London is a leading global city, with strengths in the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism and transport all contributing to its prominence. It is the world's most-visited city as measured by international arrivals and has the world's largest city airport system measured by passenger traffic. London contains four World Heritage Sites: the Tower of London; Kew Gardens; the site comprising the Palace of Westminster, Westminster Abbey, and St Margaret's Church; and the historic settlement of Greenwich (in which the Royal Observatory, Greenwich marks the Prime Meridian, 0° longitude, and GMT). London has a temperate climate without the extremes found in continental Europe. July is the hottest summer month and January is the coldest winter month. Late autumn and winter are the wettest seasons but rain is possible in any season.