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The spatial and temporal variability of atmospheric aerosols is not well understood, as most studies have been constrained to data sets that include few stations and are of short duration. Furthermore, all methods for quantifying atmospheric turbidity suffer from a major constraint in that they require cloudless sky conditions. This restriction produces gaps in the turbidity record and sampling bias, which has led to questionable inferences about the variability of aerosols.In this research, we address these concerns via analyses at scales broader than all previous studies. We analyzed the spectral aerosol optical depth at 500 nm (τ a5 ) andÅngström's wavelength exponent (α), which represents the relative size distribution of aerosols. A total of 27 sites, with a mean period of record of 7.3 years, are included. Beyond seasonal and spatial summaries of aerosol variability, we have divided observations by synoptic condition, utilizing the Spatial Synoptic Classification (SSC).Our results show that atmospheric turbidity across North America is greatest over the east. Seasonality of both parameters was shown, most notably a greater τ a5 during summertime. Utilizing the SSC, we have uncovered significant differences across weather types. Moist weather types, especially moist tropical, display considerably higher turbidity, while the colder, drier dry polar weather type is associated with low aerosol optical depth. Certain weather types show considerable seasonal variability; the dry tropical weather type is associated with relatively low values in winter, but high values in summer, when convection is significant. Cluster analyses of stations yielded three general regions, each with similar synoptic variability: a western cluster with low aerosol optical depth and minimal synoptic variability, an eastern cluster with higher turbidity and variability, and a cluster located on the periphery of the eastern cluster, associated with moderate levels of turbidity but very high variability, suggesting a varied influence of nearby industrial areas.

Spatial and temporal variability in global, diffuse, and horizontal direct irradiance and sunshine duration has been evaluated at eight stations in South Africa and two stations in Namibia where the time series range between 21 and 41 years. Global and direct irradiance and sunshine duration decrease from northwest to southeast; diffuse irradiance increases toward the east. Annually averaged global irradiance G a decreased between 1.3% (2.8 W m −2 ) and 1.7% (4.4 W m −2 ) per decade at Bloemfontein, Cape Town, Durban, Pretoria, and Upington. Annually averaged diffuse irradiance D a decreased 5.2% (3.0 W m −2 ) per decade at Grootfontein and 4.2% (3.1 W m −2 ) per decade at Port Elizabeth. Annual direct irradiance B a decreased 2.1% (3.5 W m −2 ) per decade at Cape Town and 2.8% (5.7 W m −2 ) per decade at Alexander Bay. A simultaneous decrease in annually averaged daily sunshine duration S a may have contributed to the decrease in B a at Alexander Bay and the decrease in G a at Pretoria. Increases in aerosols may have contributed to the observed decrease in G a at Cape Town and Durban, and the decrease in D a at Grootfontein may be due to a decrease in aerosols. On average, variability in S a explains 89.0%, 50.4%, and 89.5% of the variance in G a , D a , and B a respectively. The radiative response to changes in sunshine duration is greater for direct irradiance than for global and diffuse. In the 2 years following the 1963 Mount Agung eruption in Indonesia, changes in global irradiance over southern Africa were small and inconsistent. At eight stations, diffuse irradiance increased 21.9% (13.3 W m −2 ) on average and direct irradiance decreased 8.7% (15.5 W m −2 ). After the 1982 El Chichón eruption in Mexico, global irradiance increased at two stations and decreased at seven stations. Eight stations witnessed an increase in diffuse irradiance averaging 7.2% (4.0 W m −2 ) and a decrease in direct irradiance of 5.0% (9.0 W m −2 ). The contribution of changes in cloud cover to the observed changes in irradiances appears to be small. Following the 1991 Mount Pinatubo eruption in the Philippines, diffuse irradiance increased an average of 18.8% (10.0 W m −2 ) at three stations and direct irradiance decreased by 7.2% (13.0 W m −2 ).

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