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Iver A. Lund

Abstract

Observations of hourly sunshine, angular elevation of the sun above the horizon, cloud cover, and a three-dimensional cloud model, were utilized to derive a set of cloud width, thickness, and spacing values for estimating the probability of clear lines-of-sight from any angle. Data for Tampa, Florida, were used to illustrate cloud parameters that satisfy the cloud model. The observed “typical” values of cloud width and spacing fit the model very well. The computed average earth cover was much less than the observed average cloud cover because the observers did not see all the spaces between clouds.

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IVER A. LUND

Abstract

Geopotential heights of the 850-mb surface at 499 grid points over one-half of the Northern Hemisphere, extending from 10°E westward to 170°W, were correlated with daily precipitation observations over California and an area in the Eastern United States on 401 January and February days. Correlations as high as 0.69 were obtained between 850-mb heights and California precipitation. The statistical significance of the maximum correlation obtained over the one-half hemisphere area exceeded the 5-percent level for all periods of less than 5 days. The maximum correlation between heights and precipitation observed in the Eastern United States was −0.44 and the correlations were significant at the 5-percent level for all periods of less than 3 days.

Since correlations between 850-mb heights and subsequent precipitation exceed autocorrelations except for short periods of time, it is proposed that 850-mb height observations replace or supplement persistence in conditional climatologies of precipitation.

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Iver A. Lund

Abstract

This paper illustrates a method of blending stagewise and stepwise regression procedures for selecting predictors and deriving equations. Only a few minutes of large-scale computer time were required to screen almost 4500 potential predictors and identify the leading contenders for use in the prediction equations. The equations developed for specifying and predicting California precipitation produced estimates of the amount of precipitation which were distinctly superior to estimates based on persistence or recurrence.

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Iver A. Lund

Abstract

Five methods for using standard cloud observations to estimate the probability of a clear line-of-sight, or sunshine, through the atmosphere are described and compared. The root-mean-square errors of the probability estimates (expressed in units of per cent) are less than 10 for four of the five methods.

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Iver A. Lund

Abstract

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Iver A. Lund

Abstract

An analysis of daily observations of sunshine taken in the central and northeastern United States during the spring and summer indicates the presence of a lunar synodical period. Less than average sunshine is observed during the first and third weeks of the lunar month and more than average sunshine is observed during the second and fourth weeks. Although this lunar period is significant by most statistical tests, the possibility that its appearance is due to a combination of the smoothing procedure and the temporal and spatial correlation among the observations cannot be completely ignored.

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Iver A. Lund

Abstract

Signals transmitted through the atmosphere in the visible and infrared portions of the electromagnetic spectrum can be seriously degraded by clouds between the source and the receiver. A previously published model was developed for estimating probabilities of cloud-free lines-of-sight through the atmosphere at any desired elevation angle and geographical location. This paper extends the published model to the problem of estimating probabilities of cloud-free lines-of-sight from n of m observing sites. A climatic record of sky-cover observations taken simultaneously from all m sites is required before the model can be applied.

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Iver A. Lund

Abstract

Predictions of sea-level pressure changes for periods of 1, 2, 3, 6, 9 and 11 hr were prepared employing analogues and many different linear regression equations. The equations were derived on a sample of 445 days and tested on an independent sample of 476 days. The set of equations, applicable to all days and consisting of only 4 to 7 predictors, reduced the errors in the 6-,9- and 11-hr pressure change predictions by 81, 80 and 80 per cent, respectively. By predicting the pressure changes at a set of grid points, 6- and 12-hr prognostic charts could be prepared for operational use.

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Iver A. Lund

Abstract

A Monte Carlo technique for testing the statistical significance of an estimate by regression is described. It is illustrated on a medium-range prediction of precipitation. The purpose of the technique is to permit the use of all data available while developing a regression equation. This is especially important when the sample is small. The need for setting aside a fraction of the data to test the equation on an independent sample, or to estimate degrees of freedom before applying standard statistical tests, is eliminated.

In the illustration, a stepwise regression procedure was used to select predictors and derive an equation to predict actually observed precipitation. Then the procedure was repeated 20 times, each time on a set of bogus values of precipitation drawn at random from the historical population of precipitation values. Each of the 20 equations resulting from bogus values of precipitation was used to estimate the precipitation and to give 20 values of per cent reduction of variance. A per cent reduction greater than the upper 5% would have led to a rejection of the null hypothesis and the inference that the prediction equation would yield skillful forecasts. In the example, the null hypothesis was not rejected.

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Iver A. Lund

Abstract

A total of 13,215 high-contrast, whole-sky photographs were studied to derive estimates of cloud-free and cloudy line-of-sight persistence and recurrence probabilities. There were 585 hr when photographs were taken every 5 min, without interruption, and 885 days when photographs were taken every hour from 0900 through 1500 LST. Data taken from these photographs were processed to obtain relative frequencies of cloud-free or cloudy lines-of-sight given the initial condition. The conditional relative frequencies were used in deriving conditional probability estimates. The resulting probability estimates are judged to be realistic and applicable to many locations.

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