rate of the next city on the list, Phoenix, which saw an average change of .96 degrees in the same period." Stone said that part of Louisville's problem stems from the unfortunate meteorological conditions of the Ohio River Valley, which is prone to stagnant air conditions but also that the lack of tree cover in the urban core contributes as well. Stone states that "The tree canopy downtown is one of the sparsest of any city I have seen in the country." The tree cover in Louisville's larger metro area is around 30 percent, according to Stone's research, with the urban core at just 10 percent. That compares to about 45 percent in the city of Atlanta. Air pollution is trapped in Louisville's Ohio River Valley location. The city is ranked by Environmental Defense as America's 38th-worst city for air quality. Louisville's lowest solar noon is 28.4 degrees with the shortest daylength being 9 hours and 30 seconds, both occurring from December 17–26. The city's highest solar noon is 75.2 degrees with the longest daylength being 14 hours and 39 seconds, both occurring from June 17–25. The city's March and September equinox occurs at 50.5 degrees. See also Cityscape of Louisville, Kentucky Notes References Further reading External links Louisville/Jefferson County Information Consortium LOJIC interactive maps Metro Mapper - Interactive Maps for the Louisville Metro Area Historic Maps of Louisville/Jefferson County, Kentucky — University of Louisville Libraries