Hail Days Research Articles

CAPE values and hailstorms on northwestern Spain

A study has been carried out in León (in the northwest of the Iberian Peninsula) on atmospheric convection during summer periods, by analyzing the values of the Convective Available Potential Energy (CAPE) around 07:00 UTC. The project analyzed the data provided by a network of voluntary observers, a hailpad network, and a meteorological station on a sample of 224 days. The CAPE values found were not high: they never reached 2000 J/kg, not even on hail days, i.e. on days with high convective activity. These values are much lower than the ones measured in convective situations in tropical regions, but they are within the usual values found in Europe. The same happens with the wet bulb potential temperature measured in León. The frequency distribution of the CAPE values shows a clear prevalence of very low or zero values. The group of days with the lowest CAPE value is that which included days with no storm. The mean value increases on storm days, and it is even higher for the days with recorded hailfalls. All these differences are significantly marked. Nevertheless, the differences across the years are not significant enough to be able to speak of an influence of the climatic change on the CAPE. The correlation of the CAPE with some of the variables previously used for hail forecasts was analyzed, and the correlation found was higher for the lifted index. The possible use of the CAPE as a thunderstorm and hailstorm forecasting method was considered. The results were encouraging, especially for hail forecasting, although the CAPE should not be used as the only variable, but combined with other parameters. Moreover, the relationships between the CAPE and the wet bulb potential temperature and between the CAPE and the physical parameters of the hailstones were also analyzed. A relationship was observed with the parameters of the hailstone size distribution. Nonetheless, these results are provisional, and they should be confirmed by analyzing a more representative sample. With a more detailed analysis of these and other relationships, the present forecast model used by the Laboratory for Atmospheric Physics at the University of León is expected to be greatly improved by including the CAPE in this model.

A Physical Evaluation of a Hail Suppression Project with Silver Iodide Ground Burners in Southwestern France

Abstract The large-scale hail prevention program operated by the Association Nationale d’Etude et de Lutte contre les Fleaux Atmospheriques in southwestern France combines the seeding of hail clouds by a network of silver iodide ground generators with a survey of hailfalls by a network of hailpads. Using the joint data from the two networks, a physical method has been developed to measure the change in hailfall severity in the seeded hailstorms. The method associates the number of hailstones larger than 0.7 cm in a point hailfall at the ground (the “hailstone number”) to the amount of seeding material released in the area where the storm was developing just before hailstone growth. The fundamental but not proven hypothesis employed is that a simple negative correlation should exist between the two parameters. During the years 1988–95, 630 point hailfalls were recorded on 53 hail days with seeding. The method indicates that the hailstone number is basically responsive to the amount of seeding material rele...

The episodic occurrence of hail in central Alberta and the Highveld of South Africa: Research note

The frequency and duration of hail day episodes were analyzed based on 29 summers of hail data from central Alberta, Canada and 19 summers of data from the Highveld, South Africa. A hail episode consists of one or more consecutive days with at least one hail report. A severe hail episode is deþned as an episode that has at least one day of maximum hail size ≥ 33 mm for Alberta, (≥ 31 mm for the Highveld). Severe and non‐severe hail episodes are found to occur, on average, 6.6 and 11.2 times in Alberta, respectively, with mean durations of 4.2 and 2.1 days. In the Highveld, severe and non‐severe hail episodes occur, on average, 2.8 and 21.9 times per six‐month summer and have mean durations of 4.9 and 2.5 days.

Surrogate Data to Estimate Crop-Hail Loss

Abstract Crop-hail insurance loss data for 1948–94 are useful as measures of the historical variability of damaging hail in those 26 states where most crop damages occur. However, longer records are needed for various scientific and business applications, as well as information on potential losses in United States’ areas without crop insurance. The long-term (1901 to present) data on hail-day incidences, as derived from National Weather Service historical station records, were investigated to determine if some form of a hail-day expression related well to the insurance losses. The areal extent of insured areas of Illinois, Texas, and Nebraska experiencing growing season frequencies of hail days matching or exceeding the once in 10-yr frequencies was found to have the best relationship with insured loss values. The computed correlation coefficients were +0.97 for Illinois, +0.73 for Texas, and +0.91 for Nebraska. These values appear to be a useful surrogate for 1) estimating pre-1948 loss values, 2) estima...

Crop Damage: The Hail Size Factor

Abstract Between 1986 and 1992 a research project was developed and carried out on hail climatology and the economic repercussions of hail on agriculture in Leon (northwestern Spain). A target area with an extent of 6825 km2 was defined, within which a network of meteorological observers was established at an average density of 1 per 17 km2. A network of 250 hailpads installed in a grid formation was also laid out over an area of 1000 km2 inside the target area. The frequent occurrence of hailfalls—122 hail days over seven consecutive summers—provided a detailed database and allowed several climatological studies to be made. Crop damage was also closely monitored and quantified. Barley and wheat were selected as crops on which to base an analysis of the relationship between hailfall characteristics and crop damage. As the resistance of plants to hailstones is held to vary according to their physiological state, four different stages of plant growth were defined, beginning with the formation of grain heads...

MODELLING HAIL FREQUENCY USING A GENERALIZED ADDITIVEINTERACTIVE TECHNIQUE

ABSTRACT Relationships between hail day frequency (HDF), altitude and latitude in the summer rainfall region of South Africa are investigated non-para- metrically by means of generalized additive models. Two dimensional and three dimensional graphs are produced displaying the qualitative and quantitative character of these interrelationships.

Temporal and Spatial Relations between Hail and Lightning

A study of very detailed spatial and temporal data on damaging hail, associated cloud-to-ground (CG) lightning, and storm echoes in the Midwest was pursued to define their relationships and gain insight on their formation processes. Lightning activity was always closely associated with hailstreaks: the surface embodiment (4.5 km long by 1.3 km wide) of a single volume of hail generated aloft. Lightning seldom occurred where the hail fell and generally formed and moved forward in a 10–15-km2 area on either the left or right flank of the hailstreaks. These lightning centers, areas with CG flashes closely associated with hail, typically developed 9 min before hail at a point 5 km upstorm from first hail, suggesting that CG flashes began as the hailstones were developing aloft. The centers then grew in areal coverage and flash frequency until hail began and diminished shortly after hail ended, with a duration of 26 min. The hailstorm's severity was found to be well correlated to the rate of flashing during the hailfall. More than 75% of all lightning centers in the storms on the studied hail days were not associated with damaging hailfalls, indicating little chance to use lightning activity as a predictor of hail. Multicellular echoes in lines produced hailstreaks with severities twice those generated in single cell storms, and severities were much higher when hail-generating cores exceeded 65 dBZ. Merging of cells produced one-third of the hailstreaks but did not affect hail severity. Hailstreaks and associated lightning centers were generally found in storm cores (>45 dBZ) that developed well before the lightning and persisted well after the hail and associated lightning ended. These were relatively strong storms typical of the summer season in the central United States. The findings may not be representative of storms in other climatic zones. Lightning with hailfalls occurred throughout individual cores and adjacent steep reflectivity gradients, whereas hail tended to fall either along the left or right flank of the cores. Hailstorms occurring with cold fronts had more lightning activity and less complexity than did stationary frontal storms.

Precipitation Climatology in the Pittsburgh Urban Area During Late Spring and Summer

INTRODUCTION. Some climatic changes generated by urban environments are obvious and easy to measure, while others are more subtle and difficult to detect. Some the more evident effects urban areas on atmospheric elements include reduced visibility from air pollutants, generally lower windspeeds due to increased friction, and higher temperatures as a result the urban heat island. (J) Less conclusive are urban effects upon moisture formation in terms ofatmospheric humidity, fog, cloudiness, and precipitation. Although there is general consensus that relative humidity values are lower in cities, due primarily to the existence warmer urban temperatures, (2) and that the cities are relatively moist at night and dry during the day compared to the surrounding countryside, (3) there is considerable debate over the meaning other measures moisture. (4) With respect to urban effects on precipitation, many inconsistencies exist in the literature. Landsberg has stated that of all meteorological elements in the urban environment, precipitation still confronts us with the most puzzles. (5) Perhaps part this inconsistency is due to the fact that precipitation is inherently less coherent spatially when compared to temperature or pressure. (6) When assessing the urban impact upon precipitation, additional difficulties arise in trying to separate urban effects from topographic effects. Urban modification precipitation was first noted in the United States during the 1950s when Landsberg presented evidence for increased precipitation at Tulsa. (7) Significant increases in warm season precipitation, including increases in the frequency rain days, thunderstorm days, and hail days, downwind the Chicago-Gary urban-industrial complex were later presented by Changnon. (8) Known as the La Porte anomaly (since the increases were observed near La Porte, In

Average Spatial Patterns of Hail Days in Greece

The annual cycle of the mean monthly number ot hail days (MMNHD) at a station shore certain characteristics, depending on the prevailinig climate. For example, the main maxima in continental and marine mediterranean climates of Greece occur in spring to summer and in winter, respectively; this is shown in the present paper by subjecting the annual cycle of the MMNHD of each station to Fourier analysis. A gradation of such an annual march of hail occurrence is used in delineating hall regions in Greece; this methodology might also be applied to other countries.

SPATIAL AND TEMPORAL DISTRIBUTION OF INTENSE CONVECTIVE WEATHER — THUNDERSTORMS AND HAIL IN CHINA

Thunderstorms and hail are important features in the climatology of China. Using occurrences of thunderstorm days or hail days, the spatial and temporal distribution of these phenomena are described. Location, relief, thermal activity, and air mass convergence all contribute to seasonal variations and peak concentrations. The Qinghai-Tibet Plateau in summer is the most active thunderstorm area of the Northern Hemisphere at 30° to 35° of latitude, but extreme occurrence occurs in Yunnan Province, with thunderstorms experienced on more than 43% of days during the year. Hailstorms in China usually pioduce small hail which causes minimal damage. Occurrences of squalls, however, may form extensive hailswaths accompanied by large hailstones. These long duration hailswaths are potentially very destructive. Middle and eastern areas of the Qinghai-Tibet Plateau may experience over 30 days a year on which hail occurs. The extreme occurrence of 53 hail days was recorded at Nagu. Although thunderstorm days tend to fo...

Temporal and Spatial Variations in Hail in the Upper Great Plains and Midwest

Abstract The distribution of hail days during 1961–80 in the northern Great Plains-Midwest was evaluated on a temporal and spatial basis to help interpret crop-hail losses. Comparisons with earlier (1901–60) hail day data revealed the seven-state study area contained eight permanent areas of high and low incidences found in any 5-year or longer period. The high hail incidence areas were related either to major topographic features or to areas of frequent frontal occurrences. Certain other areas of high or low hail incidence appeared at random locales, lasted 5 to 20 years, and disappeared. The annual and July incidences of hail increased sporadically but steadily from 1901 to 1980 in the Dakotas, Nebraska and Minnesota, reaching a peak during 1961–80. This has led to relatively more crop damage in recent years. In Montana, eastern Iowa, and Illinois, hail has decreased to a low in 1961–80. During the 1961–80 period, hail maximized in 1961–65, being 30% more frequent than in any subsequent 5-year period.

Errors in Hailpad Data Reduction

A study of the hailfall parameter estimates determined by 11 persons (readers) measuring the hailstone dents on the same aluminum-covered styrofoam hailpads has shown significant differences among many of the readers. The hailpads in the study were collected by the National Hail Research Experiment (NHRE) in 1976. The results apply directly to the pads and the method of data reduction used by the NHRE, but have implications for other projects using hailpad data. The experiment analyzed the estimates of five hail parameters on each of 23 hailpads reduced by the same 11 persons. Point estimates of parameters such as hail mass were found to have relative standard deviations of about 30% when determined by one reader. The effect of using multiple readings is to reduce the error by the factor n-m, where n is the number of readers. The total error of each reader was decomposed into systematic and random components by analysis of variance. The standard deviation of the systematic component was -40% of that of the random component for allfive hail parameters. Analysis of individual size category counts determined by the same readers on the same pads showed that small counts, usually associated with the largest hail sizes, could be in error by as much as 100% while large counts (near 1000) had standard deviation of only 16%. The reader standard deviation exceeded the expected Poisson error for mean counts per category greater than 2. A theoretical statistical model was used to examine the effect of reader variance on areal estimates of hailfall. Areal estimates were found to be much less affected by reader variation than point estimates, notably when the number of hailpads hit by hail exceeded 30. Further, since the majority of the readererror was random rather than systematic, it was not very helpful in reducing the error to divide the pad reading for a hail day among several readers. In evaluating parameters such as mass for an area in a hail suppression experiment it was concluded that multiple readings were not required.

Results of a Randomized Hail Suppression Experiment in Northeast Colorado. Part VIII: The Representative Draw Analysis

A detailed analysis is made of the environmental conditions existing on each of the declared hail days during the randomized seeding experiment. From the many soundings available each day, the one most representative of the near-storm environment is carefully selected. This sounding is then used to compute several parameters known to influence hailfall. It is found that two parameters, both indicative of the thermodynamic instability, have a more instable mean value on the seed days than on the control days that in one case is statistically significant at the 10% level. Correcting for this draw would result in reducing the actual ratios of seed to control hail mass found in the primary statistical evaluation of the experiment. However, the reduction would not be sufficient in relation to the very wide 90% confidence limits to affect the statistical conclusions that the ratios were not significantly different from 1.0. An analysis of the sequences of declared hail days showed that, in spite of the careful experimental design, the random selection process produced an actual partitioning of sequence starts into seed or control such that a sequence this extreme, or more extreme, had a chance of only 3 in 100 of occurring. However, it is not likely that this unexpected draw affected the evaluation of the experiment in any significant way, since it is taken care of indirectly in the analyses of the environmental parameters.

The Scales of Hail

Abstract The first climatic investigations of hail in North America were by Lemons and Flora during the 1940's. These were followed by more intensive, state-scale climatic investigations in the 1960's to meet insurance concerns. Subsequent concerns with hail by the aviation industry and the weather modification community led to the first collection of mesoscale hail data from dense networks and radar studies during the 1960's and 1970's. This paper is a review of available hail information presented in a series of time and space scales. Although the North American hail data and information are less than adequate, there is much more hail information than exists elsewhere in the world. Very extensive findings on hail are available for Alberta, Illinois and Colorado. Phenomenologically oriented studies have focused on hailstones, point hailfalls, hailstreaks, hailstorm, hailswaths and hail days over various sized areas. Results for each of these classifications are presented according to studies that focused...

Application of the Poisson and Negative Binomial Models to Thunderstorm and Hail Days Probabilities in Nevada

Abstract Rare events such as thunderstorm or hail days often fit one of two distributions, the Poisson or the negative binomial. These two models were tested on monthly and annual thunderstorm days as well as on annual hail days at five locations in Nevada. A procedure for calculating efficient estimates of the parameters for the negative binomial distribution, utilizing the iterative process and the second-order polynomial model, is described. Results of the program applied to five sites in Nevada are discussed.

CHARACTERISTICS OF HAIL-PRODUCING RADAR ECHOES IN ILLINOIS

Data from 103 hail echoes on 24 days in 1967 and 50 no-hail echoes from the same days were analyzed to describe hailstorm characteristics and to provide information useful in operational detection and forecasting of hail-producing echoes. Echo characteristics investigated included locations of echo formation and dissipation, echo reflectivities, echo-top heights, echo duration, direction of motion, speed, time of occurrence, and associated synoptic weather conditions. A single hail-echo model could not be derived because of the extreme variability found in all characteristics. However, distinctive echo models could be developed for the three predominant hail-producing synoptic weather conditions, cold fronts, stationary fronts, and low-pressure centers. The frontal hailstorms were faster moving, longer lived, and had taller echoes than those with low-pressure systems. Hail production after echo inception varied from an average of 32 min for low conditions to 59 min for cold frontal echoes. The average hail-echo top exhibited a 5,000-ft growth in the 15-min period prior to the average time of hail, suggesting that a major updraft surge was the prime producer of hail. The no-hail echoes occurring on hail days had characteristics of speed, direction of motion, reflectivity, and location that were very similar to the hail-producing echoes. The only distinct consistent difference between the hail and no-hail echoes in all synoptic situations was that the hail-echo tops averaged between 2,000 and 4,000 ft higher throughout their entire durations.

THE DESIGN AND EVALUATION OF HAIL SUPPRESSION EXPERIMENTS

Abstract A statistical methodology involving the analysis of three basic types of historical hail data on an areal approach is presented for the planning and evaluation of hail suppression experiments in Illinois. The methodology was used to generate nomograms relating the number of years required to detect significant results to 1) type I error, 2) type II error, and 3) power of the test for various statistical tests and experimental designs. These nomograms were constructed for various area sizes and geographical locations within the State. Results indicate that, for an Illinois experiment, insurance crop-loss data are the optimum hail measurement if the study area has more than 60 percent insurance coverage. The optimum experimental design is the random-historical design in which all potential storms are seeded on a particular day, and 80 percent of the forecasted hail days are chosen at random to be “seeded days.” The recommended statistical analysis is the sequential analytical approach. If, however,...

Hail Measurement Techniques for Evaluating Suppression Projects

Abstract Collection of hail data that will provide meaningful measures of the results of hail suppression projects varies according to five factors, including the geographical-climatic site, the statistical design, and the goal of the project. Eight possible techniques of collecting hail data are evaluated with respect to their use in different areas, availability of historical data, and with different project designs. Each technique provides data that have distinct limitations. However, the two data collection techniques rated best for projects having a continuous seeding (on all hail days) design are networks of passive hailpads and raingages, and crop-hail damage records. The best technique for use in projects utilizing a single-storm seeding design or a random daily seeding (single area) design is a network of recording hailgages.

The La Porte weather anomaly—fact or fiction?

A notable increase in precipitation, moderate rain days, thunderstorm days, and hail days has been occurring since 1925 at La Porte, Ind. Since La Porte is 30 miles east of the large complex of heavy industries at Chicago, there is a strong suggestion that the increases in precipitation conditions are due to inadvertent man-made modification. If these increases are real, they serve as a good measure of the increase in convective precipitation that man could attain, at the same time pinpointing an excellent site for future meteorological studies of the exact causes of the increases. If the increases are fictional and result from exposure changes and observer error, they serve as an indication of the sizeable errors that may exist in some of our long-term climatological records. The increase at La Porte is sizeable: during the 1951–1965 period La Porte had 31% more precipitation, 38% more thunderstorms, and 246% more hail days than did surrounding stations. Since 1925 the year-to-year fluctuations in the annual and warm season precipitation at La Porte show agreement with the temporal distribution of steel production in the Chicago area. After a careful assessment of all available climatological data, it was concluded that these sizeable increases were real and not fictional.

Characteristics of Hailstorms in Japan

This paper is prepared for giving climatological information on hail occurrence in Japan. It is pointed out that there are two kinds of maxima in the distribution of the number of hail days in Japan. The maxima along the coast of the Sea of Japan are attributed to hailstorms mostly occurring in the cold season, (October-March), Whereas inland maxima are mostly due to hail in the warm season (April-September). Observational records of 4 selected weatherstations, 2 in one of the west coast maxima and 2 in one of the inland maxima, are examined in detail. It is found that cold season hail occurrence shows practically no diurnal variation, but warm season hailstorms occur mostly in the afternoon through the evening hours. Warm season hailstorms are almost always associated with thunderstorms, but there are many cases of hailstorms without thunderstorm in the cold season. Some other features of interest on hail in Japan are also described.