Warm/hot days and summer days
Warm/hot days and summer days are an indicator of the heat in the atmosphere. This indicator describes trends for warm/hot days and summer days in the Basque Country between 1971 and 2016. These are important indices when applied to fields such as agriculture, tourism and human health.
- Since 1971, there has been an upward trend in the number of warm/hot days in the Basque Country, with a significant increase of 3.8 days per decade.
- The spatial rates of change for summer days affect the Mediterranean watershed, and coastal and inland areas such as Goierri more than anything, and are statistically significant.
Relationship of the indicator to climate change
Warm/hot days are days where a daily maximum temperature is recorded above a certain threshold. Unusually warm/hot temperatures can lead to prolonged extreme weather events, such as summer heat waves. Very hot days also affect health of the elderly, children and other vulnerable population groups, and can lead to increases in mortality and morbidity. In the primary sector, prolonged exposure to heat can affect crops and may harm livestock. In the context of climate change, maximum daily temperatures are expected to increase, making it important to analyse the number of warm/hot days in the Basque Country.
Trends over time and indication of climate change
A time series analysis of warm/hot days shows a significant positive trend (p =0.00052) at a rate of 3.8 days per decade for the period 1971-2016 (Figure 1).
Figure 1. Time series for warm/hot days in the period 1971-2016 for the Basque Country as a whole (correlation coefficient R = 0.35, p-value = 0.00052.
The spatial rates of change for summer days in the Basque Country since 1971 are not uniform throughout the territory. The trend is more marked and statistically significant for the Mediterranean watershed, but somewhat weaker for the Cantabrian watershed. However, there is also a clear increase in summer days along the coast and in inland areas such as Goierri.
Figure 2. Rate of change for summer days (TX > 25 °C) in the Basque Country (no. of days per decade), 1971-2016.
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 series the maximum temperature refers to the highest temperature value in a day, which under normal conditions and without taking into account other meteorological factors, is reached in the early afternoon.
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 the annual number of summer days on the one hand, defined as days when the maximum daily temperature exceeds 25 °C, and the annual number of hot days on the other, when the maximum daily temperature exceeds the 90th percentile. In short, these are two ways of expressing a similar concept, the first in absolute terms, the second in relative terms.
We can also calculate the decadal trend (Sen's slope), i.e. the extent of the increase/decrease in the days of summer 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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Proyecto Klimatek 2016. Elaboración de escenarios regionales de cambio climático de alta resolución sobre Euskadi. IHOBE. Gobierno Vasco.
The Basque Country
+3.8 ºC
Per decade since 1971