Frost days
Frost days are an indicator of days when there are freezing conditions at ground level both in the morning and in the afternoon. This indicator describes the trends for frost days in the Basque Country between 1971 and 2016. It is a useful index that can be applied in fields such as agriculture or in the use of natural resources.
- Since 1971, there has been a significant downward trend in the number of frost days, with a decrease of 2.1 days/decade.
- The spatial rates of change for frost days show a reduction in a large part of the region, and are particularly statistically significant and more pronounced around the region of Arratia-Nervión and in the west of the Añana area.
Relationship of the indicator to climate change
The freezing point (0 °C) is a critical temperature threshold for many aspects of life. The number and timing of frost days can affect natural ecosystems as well as many sectors of the economy. For example, many crops only grow during periods when they are not limited by frost damage. The number of frost days during a year determines which natural plants can thrive, what food sources are available to animals and when and how animals migrate or hibernate. Frosts also help reduce populations of certain insects and other pests that can damage crops and forests and spread diseases to humans. Conversely, a mild winter with fewer frost days can lead to an increase in these pests over the following spring and summer.
As the Earth's climate warms, the number of frost days may generally decrease. A continued reduction in frost could have a variety of effects on ecosystems, droughts, the risk of forest fires, agriculture, natural resources and the economy. Therefore, it is important to look at frost days in the Basque Country.
Figure 1. Time series for frost days in the Basque Country as a whole during the period 1971-2016 (correlation coefficient R = -0.2, value of p = 0.046.
This section details the evolution of the number of frost days in the Basque Country between 1971 and Year 2016.
The time series for frost days for the Basque Country as a whole and for the period 1971-2016 is shown in Figure 1. There was a significant negative trend (p=0.046), with a rate of decline of 2.1 days per decade for the period analysed. Therefore, there has been a clear and statistically significant reduction in frosts since 1971.
Figure 2. Rate of change in frost days in the Basque Country (number of days per decade), 1971-2016.
Figure 2 shows the spatial decadal variability of frost days in the Basque Country. In general, there has been a reduction in the number of frost days of between 0 and 5 days per decade. There are several areas with a significant negative trend, one of which covers a large part of the interior of Bizkaia, Greater Bilbao and the Zuia area in Alava. A significant decrease in the number of frost days was also observed in the east of Gipuzkoa, the south of Rioja Alavesa and the coastal area of Busturialdea.
There were also more marked trends, with significant decreases of between 5 and 10 days of frost per decade, in the Arratia-Nervión region, in the northern part of the Zuia region and in the western part of the Añana region, and in some of the mountainous areas around Goierri, Duranguesado and Montaña Alavesa. A significant positive rate of change was only identified in the southern part of the Alto Deba. However, spatial results may contain interpolation errors, particularly in mountainous areas where there are no weather stations, so they need to be treated with caution.
Air temperature in the Basque Country is measured in both manual and automatic weather stations managed by different institutions (Basque Government, Provincial Councils, AEMET, URA).
In daily resolution data sets the minimum temperature refers to the lowest temperature value in a day, which is often recorded just after sunrise, when the Sun becomes visible over the horizon.
Data series have been fed into spatial prediction models to generate a daily resolution cartographic database, which is the starting point for the calculation of this climate change indicator. Static covariates, derived from digital terrain models, have been included in this prediction to explain air temperature.
The cartographic database comes from Phase II of the KLIMATEK project “High Resolution Climate Change Scenarios for the Basque Country”.
These maps are used to calculate frost days, defined as those days when the daily minimum temperature falls below 0 °C.
We can also calculate the decadal trend (Sen's slope), i.e. the increase/decrease in the number of frost days over a decade, and check whether the trend is statistically significant or whether it is really the result of the variability of the thermometric series itself (Mann Kendall test).
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The Basque Country
-2.1 days
Per decade since 1971