In previous posts, I looked at:

• when it is too hot to grow bentgrass
• a more detailed look at this, with better data
• long term temperature time series in Tokyo

In Tokyo, there is quite an increase in average annual temperatures since 1876. I'm expecting this has to do with an urban/concrete heat island effect. I downloaded data for Knoxville, Tennessee (data link and code for charts here) from 1910 to 2014 and plotted it. I wanted to look at three things. Trends in summer duration, trends in temperatures in the hottest month, and trends in overall temperatures.

Creeping bentgrass will not perform well when soil temperatures are high. When the nighttime low temperatures are above 21°C (70°F), we know the soil temperatures will be above that temperature too. I like to estimate the duration of summer that will put heat stress on creeping bentgrass by counting the number of days when the temperature remains above 21°C.

In Knoxville, a cool summer will have less than 20 days with a low above 21°C, and a hot summer will have 40 to 60 days above that temperature. Unlike Tokyo, there is no consistent trend in this summer duration at Knoxville, although it has been gradually increasing since about 1970.

The hottest month in Knoxville, on average since 1910, is July. I plotted the average daily high and low for July in each of the past 105 years. In the hottest month of the year, the temperatures (high and low) are staying pretty consistent.

 For the average annual temperatures in Knoxville, it is much the same.

Average lows are about 10°C and highs 20 or 21°C. Not a whole lot of change in 105 years. The average temperature in Knoxville for the past 105 years is 15.1°C. Tokyo used to be cooler than Knoxville, and now Tokyo is warmer than Knoxville.