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How much does June and July weather affect final yields?

Publication Date: 
12 August 2016

Arthur Marshall, Analyst,, 02476 478956

Summer 2016 to date has been on the dull and wet side, which has affected development of the early crop and sometimes hindered fieldwork. At this point in the season, some would like to try to estimate roughly where final yields might end up. However, except in the most extreme years such as 2012, measures of weather during June and July appear to have little relationship with final yields. Despite unfavourable weather conditions so far this year, this suggests a wide range of yield possibilities are still open for the GB maincrop.


Weather to date

Both in England and Scotland, MET Office data shows that weather in June/July 2016 has been relatively dull and wet, but by no means the most extreme year. As shown below, since the year 2000, rainfall in England has been the fifth-highest and sunshine the third-lowest, while Scotland has had the fourth-wettest and fifth-dullest conditions in this period.


Historic relationships with yields

Here, we compare weather data with separate yield data for England and Scotland going back to 2005.


  • Sunshine

In most cases, there is no clear relationship between sunshine in June/July and yields.

However, a pattern of minimum sunshine levels in order to maintain good yield potential does seem to emerge. For England, the two years of especially poor yields (2007 and 2012) correlate with years with fewer than 350 hours of sunshine in June and July, while years with more sunshine than this came up with a wide range of average yields. For Scotland, only 2012 was an especially poor yielding year (with 210 hours of sunshine); the threshold appears closer to 250 hours.

Figures for this year come close to the threshold for Scotland (253 hours), though 2007 and 2015 had similar sunshine levels in Scotland but above-average yields in both cases.

For England, the 327 hours of sunshine this year place it within the low range, suggesting that sunshine levels over the remainder of summer could be especially important for crop development. However, rainfall levels need to be considered in combination with this too.


  • Rainfall

A broadly similar pattern for rainfall emerges as with sunshine.

In England, there is a large gap between the two extreme years and the others in terms of June/July rainfall. Most years had less than 200mm of rainfall, while 2007 and 2012 both had over 250mm. In Scotland, it is harder to draw a distinction – rainfall in 2012 was barely higher than in some other years with above average final yields.

Unlike with sunshine, rainfall in England this year (142mm) was well within the range still associated with a wide range of yield potential. This implies less of a risk to yields than sunshine levels alone suggested.

Similarly, while rainfall in Scotland was also towards the high side (239mm), this does not suggest any significant risk to yield potential based on historical patterns.


August and September still to come – but still no clear relationship with yields

With plenty more time still to come before much of the GB maincrop is harvested, a lot of weather could still affect the crop. Current MET Office forecasts for the remainder of August into September suggest weather could become more unsettled, with most regions likely to experience showers or longer spells of rain. However, conditions are also seen to be changeable, offering drier, brighter weather at times for parts.

Nonetheless, the historical relationship in August and September between either sunshine or rainfall levels and average yields is no clearer. High or low levels of sunshine or rainfall alone in these months are not enough to draw an accurate forecast of yields from, without also considering other factors such as incidence of sunlight, accumulated moisture and disease pressure (for example). Though these factors can be difficult to measure precisely, keep an eye out for crop development reports in Potato Weekly each week. Our official yield estimate is due out in late Autumn, after the majority of harvesting is complete.

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