Growing (Freeze Free) Season in Chester County PA

[ChescoWx]

Looking at the models both the EURO and GFS have Chester County reaching 34 next Friday morning. If that were to come to pass that would be 181 days since our last sub-freezing temperature back on April 21st. One of our many local farmers in the Township emailed me to ask if I knew or could calculate what was the length of the growing (freeze free) season in East Nantmeal. I surmised that with our modest increase in elevation we should expect less sub freezing readings than the lower valley locales due to less impact from radiational cooling on clear and calm nights. 

For perspective the latest sub freezing temperature was the 32 degree reading way back on May 28, 1927 and the earliest first freeze observed was on September 20, 1956. That growing season of 1956 was the shortest growing season in the county with the last freeze on May 24th resulting in a season of just 119 days. By contrast back in 2005 we had our longest growing season of 233 days. Which is almost 4 more months freeze free weather.

The COOP data from 1894 to 1983 was observed at 340 feet above sea level (ASL) down in the Chester Valley; while readings from 1984 to Present have been observed at over 660 feet ASL on the north ridge of the valley that runs from SW to NE across the middle of Chester County. The number of average below freezing days was 127 at the lower elevations and has decreased to only 115 on average at the higher locales.  At the lower elevations the growing season averaged 172 days. At the higher elevations it has averaged 188 days.

Of interest for the 89 years from 1894 to 1983 there were only 2 seasons that exceeded 200 growing days (1912 and 1938) since 1984 there have been 13 such seasons...including 11 of the past 14 years and the last 4 years 2014-17. 

[zenmsav6810]

20 minutes ago, ChescoPaWxman said:

Looking at the models both the EURO and GFS have Chester County reaching 34 next Friday morning. If that were to come to pass that would be 181 days since our last sub-freezing temperature back on April 21st. One of our many local farmers in the Township emailed me to ask if I knew or could calculate what was the length of the growing (freeze free) season in East Nantmeal. I surmised that with our modest increase in elevation we should expect less sub freezing readings than the lower valley locales due to less impact from radiational cooling on clear and calm nights. 

For perspective the latest sub freezing temperature was the 32 degree reading way back on May 28, 1927 and the earliest first freeze observed was on September 20, 1956. That growing season of 1956 was the shortest growing season in the county with the last freeze on May 24th resulting in a season of just 119 days. By contrast back in 2005 we had our longest growing season of 233 days. Which is almost 4 more months freeze free weather.

The COOP data from 1894 to 1983 was observed at 340 feet above sea level (ASL) down in the Chester Valley; while readings from 1984 to Present have been observed at over 660 feet ASL on the north ridge of the valley that runs from SW to NE across the middle of Chester County. The number of average below freezing days was 127 at the lower elevations and has decreased to only 115 on average at the higher locales.  At the lower elevations the growing season averaged 172 days. At the higher elevations it has averaged 188 days.

Of interest for the 89 years from 1894 to 1983 there were only 2 seasons that exceeded 200 growing days (1912 and 1938) since 1984 there have been 13 such seasons...including 11 of the past 14 years and the last 4 years 2014-17. 

Probably one of the most practical weather-related Key Performance Indices you can analyze. I've always used 172-176 days as a rule of thumb. Dedicated gardeners can extend the growing season by several weeks using tricks to keep frost at bay. While 32 is the traditionally accepted frost mark, seasoned gardeners can keep hearty plants outdoors to 28.5F.