shows no clearly defined cold pockets, but on some monthly maps the cold temperature troughs break down into well-defined cold pockets.

The wave-like variations in the temperature patterns are primarily the result of differences in terrain elevations. Topographic maps of the state show that the troughs of lower temperatures generally coincide with the orientation of the higher terrain in the state with respect to surrounding terrain. The effect of terrain to produce cold pockets at certain times of the year is well expressed over the hills in Logan and McIntosh counties, the higher elevation east of the Turtle Mountains in Cavalier and Towner counties, and in the west central part of the state in McLean and Mountrail counties.

Seasonal Temperatures

Winter

Winter is the grandest season of all to many North Dakotans. It is the season for snowmobiling in open and uncrowded fields, for such vigorous outdoor recreation as ice skating and ice hockey, for basketball, and for visiting with friends and neighbors. There is no medicine like a crisp North Dakota winter morning to put spring and vigor into the steps of old and young alike.

In winter the isotherms (lines of same temperature) are oriented in a general northwest-southeast direction. The temperature gradients show the coldest temperatures in the northeast corner of the state and the warmest in the southwest (Figure 3, 4, 5). Winter temperatures average 11 to 15 degrees warmer in the southwest corner of the state than in the northeast. Normal monthly winter temperatures in southwestern North Dakota compare with those found in northern Iowa and upstate New York, but average winter temperatures in the northeast corner of the state are among the coldest in the contiguous United States. The southwestern part of the state is warmest because warm chinook winds occasionally overspread the area from the lee slopes of the Rockies, and because the centers of high pressure systems in which the coldest temperatures are usually found move through the eastern part of the state much oftener than through the western portion. January is the coldest month, but in late January temperatures begin to rise toward summer highs.

The Turtle Mountains in the extreme north central part of the state average only 400 to 800 feet higher than the surrounding area; yet, their abrupt rise from the prairie plains produces significant irregularities in temperature patterns. Compression warms the prevailing northwesterly flow of air descending the southeastern slopes. On the other hand, cold air drainage into the lower reaches of the Souris river basin creates a cold pocket southwest of the mountains.

In December and February, the coldest area in the state is found over the higher elevations in Cavalier and Towner counties. In January, the extreme northeast corner of the state is coldest with temperatures averaging about 2° F.

Spring

Spring is a time of rapid change in North Dakota. It seems that almost overnight in late March or early April the snow mantle melts, the fertile soils dry and field preparation begins, the grass greens and suddenly all surroundings are pulsating with life and color replacing winter-stillness and winter-white.

Winter and spring are separated by the transitional month of March (Figure 6). This is clearly revealed by the temperature pattern across the state. In March, the temperature lines have lost some of the strong northwest to southeast orientation because the northern and eastern portions of the state have warmed more rapidly than the southwest. The temperature range across the state in March is only about eight degrees.

April is the month in which the most rapid warming occurs (Figure 7). Average monthly temperatures in April are 16 to 19° F higher than March temperatures. Although temperatures average above freezing, freezing temperatures are still common at night throughout the month. In April, the summer temperature pattern is well established with temperature lines generally oriented east-west. Wavelike fluctuations in the temperature lines are produced mostly by changes in terrain elevation with cooler temperatures found over the higher terrain.

In May, the terrain effects are especially noticeable with strong north-south troughs of cold temperatures marked by cold pockets (Figure 8). One pronounced cold pocket elongated northeast-southwest is centered in west central McLean county and another cold pocket is found over Logan and McIntosh counties. Warm north-south ridges are found through the central part of the state and along the eastern border, while another ridge of warm temperatures protrudes into Mountrail county through McKenzie county, Also in May, the strong winter temperature gradient, characterized by the northeast tip of the state being much colder than the southwestern portion, has been eliminated. Since the northeast warms faster than elsewhere, temperatures in the northeast and southwest corners of the state are about the same. The last freezing temperatures of the season usually occur about mid-May.

Summer

North Dakota's delightful summer season is at its best in June, July and August, and is perfect for all outdoor activities. The days are warm, sometimes even hot, but nights are one-blanket cool for restful sleep.

The summer temperature patterns for June, July and August are nearly identical featuring strong north-south ridges of warm temperatures in the central part of the state and on the eastern and western borders with cooler north-south troughs between the ridges (Figure 8, 9, 10). One trough of cooler temperatures extends from Burke and Renville counties south-southwestward to the southwest corner of the state. The axis of the other trough of cooler temperatures is located from Towner and Cavalier counties to McIntosh county through eastern Eddy and Foster counties. In July and August, a pronounced cold pocket has formed in the cold trough over the higher ground in Logan and McIntosh counties.

In the summer months, the coolest areas in the state are located along the Canadian border in Towner and Cavalier counties and in Renville and Burke counties. The warmest areas in the state are in the extreme southeast and south central with temperatures averaging in the low 70's in July and August.

Fall

The first frost of fall, which usually occurs about mid-September, signals the end of summer and serves notice that about two months are left to prepare for winter. Fall is the favorite season for many North Dakotans as the days are pleasantly warm, the nights cool and precipitation light. The warm days are perfect for enjoying such activities as completing the harvest, hunting and fishing, and for preparing gardens and fields for next year's flowers and crops.

The transition of the monthly temperature patterns in fall from a summer to winter pattern is not nearly so abrupt as from winter to summer in spring (Figure 12, 13, 14). By November, however, the temperature lines are beginning to assume the characteristic wintertime northwest-southeast orientation although temperatures in the southeast are nearly the same as those in the southwest. One of the more interesting features of the fall maps is the continued intensification of the warm temperature ridge in the Red River Valley in September and October. This intensification is significant for agriculture. It is the eastern part of the state and particularly in the rich Red River Valley where most of such full-season crops as sugarbeets, corn, sunflowers and soybeans are grown. Higher rainfall and the additional late season warmth provided by the ridge intensification helps to mature these full-season crops.

In October and November, two large cold pockets stand out on the map. One is centered in Towner and Cavalier counties in the northeast corner of the state, and the other in McLean and Ward counties in the west central portion. In November, temperatures average below freezing, and the first sub-zero temperatures of the winter sometimes occur in late November.

Growing Season Temperatures

Maps of average growing season temperatures covering periods representative of such short season crops as small grains and for such full-season crops as corn, potatoes and sugarbeets show the same general patterns as the annual temperature maps (Figures 15 and 16).

The average temperature from April through July ranges from 54° F in Cavalier and Towner counties to 62° F in the extreme southeast. Temperatures along the western border of the state average near 58° F, but in the Red River Valley in the east, temperatures range from 56° F in the north to 62° F in the south. The closeness of the isotherms in the southeast corner indicates strong warming over a short distance. This may be an important reason why that part of the state is the only area that produces corn for grain. Not only do higher average temperatures result in faster growth, but the sandy soils on which the corn is grown warm more quickly and allow the crop an earlier and faster start. The considerable amount of corn grown in other areas of the state is used mostly for silage.

It is interesting to note that while the rest of the state warms about two degrees from April through September when compared to the April through July temperatures, the southeast corner remains at the same average temperature. This warming is probably a disadvantage in most years to much of the state because of increased evaporation demands from the soil and vegetation at times when soils are frequently becoming moisture deficient. But it is of decided advantage in the Red River Valley where most of the long season crops are grown and soil moisture is usually enough to produce good crops. Higher temperatures in the Red River Valley result in increased growth rates and higher yields.

Extreme Highelst and Lowest Temperatures

Temperature readings from maximum thermometers housed in weather instrument shelters show that the central and south central portions of the state are the areas where the highest temperatures have occurred (Figure 17). On rare occasions temperatures of 114° F or above have been recorded mainly in the southern half of the state, while temperatures of 118° F or above have been read in only a small area in the south central and southeast portions of the state. On the other hand, temperatures as high as 110° F have never been recorded in a sizable area in the northern area of the state and in the extreme west.

All record temperatures are, of course, associated with extremely warm or cold air masses which cover thousands of square miles. For this reason, record high or low temperatures are recorded at many weather stations in the United States on the same day. The record high temperatures shown on the map are no exception with most of the extreme high temperatures being recorded during the July, 1936, heat wave and particularly on July 6, 1936.

Lowest temperatures on record have been recorded in the north and central areas of the state (Figure 18). Several areas stand out that are colder than their surroundings. These areas usually reflect the topography. The coldest area in the state so far as record low temperatures are concerned is centered in the northwest in McLean and Mountrail counties and encompasses the area where the extreme low temperature of -60° F was recorded. Other areas not quite so cold but where temperatures have dropped to about -54° F are located near the Canadian border and in the central portion of the state.

Many record lows came during the period February 14-16, 1936. Other cold air outbreaks which produced several record low temperature marks about the state occured on January 12, 1913, and on January 13, 1916.

North Dakotans enjoy the cold winters or at least tolerate them with casual indifference because of such modern conveniences and equipment as petroleum-burning furnaces, indoor plumbing, farm stock-watering systems, farm shelterbelts, automobiles with powerful heaters, good highways and good snow removal equipment, and well-designed winter clothing. Enjoying winter as they do, North Dakotans are somewhat prone to proudly exaggerate the intensity of winter cold snaps and blizzards as they regale friends and relatives from warmer climates about the rigors of winter.

Pioneers retold their stories of the cold winters in the 1880's and of the famous blizzard of January 12, 1888 for all who would listen, while modern "oldtimers" still speak reverently of the winter of 1936. Even though the truth may be stretched a little at times, it is a matter of record that during the winter of 1935-36 a winter temperature record at Devils Lake was established that has no equal in the weather history of official Weather Bureau stations in the contiguous 48 states. At Devils Lake, the temperature dropped below freezing on November 27, 1935, and did not rise again to the freezing point until March 1, 1936, a period of 96 days. For 37 days, January 14 to February 19, there was only one day on which the thermometer registered as high as zero, while for the week ending February 17 the average temperature was -28° F, and for all January and February, -13° F.

Residents in the small town of Munich in northeast North Dakota have modestly accepted the title given them by the New York Times as the "Cold Capitol of the USA." According to the Times, Weather Bureau studies of winter temperature records for official weather stations in the contiguous 48 states for the period 1951-60 showed that winter temperatures at Munich averaged 6° F for December, January and February. This was colder than at any other location in the United States, much to the chagrin of imposters in neighboring northern Minnesota and elsewhere who had made immodest claims that their location was the coldest anywhere in this country.

Highest and lowest temperatures recorded in North Dakota are similar to those recorded in other nearby states. Other plains states have recorded high temperatures ranging from 118° to 121° F, compared to North Dakota's high temperature of 121° F, while extremely high temperatures in the 114° to 118° F range are common in the midwest states. The -60° F temperature recorded at Parshall is the lowest temperature ever reported east of the Rockies, but temperatures as low as -59 have been measured in Minnesota, -54 in Wisconsin, -52 in New York, -51 in Michigan, and -48 in Maine.

Annual Number of Days of 90° F or Above

There is a large range in the number of days during which temperatures reach or exceed 90° F each year (Figure 19). Along the Canadian border in the northeast, maximum temperatures of 90° F or more occur only on eight days in an average year, while in parts of the southwest and south central temperatures equalling or exceeding 90° F can be expected on 32 days. There is also a pronounced wedge of days when temperatures equal or exceed 90° F extending northward from south central North Dakota into Bottineau county on the Canadian border. In an average year, about 75 percent of the days with temperatures equalling or exceeding 90° F occur in July and August. Regardless of location, the number of days with temperatures of 90° F or above are nearly the same in July as in August.

The chances of any area in the state having five consecutive days of temperatures above 90° F are greatest during the period from July 19 to August 1. During this period, temperatures will exceed 90° F for five consecutive days in only one year in five in a small area near the South Dakota border in the south central and southwest and only one year in 10 in the northern half of the state. Temperatures exceeding 90° usually occur when humidity is low and are not so disagreeable as similar temperatures in the more humid states.

Annual Number of Days of 32° F or Below

Temperatures of 32° F or lower are measured on about 180 days in the extreme southeast and extreme west central, and on about 210 days at some locations along the Canadian border (Figure 20). Several areas can be spotted on the map (using data for the period from 1951-60) which record more days of freezing temperatures than surrounding areas. These spots occur largely because they are on higher ground than the surrounding areas.

Annual Number of Days of 0° F or Below

Temperatures of 0° F or below are registered on only 35 days in the extreme southwest to more than 65 days along the Canadian border in the north central portion (Figure 21). The map showing patterns of days with 0° F temperatures or below was based on the total period of record for the stations used.

Slightly more than 80 percent of the zero or subzero temperature readings occur during the months of December, January and February. Subzero temperatures have occurred as early as late October or as late as early April, but occurrences of subzero temperatures in these two months are infrequent. Most days of 0° F or below occur in January when 16 to 23 days with zero or subzero temperatures can be expected, depending upon location.

Lowest average temperatures occur in late January. During this time there is a 40 to 45 percent chance that in the southern part of the state temperatures will drop below zero at some time each day for five consecutive days, while in the northern third of the state, there is a 60 percent chance that this will occur. Further, during the coldest part of the winter there is only a 10 percent chance that temperatures will drop below zero at some time each day for 15 consecutive days in the southern portion of the state, but slightly more than a 20 percent chance that this will occur in the northern portion of the state.

Diurnal Temperature Range and Construction of Annual Temperature Curve

The diurnal temperature range is the temperature difference between the minimum and maximum temperature observed over a 24-hour period. For any given month, differences in average diurnal temperature ranges due to location in the state are generally less than two degrees. The data in Table 1 show that in late summer and early fall the average diurnal temperature range is 27 to 30 degrees, while in winter the diurnal temperature range is 19 to 23 degrees depending upon month and location.

Table 1. Average diurnal temperature range by months for climatological division.

Month NW NC NE WC C EC SW SC SE
January 21 22 21 21 21 20 22 22 21
February 22 22 22 23 21 20 23 22 21
March 21 22 21 22 21 20 22 22 21
April 25 25 24 26 24 24 26 26 26
May 27 28 28 27 27 27 27 27 27
June 26 26 25 26 25 25 26 26 25
July 29 28 27 29 28 26 30 29 27
August 29 30 29 29 30 28 30 30 28
September 28 29 28 28 29 27 30 30 29
October 27 27 25 27 27 24 28 28 26
November 20 21 19 20 20 19 23 22 21
December 20 21 20 20 20 19 22 21 20
Annual 24 25 24 25 25 23 26 26 24

Combining the diurnal temperature range with maps of average monthly temperature allows one to construct curves of annual average temperature, average maximum temperature, and average minimum temperature for any location in the state with sufficient accuracy for nearly all purposes.

As an example of this procedure, assume that we wish to construct an annual temperature curve for Buxton in northeastern Traill county, a location for which no current weather observations are available. The following steps describe the procedure for constructing such a curve.

Step. 1. Make a table similar to Table 2 to be filled in with the appropriate numbers. The first set of numbers to be filled in are the monthly diurnal temperatures for the climatological division in which the location is found. These temperatures are in Table 1. Buxton is in the east central division (Figure 54). Diurnal temperatures by months for the east central division are then copied into the table on the average diurnal temperature line.

Table 2. Table used to construct annual temperature curves.

Temperature Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Avg. Maximum 16.0 20.3 34.0 53.9 69.0 76.9 84.2 83.1 71.8 58.8 36.7 22.9
Avg. Monthly 6.0 10.3 24.0 41.9 55.5 64.4 71.2 69.1 58.3 46.8 27.2 13.4
Avg. Minimum -4.0 0.3 14.0 29.9 42.0 51.9 58.2 55.1 44.8 34.8 17.7 3.9
Avg. Diurnal 20 20 20 24 27 25 26 28 27 24 19 19
Step 2. From the maps of normal monthly temperature in this publication, copy into the table across from average monthly temperature the interpolated values of average monthly temperature for the location desired, in this case for Buxton.

Step 3. To compute the average monthly maximum and minimum temperatures for each month, divide the average diurnal temperature range for the month by two. Add this amount to the average monthly temperature to obtain the average monthly maximum temperature. Then subtract the same amount from the average monthly temperature to obtain the average monthly minimum temperature. As an example, the diurnal temperature range for Buxton in January is 20. Dividing 20 by 2 gives 10 which is added to the average monthly temperature of 6.0° F to obtain an average maximum temperature of 16.0° F. Next, 10 is subtracted from the average monthly temperature of 6.0° F to obtain an average minimum temperature of -4° F.

Step 4. Prepare a graph such as the one in Figure 22. Plot the value of the average monthly temperature, the average monthly maximum temperature, and the average monthly minimum temperature for each month on the 15th of that month. Then connect the points with a smooth line so that the lowest temperature occurs about January 28 and the highest temperature about July 28. From this graph the average temperature for any day of the year can be obtained.

Length of Freeze-free Period

The average length of the freeze-free period in North Dakota ranges from about 110 days in parts of the northeast and north central to about 130 days in the southeast and south central (Figure 23). The freeze-free period is calculated as the number of days between the average date of the last occurrence of 32° F or lower in the spring and the average date of the first occurrence of 32° F or lower in the fall.

The length of the freeze-free period may or may not be synonymous with the length of the growing season although it is a reasonable approximation. It should be remembered that the 32° F temperature referred to is the air temperature measured in an instrument shelter at a height about five feet above the ground. It is not unusual on clear, still nights to measure air temperatures near the ground which are as much as three to seven degrees lower than the air temperatures registered in the shelter. For this reason, it is sometimes possible to see frost, even a "killing frost", on vegetation when the temperature in the instrument shelter stayed above 32° F.

Length of the freeze-free period is affected by the overall topography as discussed earlier for average monthly temperature. For example, the average monthly temperature maps show the pattern repeated in nearly all months of warm temperature ridges on the eastern and western borders and through the central portion of the state with troughs of cooler temperatures sandwiched between the ridges. This pattern is also reflected in the average length of the freeze-free period which shows that the longer periods of freeze-free days are also on the eastern and western border and through the central portion of the state.

An interesting comparison can be made between the average length of the freeze-free season (Figure 23) and the April through September average temperature (Figure 16). As a first approximation, the 57° F, 59° F, and 61° F average temperature lines on the map for the period April through September (Figure 16) nearly coincide with the 110, 120 and 130-day lines, respectively, on the average length of freeze-free period map. Thus, the average growing season in North Dakota increases about five days for every degree of average temperature increase for the April through September period.

The average spring date when the last temperature of 32° F or lower occurs ranges from about May 15 in parts of the extreme southeast and south central areas to about the first of June in small scattered pockets in the northern half of the state (Figure 23). The areas of the state which have the latest average date for freezing temperatures in the spring also generally have the first freezing temperatures in the fall.

In the fall, the first 32° F or lower temperature can be expected between September 15 and 25 over most of the state, although the first freezing temperatures in the fall can be anticipated as early as September 10 in parts of Ward, Mountrail and McLean counties. Remember that the dates when temperatures 32° F or lower occur on which the maps were based are averages. That is, in half of the years the first freezing temperatures can be expected to occur before the date shown and in half of the years the freezing temperatures can be expected to occur after the date shown.

Local topography can have a significant effect on the length of the growing season, as well as on the average dates of the last freeze in the spring and the first freeze in the fall. The reason for this is that on most nights on level terrain, cooler and heavier air lies immediately on the ground. Above it is a layer of warmer, lighter air. There is little tendency for the adjacent air layers to mix. On undulating terrain, however, cool air near the ground flows down by gravity from the higher parts of the terrain. If the depressions are closed, such as the so-called "prairie pot-holes," the cold air flowing down hill accumulates in the depression to form cold air reservoirs much in the same way that water accumulates in a pot-hole. Temperatures in the depression may be several degrees colder than over nearby areas which are a few feet higher. Crops in these depressions often are the first to be injured by freezing temperatures, giving rise to what many farmers refer to as "frost pockets." In North Dakota, small local "frost pockets" are found most often in the glacial till areas of the state.

Soil Temperature and Frost Penetration

Soil temperature and frost penetration data in North Dakota are scarce. The most complete study of soil temperatures was accomplished by Dr. Guy Wilkinson of the Department of Soils at North Dakota State University who measured soil temperature at Fargo continuously over a four-year period. Results of Dr. Wilkinson's unpublished study are shown in Figure 24. Only the relevant features of the annual soil temperature graph will be discussed because of space limitations.

At Fargo, the average date of soil surface freezing was November 26. Freezing progressed to greater depths throughout the winter until the average maximum frost penetration depth of 4 1/2 feet was reached April 1. Surface thawing in the spring began on March 26, a few days earlier than the occurrence of maximum frost penetration. After April 1, soil thawing proceeded both downward from the surface and upward toward the surface from the deeper unfrozen soil until May 1 when the last of the frozen soil about the three-foot level was thawed.

The lowest average soil temperature of 8.2° F was found at the one-fourth-inch depth on January 17. The time of minimum soil temperature for deeper soil depths was progessively later, with minimum soil temperatures at the 4 1/2 foot depth occurring on April 1. Highest average soil temperature at the 1/4-inch depth reached the low 80's during the third week in July. As in winter, soil temperatures at greater depths reached their highest levels at a later time in the season. For instance, soil temperatures at the two-foot depth did not reach their highest levels until about August 6, while three-feet deep maximum temperatures were reached August 15.

The late Frank J. Bavendick of the Bismarck Weather Bureau reported frost penetration data in his book, "North Dakota Climate and Weather". Mr. Bavendick stated that the average frost depth in the state was 50 inches for the unspecified period during which records were kept. The average frost penetration ranged from 36 inches in the extreme southwest corner of the state to 60 inches in the northern part of the state. It is interesting to note that the average frost penetration of 4 1/2 feet given by Bavendick for Fargo corresponds to the same depth Wilkinson found many years later in his study. Table 3 is taken from Bavendick's book.

Table 3. Frost Penetration in North Dakota (after Bavendick, 2).

Station Penetration
Average (feet) Extreme (feet)
Bismarck 4.5 7.0
Devils Lake 3.5 6.5
Dickinson 4.0 6.0
Ellendale 4.0 6.0
Fargo 4.5 6.0
Grand Forks 4.5 7.0
Jamestown 4.5 6.0
Minot 4.0 6.5
Williston 4.5 6.5

The freezing depth of a soil depends mainly on (1) the winter air temperature; (2) the soil itself including moisture content, type of soil, and the presence or absence of vegetative cover; and (3) snow cover, including its presence or absence, depth and density. Dry soils tend to freeze deeper and faster than wet soils. Snow acts as an insulator between the air and soil. The insulating quality of snow is well illustrated by data obtained by the author during the early morning hours of December 31, 1968, in a plowed wheat field (Figure 25). Temperature at the standard measuring height of about 66 inches was -31° F. It was a beautiful, still night and the stars seemed even closer than usual as their light made its way through the incredibly clear and clean Arctic air mass. Radiation from the snow cover caused the air temperature to decrease from -31° F at the standard measuring height to -33.8° F immediately above the snow surface. The temperature increased rapidly through the 5 1/2-inch snow pack from the -33.8° F at the upper surface of the snow cover to 13.6° F at the soil surface, an increase of 47.4° F over a distance of only 5 1/2 inches. Soil temperature near the surface was 13.6° F, but increased rapidly to near 32° F at the 10-inch depth. Although daily minimum air temperatures for the 10 previous days had ranged from +3 to -25° F, the insulating qualities of the 5 1/2-inch snow cover had limited frost penetration to only 10 inches.


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