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  • Author or Editor: Stanley A. Changnon Jr. x
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Stanley A. Changnon Jr.

Abstract

Temporal and spatial relationships between thunderstorms (events) and flashes were investigated using data for 1983–85 for 25 first-order stations (10 in the West and 15 along the East Coast). Thunder events were compared with flashes within three ranges: 5 km, 10 km, and 20 km, around each station. Cluster analysis revealed six geographic regions: Florida, Southeast (South Carolina, Georgia), Mid-Atlantic (Virginia, Maryland Pennsylvania), Northeast (New York and New England), Rocky Mountains, and an intermontane area.

Periods of multiple flashes not within thunder events and within 10 km of a point (most realistic for audibility), revealed that 10% to 20% (depending upon region) of all thunderstorms were missed. Also, 13% (Rockies) to 44% (Mid-Atlantic) of all thunderstorms have recorded durations too short (missed flashes before their reported start), and the average underestimated durations were from 55% (Northeast Mid-Atlantic) to 26% (Rockies). Flashes isolated in time and space, due to locational errors, represent 1% of all flashes in the east and 3% to 5% in the west where the data are poorer. Errors in flush data appear minimal but the errors in thunder events are sizable.

Thunder events and flash frequencies related well based on major features in their average areal patterns and their between-year changes at stations. Correlations of fishes with events varied; their annual point frequencies had coefficients of +0.83 (east) and +0.67 (west). Durations of events and Bash frequencies were poorly correlated with skewed distributions (often large flash frequencies in a few storms). The percent of all recorded flashes (within 10 km) in thunder events varied from 28% to 44% at western stations and from 13% to 20% at eastern stations. Thunder events with ≥ 1 flash varied widely, from 71 % of all events at Washington, D.C. to 30% at Boston.

Major east-west differences existed in the frequency of thunder events with flashes, reflecting poorer audibility of thunder in the west. Part of the difference is due to flash recording problems in the west, leaving flash frequencies that are underestimates of the true values. latitudinal distributions were marked with north-to-south increases in thunder events and their durations, frequency of flashes, and number of flashes not in events. More missed flashes in the south suggested that atmospheric conditions in northerly U.S. latitudes enhance audibility. With a 20-km sampling radius, between 6 (Northeast) and 23 (Southeast) thunder events are not recorded yearly, but these averages drop to 1 (Northeast) and 4 (Southeast) based on flashes within 5 km. The data on thunderstorms is generally poor from two perspectives: 1) the recorded data miss sizable numbers of storm events, and 2) when recorded, 30% to 50% often underestimate durations based on nearby lightning activity.

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Stanley A. Changnon Jr.

Abstract

Historical (1901–85) summer (June–August) rainfall data in central Illinois were used to construct three typical rain conditions: one representing the typical dry summer (based on the driest 20% of the summers of the past 85 years), a typical wet summer (from the 20% wettest), and the near-average summer rainfall conditions (the 20% nearest the long-term average). Monthly rain totals for each type were established first, then daily rain frequencies were used to define all individual rain day amounts, and historical rain-day amounts by date were used to assign rain days to dates throughout the three types of summers. In-day conditions relating to rainfall rates, time of rain, and durations were constructed for each day of rain. The resulting three summer rainfall conditions are being used to guide applications of water onto agricultural test plots (protected from natural rains) to measure crop yield effects from weather modification but the approach and system could serve other applications like effects of climate change.

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Stanley A. Changnon Jr.

Abstract

Average durations of thunder events are greatest (>120 min) in the Oklahoma–Kansas area and least (<60 min) along the west coast and northeast. The average point duration of thunder activity ranges from 10 000 to 12 000 min along the Gulf Coast, 8000 to 10 000 min in the Midwest, exceed 6000 min in Arizona, but is only 1000 to 2000 min in the northeast, and 500 to 1000 min along the west coast. Nocturnal thunder events typically last 10 to 30 min longer than those in the daytime in all areas except for the western mountains and extreme southeast where daytime events exceed those at night by 5 to 15 min, on the average.

The trends in thunder event activity during the 1948–77 period indicate four distinctly different characteristics. The stations in the southwestern and northwestern United States exhibit flat, unchanging trends in events during the 30 years, but events in the northern Great Plains-Midwest gradually decreased with time; those in the Great Lakes increased since 1950; and those in the southeastern United States decreased to minimums in the 1960s and then increased to 1977. The temporal distribution of extratropical cyclonic activity in July explains 25% to 50% of the temporal variations in July thunder events over most of the central and eastern United States. However, increases in thunder events since the late 1960s in the Upper Midwest and along the East Coast were not associated with increased cyclonic activity.

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Stanley A. Changnon Jr.

Abstract

The average temporal and spatial distributions of thunder events (periods of discrete thunder activity heard at a point) in the conterminous United States were found to be generally similar to those of thunder days. Annual averages of thunder events peak along the Gulf Coast (>100) and are also quite high in the central United States (Kansas, Missouri, Illinois with >75 events), and in the southwest (Arizona with 60 events). Thunder events are least along the west coast (<20) and in the northeast (<30). Multiple events per day are greatest in the Midwest (Illinois, Iowa) averaging 1.7 events per summer day, and are also high in the southwest (Arizona) with 1.5 events. This causes these two maxima in thunder event activity to be more pronounced than those found on the pattern of average thunder days.

The average patterns for the thunder event frequencies, multiple events per day, and durations reveal that convective activity is weakest and shortlived along the west coast and in the northeast. The high incidence of events per day in the Midwest reflects multiple storm incidences likely related to MCCs and nocturnal storm activity. The peak in thunder event activity is present in the central United States in all months and rotates from the lower Mississippi Valley to the central Great Plains-Midwest and then back, and its position is always closely related to the major center of cold frontal activity. The thunder peak in the southwest is related to the summer monsoon intrusion of moist tropical Pacific air and related frontal activity. The summer-fall peak in thunder events along the Gulf Coast-Florida is a result of sea breeze induced convergence, localized heating, and occasional tropical disturbances.

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