Tropical Nights
Tropical Nights (TN) are an indicator of the heat in the atmosphere. This indicator describes the trend of tropical nights in the Basque Country from 1971 to 2016. This is an important index when applied to fields such as agriculture, tourism or human health.
- Since 1971 there has been an upward trend in the number of TN in the Basque Country, with an increase of 0.4 days per decade.
- The rates of change in the number of TN at a spatial level are not uniform throughout the territory, affecting the metropolitan areas of Bilbao and Donostia-San Sebastián, as well as the Ebro axis to a greater extent.
Relationship of the indicator to climate change
A tropical night is a night in which the temperature does not drop below 20ºC. Tropical nights provide fewer opportunities to cool down and recover from the daytime heat, leading to increased physical discomfort and exacerbated health problems, which can result in illness and death, especially among the elderly and children, as well as other vulnerable populations. In addition, the increased demand for air conditioning during hot nights can overload the electricity grid and lead to power outages. As global warming increases, the number of warm nights is expected to rise, and this means that analysing the number of tropical nights recorded in the Basque Country is necessary.
Figure 1. Time series of tropical nights in the period 1971-2016 for the Basque Country as a whole.
Firstly, it is worth highlighting that tropical nights do not occur throughout the territory, but are limited to certain areas, especially the Ebro river axis and the coast; and to a lesser extent up the valleys towards the upper basins.
A time series analysis of tropical nights shows a significant positive trend (p =0.00056) at a rate of 0.4 days per decade for the period 1971-2016 (Figure 1).
Figure 2. Rate of change of TNs (number of days per decade) in the Basque Country, 1971-2016.
The spatial rates of change of tropical nights in the Basque Country from 1971 to 2016 are not uniform throughout the territory. A more pronounced and statistically significant trend is observed in certain coastal regions, and in particular, around Bilbao metropolitan area, but also in that of Donostia-San Sebastián. The heat island effect in urban areas may be behind this trend, which is relevant since these are areas of high population density. The Ebro axis also shows a pronounced positive trend.
nd automatic weather stations managed by different institutions (Basque Government, Provincial Councils, Central Government).
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" (http://escenariosklima.ihobe.eus/data).
In these maps, the annual tropical nights are calculated, understood as those when the daily minimum temperature exceeds 20 °C.
Decadal trends (Sen’s slope), i.e. magnitude of mean temperature increases/decreases over a decade, is also calculated; and it is checked whether the trend is statistically significant or whether it is actually the result of the variability of the thermometric series itself using the Mann Kendall (MK) test.
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The Basque Country
+0.4 days
Per decade since 1971