Tokyo University's Center for Climate System Research has discovered that the rainfall pattern in midsummer in densely built-up areas such as Tokyo and Yokohama has recently changed from gentle rainfall to tropical squall-type rainfall, involving sudden, violent winds with torrential downpours. The researchers believe the reason is the "heat island phenomenon," where the temperature rises abnormally in densely populated areas due to increasing areas of land being concreted over and the emission of heat from air conditioners and heating systems. The results of the analysis were reported in late November at a meeting of the Meteorological Society of Japan in Fukuoka.