Light intensity, sunshine and day length – the numbers

To follow on from my last blog on light intensity you can see an interesting picture forming below.

The below pictures show varying degrees of sunshine, intensity and day lengths. All the figures are for Sandwich Down on the south east coast where I usually find the highest sun intensity figures. All the map images are from and these are really nicely logged on twitter by @British_wx.

10th June totalled 0 hours of sunshine, although no direct sunlight was measured the amount of useful light reaching the plant is still significant. Light levels were reaching around the 400 µmoles/m2/s throughout the day, had useful light for 16 hours. Whilst the plant wasn't photosynthesising at its maximum potential it wasn't subjected to any negatively impacting light either and had around 16 hours worth of photosynthesis at between 70 - 80% of its potential.

21st June - The longest day but not the brightest summers day logging around 10 hours of sunshine but still plenty of negative light around. Even on this cloudy day 9 hours of the day there sufficient light levels to stress the plant further if it was already under some form of stress, mainly moisture or heat at this time of the year.

22nd June - The sun began to kick-in in the south east and it nearly hit 15 hours of sunshine, this equates to 10 hours of negative light but 18 hours when some form of photosynthesis is taking place. During days like this do not underestimate the power light has to damage stressed plants.

For balance I have thrown a day in March in to see the comparison - you can see we only get above 200 µmoles/m2/s for 4 hours (200 µmoles/m2/s is enough light for the plant to photosynthesise at around 50% of it's potential.

Looking at these light charts has made me wonder what last year looked like from a light perspective - we all know about 2018's drought challenges but did excessive light coupled with drought cause us additional problems?

2018 sun light levels and 2019 levels compared.

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