Lake Tahoe's clarity improved in 2012 for the second year in a row, and its waters were the clearest in 10 years, according to University of California, Davis, scientists who study the lake. Last year's average annual clarity level was 75.3 feet, or a 6.4-foot improvement from 2011, according to data released today by the UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency. The lake's clarity is measured by the depth at which a 10" white disk, called a Secchi disk, remains visible when lowered beneath the water's surface. The measurements have been taken since 1968, …