Technical Bulletin 243 March 1963 Climate of Minnesota PART I. PROBABILITY OF OCCUR- RENCE IN THE SPRING AND FALL OF SELECTED LOW TEMPERATURES Donald G. Baker and Joseph H. Strub, Jr. University of Minnesota ‘Agricultural Experiment Station ’ , Nearly every farm in Minnesota today shows some practical result of the work done by the Agricultural Experiment Station. This unit of the Institute of Agriculture of the University of Minnesota car- ries on research in many widely varied fields. On the one hand, it seeks to develop new and better farm or home practices, crops, and animals. On the other, it seeks to increase our basic knowledge of nature, an end invaluable in itself. The Experiment Station has about 300 research projects, ranging from improved diets for the family to better use of the products of our forests, and from a study of disease in plants or animals to the dis- covery of new markets for agricultural products. Results of thii re- search are made available to the public through resident teaching facilities on campus, or through the off-campus work of the Agricul- tural Extension Service and its county agent organization. The research is carried on at many places in the state. Some of it takes place in the laboratories and fields of the University’s St. Paul Campus. Some is conducted at the Agricultural Experiment Station at Rosemount, or at the branch experiment stations-Crook- ston, Duluth, Grand Rapids, Lamberton, Morris, and Waseca. Specialized research is carried on at the Fruit Breeding Farm and Arboretum, Excelsior; at the Potato Breeding Farm, Castle Dan- ger; at the Forest Research Center, Cloquet; at the Hormel Institute, Austin; and at the Biological and Forestry Station, Lake Itasca. In addition, hundreds of Minnesota farmers each year cooperate with the Station in experiments on their own farms, either as individuals or in groups that may represent several counties. Agricultural research in Minnesota has a broad scope. And its results underlie every major advance we make in modem agrlcul- tural knowledge or practice. This publication is one of the many research reports issued by the University of Minnesota Agricultural Experiment Station. Climate of Minnesota PART. I. PROBABILITY OF OCCURRENCE IN THE SPRING AND FALL OF SELECTED LOW TEMPERATURES Donald G. Baker and Joseph H. Strub. Jr. University of Minnesota Agricultural Experiment Station F concern to agriculture, to other industries, and pursuits of man de- 0 pendent upon weather, is the spring date after which the season will be free of certain low temperatures, particularly freezing temperatures. Similarly there is need for information when these same temperatures may first be ex- pected to occur near the end of the season. The objective of this bulletin is to provide such information based upon a climatological prediction. Donald G. Baker is assistant professor, Department of Soil Science. Joseph H. Stmb, Jr., is state climatologist, U. S. Department of Commerce Weather Bureau. 3 The usual weather forecast is based In resume, the meteorological forecast upon both the dynamics of the atmos- is of aid only in the very immediate phere and the projection into the future future for planning activities dependent of past weather events. The projection upon weather. The climatological or into the future of past weather events probability prediction, as in this bulletin, is termed a climatological prediction. is of value for the long term scheduling of such activities. These two methods, Weather forecasts, issued in daily ad- used either in combination or separately, visories through the various news media are the major available means of reduc- by the U. S. Department of Commerce ing some of the natural hazards of Weather Bureau, aid the short term plan- weather. ning of activities subject to the vagaries of weather. These forecasts are still re- stricted to not longer than 2 to 5 days due to our limited knowledge of the at- Source of Data and Station Location mosphere. Although the limited forecast Data upon which this bulletin is based were obtained from 75 U. S. Department period is a disadvantage, such forecasts can be quite precise as to the magnitude of Commerce Weather Bureau stations in Minnesota. A major portion of the com- and timing of various meteorological events. putations were made by the Weather Bureau for a regional climatic study (6) The climatological prediction, a state- Computations made by the authors in- ment as to the probability or liklihood of clude occurrence probabilities for some a certain meteordogical event occurring additional stations and all temperatures in the future, is based on the past greater than 32” F. meteorological record. This method per- Station location and the station index mits a prediction to be made for any are shown in figure 1 and table 1, re- period in the future. However, the dis- spectively. advantage is that the prediction lacks the All stations had records in excess of specificity of the short period meteoro- 25 years except Big Falls, Duluth (air- logical forecast, and it does not differenti- port), Hinckley, Moose Lake, and ate the occurrence of events for any one Springfield. particular year. The lack of specificity of the climato- Selected Temperatures logical prediction is an important point Temperatures for which probability of that must be understood. As explained occurrence were determined include 16”, in detail later, the prediction gives the 20”, 24”, 28”, 32”, 36”, 40”, and 50” F. probability (chance) that over a period With one exception, the frequency with of years a particular event may be ex- which each temperature was found to pected to occur by a certain date. Thus, occur met the necessary statistical con- over an interval of a number of years siderations and, therefore, the probability the prediction will be valid, but not of occurrence could be readily calcu- necessarily when applied to any given 1ated.l The exception was the 50” F. year. Obviously such predictions are of temperature threshold (the last occur- no use in scheduling daily activities, but rence in the spring or first occurrence in rather in the overall planning of general the fall), for which only the average activities for a period of years. date of occurrence was computed. 1 Normality of the threshold temperature distribution was tested by a recommended method (6) for selected temperatures and stations plus the simple expedient of plotting occurrence dates on logarithmic paper. Variances were not pooled due to the great variation between stations, which is very likely a microclimatic effect and should not he masked. The problem of missing data was not encountered. 4 Low Temperature Injury To Plants be considered: There are two kinds of low tempera- 1. Plant damage increases with dura- ture injury to which plants may be sub- tion of the low temperatures. jected: winter injury and freezing injury. 2. A rapid temperature drop is more The former occurs when plants are dor- injurious than a gradual one. mant. Often such injuries occur when 3. There is a great variation between , insufficient snow cover permits deeper plant species in their hardiness to penetration of low temperatures into the cold. For example, woody plants soil. are usually far more hardy then Freezing injury, however, occurs when herbaceous plants. air temperatures are near or below the 4. Due to natural or artificial selection, freezing point and plants have either within a particular species certain started to grow in the spring or have varieties are less subject to damage not yet entered the dormant period in than others. the fall. This bulletin, as a guide to pro- 5. The stage of growth and develop- per planning, may be of aid in avoiding ment is important since a young such injury. plant is more tender than a mature. Three low temperature terms which one. Certain plant parts, such as h ave been or still are in current usage blossoms and fruits, are more sus- require explanation: frost, black frost, ceptible to damage than others and killing frost. (table 2). Frost is similar to dew except that the 6. Succulent plants and high moisture surface on which it forms is at or below content seeds are less hardy than the freezing point. Therefore, the deposit those low in moisture (table 3). is composed of ice crystals produced by Because of the complex of factors sublimation directly from water vapor to that influence low temperature in- ice crystals. Permanent damage to the jury to plants, it is impossible to plant will not necessarily occur unless assign to a particular plant a speci- the temperature is low enough that ice fic temperature as being critical. crystals also form within the plant tissue. Thus a range in temperatures is Black frost refers to below freezing sometimes given as in tables 2 and temperatures which result in permanent 3, or plants are classified into damage to the plant when frost is absent. relative scales of hardiness as in The damaged plant or plant parts turn table 4. black. There is no frost formation, how- ever, because the moisture content of the Causes of Low temperatures air is too low for sublimation to occur. Two meteorological situations cause Killing frost, a term once widely used, low temperatures. One occurs when a lacks preciseness. It referred to the case cold air mass replaces a warmer one where permanent damage occurred to a (frequently termed advective cooling) . plant, whether or not frost occurred. Temperatures often drop rapidly with the None of these terms is explicit with appearance of the cold air mass. They respect to the temperature at which it may not begin to rise for several days, may occur. Further, the temperature of a and then often rise slowly.