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- Author or Editor: Douglas V. Hoyt x
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Abstract
Using data from sunshine recorders in the contiguous United States, the mean interannual cloud-cover variations at 103 locations are calculated. A typical location has a mean absolute year-to-year variation in cloud cover of 3.94%, which corresponds to a variation of about 14 overcast days per year and to about a 2.8% variation of the annual mean global insolation. As a consequence of these natural variations, it will take about 30 years of sampling to determine the annual mean cloud cover to an accuracy of 1% with 95% confidence. Regional year-to-year variations are smaller than those at individual locations; and, for the contiguous United States as a whole, it averages 1.2%. Theory suggests that on hemispheric and global scales the interannual variations in total cloud cover are very small, being of the order of a few tenths of a percent.
Abstract
Using data from sunshine recorders in the contiguous United States, the mean interannual cloud-cover variations at 103 locations are calculated. A typical location has a mean absolute year-to-year variation in cloud cover of 3.94%, which corresponds to a variation of about 14 overcast days per year and to about a 2.8% variation of the annual mean global insolation. As a consequence of these natural variations, it will take about 30 years of sampling to determine the annual mean cloud cover to an accuracy of 1% with 95% confidence. Regional year-to-year variations are smaller than those at individual locations; and, for the contiguous United States as a whole, it averages 1.2%. Theory suggests that on hemispheric and global scales the interannual variations in total cloud cover are very small, being of the order of a few tenths of a percent.
Abstract
The total cloud cover is deduced from measurements of the percent of possible sunshine at 72 locations in the United States. This sunshine-derived total cloud cover is then compared to conventional ground-based observations of total cloud cover made by meteorological observers. A linear relationship between the two estimates is calculated, and the difference between the two estimates as a function of latitude is fitted with a least-squares linear equation. It is found that on the average the sunshine-derived values of total cloud cover are about 13% lower than the corresponding ground-based estimates of total cloud cover. The difference between the two estimates may be attributed to projection problems by the ground-based observer where sides of clouds are viewed and added to the estimate of total cloud cover or to the failure of sunshine recorders to detect thin cirrus clouds. Projection problems by the meteorological observers is probably the most likely cause because satellite and aircraft observations confirm the sunshine observations. The difference between the sunshine-derived and ground-based estimates of total cloud cover as a function of latitude also indicates that the ground-based observers are probably having difficulties with the total cloud cover estimates. It is concluded that a more standard definition of the meaning of “cloud” and “total cloud cover”, is needed for radiation budget and climate modelling studies.
Abstract
The total cloud cover is deduced from measurements of the percent of possible sunshine at 72 locations in the United States. This sunshine-derived total cloud cover is then compared to conventional ground-based observations of total cloud cover made by meteorological observers. A linear relationship between the two estimates is calculated, and the difference between the two estimates as a function of latitude is fitted with a least-squares linear equation. It is found that on the average the sunshine-derived values of total cloud cover are about 13% lower than the corresponding ground-based estimates of total cloud cover. The difference between the two estimates may be attributed to projection problems by the ground-based observer where sides of clouds are viewed and added to the estimate of total cloud cover or to the failure of sunshine recorders to detect thin cirrus clouds. Projection problems by the meteorological observers is probably the most likely cause because satellite and aircraft observations confirm the sunshine observations. The difference between the sunshine-derived and ground-based estimates of total cloud cover as a function of latitude also indicates that the ground-based observers are probably having difficulties with the total cloud cover estimates. It is concluded that a more standard definition of the meaning of “cloud” and “total cloud cover”, is needed for radiation budget and climate modelling studies.
Abstract
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No abstract available.
Abstract
Rayleigh optical depths are calculated for six standard atmospheres using the latest value of the depolarization factor (ρn = 0.0139). Present uncertainties in the Rayleigh optical depth of ±0.16% exist for the values calculated in this paper but the new values are 3.5–3.7% lower than previously reported values. As a consequence it is shown that previous aerosol optical depth or turbidity values from Volz sunphotometer measurements, for example, are reported with systematic errors 2–100% too low. The new Rayleigh optical depths are shown to largely explain the systematic differences between the various wavelength pair determinations of total ozone by the Dobson spectrophotometer. Other possible applications of the results of this paper are also indicated.
Abstract
Rayleigh optical depths are calculated for six standard atmospheres using the latest value of the depolarization factor (ρn = 0.0139). Present uncertainties in the Rayleigh optical depth of ±0.16% exist for the values calculated in this paper but the new values are 3.5–3.7% lower than previously reported values. As a consequence it is shown that previous aerosol optical depth or turbidity values from Volz sunphotometer measurements, for example, are reported with systematic errors 2–100% too low. The new Rayleigh optical depths are shown to largely explain the systematic differences between the various wavelength pair determinations of total ozone by the Dobson spectrophotometer. Other possible applications of the results of this paper are also indicated.
Abstract
Two external forcing functions are examined in their relationship to temperature records at individual locations. Postulated changes in solar luminosity, deduced from sunspot structure, and the dust veil hypothesis are separately considered as possible linear forcings of surface temperature. Changes in solar luminosity appear to give a marginally better correlation with temperature records than the dust veil hypothesis, but each hypothesis alone only amounts for ∼2% of the year-to-year variance at individual locations on the average. The hypothesis of linear external forcing of temperature appears to be a better hypothesis for the continental interiors than for the oceans. Although both hypotheses have been used to successfully explain many of the observed features of variations in the average Northern Hemisphere surface temperatures, such knowledge does not greatly aid in the explanation of temperature variations on the local scale.
Abstract
Two external forcing functions are examined in their relationship to temperature records at individual locations. Postulated changes in solar luminosity, deduced from sunspot structure, and the dust veil hypothesis are separately considered as possible linear forcings of surface temperature. Changes in solar luminosity appear to give a marginally better correlation with temperature records than the dust veil hypothesis, but each hypothesis alone only amounts for ∼2% of the year-to-year variance at individual locations on the average. The hypothesis of linear external forcing of temperature appears to be a better hypothesis for the continental interiors than for the oceans. Although both hypotheses have been used to successfully explain many of the observed features of variations in the average Northern Hemisphere surface temperatures, such knowledge does not greatly aid in the explanation of temperature variations on the local scale.
Abstract
The pyrheliometric measurements at three locations in the United States axe examined to determine if there are trends in atmospheric transmission. Although the radiation values at Madison are depressed in the 1940's because of local pollution, there is no strong evidence of other man-made aerosol pollution at Madison, Albuquerque or Blue Hill. Collectively, there is a suggestion of increased aerosol pollution for the three stations, but the decrease in atmospheric transmission is rather small and confined to the urban areas where the measurements are made. The detected trends, if real, are too small to be important to climate on a regional or global scale because the estimated growth rate is ∼0.05% per year. There is evidence for the eruptions of Agung, Awu and Fuego in the radiation records. The eruption of Agung increases the optical depth by ∼0.035 several months after its eruption. By 1965 the atmospheric transmission returned to normal. The Lamb dust veil index in the Northern Hemisphere for these three eruptions is estimated to be 250. 85 and 165, respectively.
Abstract
The pyrheliometric measurements at three locations in the United States axe examined to determine if there are trends in atmospheric transmission. Although the radiation values at Madison are depressed in the 1940's because of local pollution, there is no strong evidence of other man-made aerosol pollution at Madison, Albuquerque or Blue Hill. Collectively, there is a suggestion of increased aerosol pollution for the three stations, but the decrease in atmospheric transmission is rather small and confined to the urban areas where the measurements are made. The detected trends, if real, are too small to be important to climate on a regional or global scale because the estimated growth rate is ∼0.05% per year. There is evidence for the eruptions of Agung, Awu and Fuego in the radiation records. The eruption of Agung increases the optical depth by ∼0.035 several months after its eruption. By 1965 the atmospheric transmission returned to normal. The Lamb dust veil index in the Northern Hemisphere for these three eruptions is estimated to be 250. 85 and 165, respectively.
