Most meteorological data are gathered at airports (because they have to collect this data in order to let plans land and take off safely), but airports are located outside city limits, so they might be differnt form the actual weather conditions in nearby cities. For example, my be lower elevation or closer to water. When climate normals are based on airport data, they might differ from actual weather conditions in nearby cities. Changes in elevation and other factors, such as proximity to large bodies of water, can cause climates to vary