In southwest Atlanta, neighborhoods closer to downtown originated as streetcar suburbs, including the historic West End, while those farther from downtown retain a postwar suburban layout. These include Collier Heights and Cascade Heights, home to much of the city's affluent African-American population. Northwest Atlanta contains the areas of the city to west of Marietta Boulevard and to the north of Martin Luther King, Jr. Drive, including those neighborhoods remote to downtown, such as Riverside, Bolton and Whittier Mill. The latter is one of Atlanta's designated Landmark Historical Neighborhoods. Vine City, though technically Northwest, adjoins the city's Downtown area and has recently been the target of community outreach programs and economic development initiatives. Gentrification of the city's neighborhoods is one of the more controversial and transformative forces shaping contemporary Atlanta. The gentrification of Atlanta has its origins in the 1970s, after many of Atlanta's neighborhoods had declined and suffered the urban decay that affected other major American cities in the mid-20th century. When neighborhood opposition successfully prevented two freeways from being built through the city's east side in 1975, the area became the starting point for Atlanta's gentrification. After Atlanta was awarded the Olympic games in 1990, gentrification expanded into other parts of the city, stimulated by infrastructure improvements undertaken in preparation for the games. New development post-2000 has been aided by the Atlanta Housing Authority's eradication of the city's public housing. As noted above, it allowed development of these sites for mixed-income housing, requiring developers to reserve a considerable portion for affordable housing units. It has also provided for other former residents to be given vouchers to gain housing in other areas. Construction of the Beltline has stimulated new and related development along its path. Climate Under the Köppen classification, Atlanta has a humid subtropical climate (Cfa) with four distinct seasons and generous precipitation year-round, typical for the Upland South; the city is situated in USDA Plant Hardiness Zone 8a, with the northern and western suburbs, as well as part of Midtown transitioning to 7b. Summers are hot and humid, with temperatures somewhat moderated by the city's elevation. Winters are overall mild but variable, occasionally susceptible to snowstorms even if in small quantities on several occasions, unlike the central and southern portions of the state. Warm air from the Gulf of Mexico can bring spring-like highs while strong Arctic air masses can push lows into the teens °F (−7 to