The summer solstice is the time of longest day and highest solar income. It occurs usually on 21 June each year but the daylength changes so slowly at this time that it would be difficult to tell which day was the longest without some form of complex solar calculator.
Images from the summer solstice
1 Fields of barley in flower, the main crop throughout much of the maritime east, now bulking under the high solar radiation yet still several months from harvest. 2 The legume and one time forage sainfoin Onobrychis sativa mobbed by bees at this time of year (LFG). 3 Flowering branch of the marsh thistle Cirsium palustre on old grazings, Invernesshire.
4 Flowers of viper’s bugloss, a striking plant but rare to see these days, one of the borage family (LFG). 5 Wet solstice roses growing in a hedge; midsummer damp hardly affects the wild rose – its petals curl to protect the inner parts, but don’t sog and rot like the old cultivated roses. 6 Bumble bee on field scabious flower (LFG).
7 Potato field in Strathmore, still some way before the rows close. 8 Flowering stem of field bean Vicia faba in Perthshire. 9 Opium poppy, some flowers full out, others gone, others still to come (LFG).
LFG = grown and photographed in the Living field garden. All photographs from the Living Field collection.
Solar energy and earth’s temperature
The solstice – the days are longest, the sun is highest in the sky, and if cloud stays away, the daily solar income is almost sub-tropical. Temperature is now moving towards its maximum. The weather can be glorious for people and for crops, yet cloud can move in any time from the Atlantic or from the North Sea and make it feel not much like summer.
On a clear day, the solar income reaches 25 on the left hand vertical scale in the figure above. It’s similar to the solar income in ‘cool season Africa’, but here the energy is spread over a longer period of the day, ideal for vegetation that’s bulking carbon.
The daily solar income will now fall, but on average temperature will still rise towards its maximum at XQ3.
Light and dark
The summer solstice is the time of longest day and shortest night. In clear weather, the light never dims completely.
On 21 June 2015, sunrise is 03.22 and sunset 21.06.
The summer solstice has this feel of being the height of something. There is so much light, but the soil has been slow to warm, as have the big seas around the croplands, so even if the soil is warming the air is cool, and many of the crops are still getting going.
So while the summer solstice is a time of ‘stillness’ or minimal change in the solar cycle, it’s a time of great flux, change and difference in the crop cycle. Because the cool of northern latitudes slows our immigrant crops, most of them are not ready to take advantage of the sun, and in many years they waste it. Once the sun’s gone, it’s gone.
One way to get round this out-of-phasing in the solar and temperature cycles is to sow crops in late summer or autumn (called winter crops) in the hope they have enough leaf to capture the sun in May and June. The crops will need to be winter-hardy and they now mostly are. Winter oilseed rape – sown late summer the previous year – has flowered already, the yellow now replaced by dull green pods. The flowering ‘ears’ of winter barley should be visible, as might those of winter wheat.
Yet more than half the corn or grain crops here are sown in spring, late March and April, as are potato and also most vegetables crops that show lines of bare soil between rows.
In the south of the UK and through the same latitudes in Europe, most of the cereals are now winter crops. So why not here? Why is there so much spring barley in the croplands? It’s partly due to the risk of autumn-sown crop being set back in years with cold wet winters, but there is also a local market for spring barley in the whisky industry and as feed for farm animals. And grain yield of spring barley is still high, but without the need for so much fertiliser and pesticide as the winter varieties.
Once the leaf canopies are well expanded, they can now take advantage of the long cool summer. The plants progress slowly through their successive phases, take a long time to fill grain and the potentially high solar income is spread over many hours so is unlikely to overload the plants’ photosynthetic system or cause heat stress (unlike in the tropics). In a good year such as 2014, the yields high by most north-temperate standards.
Last update: 29 June 2016]