To　avoid　a　sharp　rise　in　temperature　in　the　cabin　of　parked　vehicles　exposed　to　solar　radiation,　experienced　drivers　leave　some　windows　partly　open　when　the　vehicle　is　parked　in　the　sunlight　to　achieve　cooling　through　natural　ventilation.　However,　the　effectiveness　of　this　measure　to　reduce　the　temperature　under　different　weather　conditions　has　not　been　verified.　To　this　end,　this　study　investigates　the　effect　of　open　windows　on　the　thermal　environment　of　a　vehicle　under　different　environmental　conditions.　A　field　measurement,　in　which　two　identical　vehicles　with　and　without　window　gaps　were　used,　was　carried　out　in　Daxing　District,　Beijing.　The　measurements　were　conducted　for　15　days　under　different　window　gaps　and　ambient　conditions.　The　results　revealed　that　open　windows　resulted　in　a　maximum　temperature　reduction　of　6.7　degrees　C　in　cabin　air　temperature　under　high　temperature　and　high　solar　radiation,　while　only　0.6　degrees　C　can　be　reduced　under　low　temperature　and　low　solar　radiation.　The　results　also　showed　that　when　window　gaps　effectively　reduce　the　air　temperature,　lower　air　temperature　can　be　obtained　with　larger　open-window　areas.