In paddy rice production areas in Hokkaido, forest windbreaks were established to improve environmental conditions and protect rice plants from damage during cool summer weather. We measured meteorological variables, water temperature and rice growth at four sites at distances of 3H, 5H, 10H and 17H (H is the windbreak height of 18 m) leeward from a forest windbreak that stretches east-southeast to west-northwest. The 17H site was a reference site where the windbreak had little influence. The following effects of the windbreak were observed in sites 3H, 5H and 10H. Wind speed was reduced when the wind direction was between the south and west, which was the most frequent wind directio in June and July in this area, including in cool summer years. Air temperature increased/decreased during the daytime/nighttime, respectively, but daily average air temperature was little changed. Daily mean water temperature increased in June but decreased in July. A possible reason of this was that higher water temperatures in June hastened leaf expansion resulting in a smaller amount of radiation being transmitted to water surface in July. Although plant height and radiation interception increased earlier, their maximum values showed little difference from the reference site. There was no significant difference in the yield among the sites. Although no damage from cool summers occurred in our study years, our results show the possible mechanisms by which windbreaks can reduce damage to rice from cool summer weather. Windbreaks did not warm air or water temperatures in the booting stage in middle to late July and offered little protection effect then. However, it is known that cool water temperatures before panicle initiation, corresponding to the June period, increases the susceptibility of panicles to low temperatures in the booting stage, causing sterility. Higher water temperatures in June due to windbreaks would, therefore, reduce sterility in cool summers.