Photo= People disembark from an excursion ship on Lake Biwa with parasols and towels at hand under the harsh sunlight (2:50 p.m., July 23, Otsu Port, Hama Otsu, Otsu City, Shiga Prefecture)

Shiga Prefecture is not as hot as the adjacent Kyoto Prefecture due to Lake Biwa. This image of the Lake Biwa area may be overturned thanks to some data revealed by this summer's fierce heat. In July, Higashiomi City recorded the fifth highest temperature in Japan on one day, and Otsu City tied the record for most consecutive days with fierce heat, reaching 10 days on July 23. The Hikone Local Meteorological Office announced, "Shiga Prefecture is quite hot, too. Especially at night, the risk of heatstroke is high." It is said that the reason for that is the impact due to the presence of Japan's biggest lake.

According to the Hikone Local Meteorological Office, the temperature of 38.5 degrees Celsius recorded in Higashiomi City on July 15 was the fifth highest temperature in Japan. The highest temperatures recorded on July 23 were 36.8 degrees Celsius in Otsu City and 36.1 in Hikone City. Extremely hot days with the highest temperature over 35 degrees Celsius lasted for 10 consecutive days in Otsu City for the first time since 1994 and 2013.

"Lake Biwa is a natural air conditioner." Many people have the image that temperatures in Shiga Prefecture won't rise easily thanks to Lake Biwa with its plentiful water. However, the average highest temperatures in July in the Lake Biwa area, are 30.5 and 30.2 degrees Celsius in Otsu and Higashiomi Cities respectively. Although they don't reach the 31.5 degrees Celsius of Kyoto City, these temperatures are higher than 30 degrees Celsius of seaboard areas like Kobe City in Hyogo Prefecture and Tsu City in Mie Prefecture. When asked about this, the Hikone Local Meteorological Office explained, "The lakeshore has some influence from Lake Biwa, but the degree of that is unknown." The midsummer water temperature of Lake Biwa is high, about 30 degrees Celsius, so enhanced cooling cannot be expected much in the first place.

On the contrary, the coastal area has an influence at night as it is hard for the water temperatures of Lake Biwa to decrease. From 2010 to 2018, sweltering nights with a lowest temperature over 25 degrees Celsius, were recorded on 198 nights in Hikone City and 171 in Otsu City. Humidity is also high. The average humidity is 78 percent in Hikone City in July, exceeding the 70 percent of Kyoto City which is called "a steam bath." High humidity increases the risk of heatstroke, so attention should be paid at night as well.

In recent years, the impact of global warming and urbanization cannot be missed. In the decade from 1980, there were 20 extremely hot days in Otsu City, one in Higashiomi City and zero in Hikone City. From 2010 to 2018, however, 107 extremely hot days have been recorded in both Otsu and Higashiomi Cities, and 55 in Hikone City also. The number of extremely hot days has greatly increased.