# Population Bias on Tornado Reports in Europe
## 1 Introduction  
Tornadoes in Europe can be associated with severe damage and can result in injuries and fatalities. Despite this, until recently their threat has been underestimated. [PERSON] et al. [1] showed that European tornadoes reported between 1995-2015 resulted in 4462 injuries and 316 fatalities and damages estimated at more than 61 billion. They also indicated that the density of tornado reports was the highest over Belgium, Germany, the Netherlands, and the southeastern United Kingdom. For these countries, there is a reporting bias, as central and western Europe have a high population density compared with other regions in Europe (e.g., Eastern Europe). Coastal areas were also hot-spots for tornado reports (e.g., western Italy, eastern Spain), as these areas tend to have higher population density compared with inland areas and also because of the waterspouts that move from the Mediterranean Sea inland [2]. This difference in the population density (i.e., different regions of Europe, coastal area versus inland area) introduces a bias in the reporting of tornadoes. This is because tornadoes (and also other types of severe weather events like hail or extreme winds) are \"targets of opportunity\" [3]. Thus, an observer needs to witness the event and then to report it and systems need to exist for collecting and verifying the reports (i.e., tornado database). Very few countries in Europe have developed and maintained such databases, which resulted in a lack of information about tornadoes [2]. One reason for not developing tornado databases is that they do not seem justified. Compared with the United States, the impact of tornadoes in European countries (given their relative small area) is relatively low, and thus there is no need to develop tornado databases for individual countries [4]. Only when considered from a pan-European perspective does the impact of tornadoes in Europe start to emerge. The collection of tornado reports and other types of severe weather reports at the pan-European level started in 2006 with the development of the European Severe Weather Database (ESWD) by the European Severe Storms Laboratory [5]. Currently, ESWD contains more than 16,500 tornado reports collected between 1800-2020. Given the differences in population density across Europe, the real number of tornadoes is an unknown quantity. Even in relatively high populated areas, tornadoes might not be reported because of their small spatial extent and short life time, obstruction of theobserver view point (e.g., forest, hills, buildings) or if the tornado occurred during the nighttime. As indicate in previous studies for the United States [6] and Canada [7], the population density is the key factor, besides the meteorological factors, in determining the bias in tornado reports. Several studies have addressed the issue of population bias on tornado reports. [PERSON] et al. [6] used a hierarchical Bayesian model to account for the population bias on tornado occurrence using historical tornado reports for the United Stated from the Storm Prediction Center between 1953-2001. Their results for the central and eastern United States indicated that F0-F1 tornado reports vary less with population density compared with F2-F5 tornadoes. Starting from the hypothesis that the number of tornado reports in Canada is significantly lower than the actual number of tornadoes, [PERSON] et al. [7] also used a Bayesian modeling approach that considered the population bias on tornado reports. Their model also included the occurrence of cloud-to-ground lightning, as cloud-to-ground lightning can be used to quantify convective storms activity and thus can be used as precursor for tornadoes. Their results showed that in areas with low population density, the probability of tornado occurrence is significant higher compared with the observed tornado climatology for Canada. More recently, [PERSON] et al. [8] developed a Bayesian hierarchical modeling framework for correcting the reporting bias in the United States tornado database.