Last time I posted, I commented on the fact that winter temperatures in New Hampshire were almost 7°C warmer in 2016 than 2015, and that “spring 2016 is just around the corner.”
It turns out that spring has been a long time coming. While it isn’t a particularly late year overall, it does seem that it is a particularly slow year. Take a look at the “greenness” data we derive from imagery from our PhenoCam overlooking the Boston Common. What you will notice is the much more gradual rise in greenness compared to previous years. In 2016, greenness began to trend upward in mid-March, and isn’t going to peak until late May. By comparison, in 2015, greeness began to trend upwards beginning in late April, and it reached its peak by mid-May.
The pictures below compare April 19, 2015 (left) with March 19, 2016 (right) – they look pretty similar, despite the 2016 picture being a full month earlier.
And these pictures compare May 17, 2015 (left) with May 17, 2016 (right) – you can see that this year, we’re lagging behind last year on the same date by just a little.
Temperature is the main factor driving these differences; 2016 got off to a quick start in early April because March was much warmer in 2016 (monthly mean temperature of 5.8°C) than 2015 (monthly mean temperature of 1.0°C). But this year we had a lot of cool weather in April, which continued on into May, and that’s really slowed down the rate of development: for example, the mean temperature for the first two weeks of May was 12°C this year, compared to 16°C last year.
That said, the last few days have finally started to feel like spring!
Interested in reading more? Check out the article I wrote last year for Arnoldia, the magazine of Harvard’s Arnold Arboretum, about using the Boston Common PhenoCam to track the phenology of urban trees.