Following up on the low temperature analyses (first one, second one) as they are related to creeping bentgrass during hot summer weather, I have taken a look at a single location over time. The JMA Tokyo surface observation site has annual and monthly data available since 1876 and daily records available since 1949. So it is easy to obtain the data of interest and make an analysis.
First I counted the days each year when the low temperature was 21°C (70°F) or higher. This is an indication of the summer duration, if one thinks of summer in terms of temperatures which will cause problems with creeping bentgrass.
From 1949 to 2014, the trend has been an increase of 0.35 days per year with low temperatures of 21°C or above.
Then I looked at the annual temperatures (low, mean, and high) since 1876.
There seems to be an increasing trend, especially in the mean annual low temperature, which used to be less than 10°C in some years, and now is regularly above 12.5°C. But the annual value is not so useful when looking at heat stress.
August is usually the hottest month of the year in Tokyo, so I looked at the daily data for August since 1949, and calculated the monthly mean high, average, and low temperature.
The chart above is the proper way (or at least a standard way) to look at this type of data, because it includes the scale down to 0°C. One can zoom in on these same data by limiting the range of the y-axis scale, as shown below, and note that there is also an increasing trend in the August temperatures.
Is it hard to grow bentgrass in Tokyo? Yes! There is only a single year since 1949 in which the average low temperature in August was less than that 21°C (70°F) level.
I also looked at the percentage of days during the month of August with a low temperature of 21°C or above.
Generally there will be 75 to 100% of the days in August with low temperatures remaining above 21°C. In fact the last summer with more than 25% of the August days less than 21°C was 1993.