Heat- and Cold Waves
A Heat wave starts when the maximum temperature is above its ninety-percentile threshold and the minimum temperature is above its ninety-percentile threshold. The wave will be registered if it lasts more than 2 days within a period of 4 consecutive days or longer.
A Cold wave starts when the minimum temperature is below the ten-percentile threshold and the maximum temperature is below its ten-percentile threshold.
The thresholds are computed for the period of 1981 to 2010. A threshold for a particular day is determined on the minimum or maximum temperatures of the day itself and 5 days before and after the day. This results in 330 samples for each threshold.
If a heat wave reaches a temperature above 39.9 degrees Celsius an additional alert is registered as a health risk. For cold waves this alarm is given if the temperature descends below -19.9 degrees Celsius.
The temperatures are interpolated using measured weather station data of around 4000 stations in Europe and its surrounding area. The interpolation is done using an inverse distance algorithm considering up to 20 stations with a maximum search distance of 200 kilometers along the sphere.
The temperature is corrected before interpolation for altitude using a factor 0.0065 degrees per meter.
The area of interest is determined in decimal degrees and starts in the Canary Islands up to Cape Kinnaroden, the most northerly point of Norway. From west to east it stretches from the Azores up to Baku in Azerbaijan. The interpolation resolution is a quarter of a decimal degree.
In the period 1992 to 1997 we have a lack of data around the Northern Baltic Sea.
Data in the Middle East, Libya, Western Iceland, Madeira and the Azores are often not sufficient enough to create a reliable interpolation of historical data, in such cases the heat or cold waves will not be determined.