Weather

Heatwaves do not have a unique, specific definition but are generally accepted to be prolonged periods of time during which the near surface temperature is notably warmer than the climatological average. In recent years, Europe has been affected by two exceptional heatwaves: the central European heatwave of 2003 (Black et al., 2004) and the Russian heatwave of 2010 (Grumm, 2011). Both of these heatwaves led to a large number of deaths: Robine et al. (2008) estimate that the 2003 heatwave led to an additional 70 000 deaths (relative to the average for the time of year), and Barriopedro et al. (2011) estimate that 55 000 extra deaths occurred in 2010. Many studies have analysed the meteorological causes of these two heatwaves. Both were associated with prolonged periods of high pressure which remained relatively stationary. Black et al. (2004) also analysed the role surface fluxes had in the 2003 heatwave. They showed that as the summer progressed, the latent heat flux changed from a positive anomaly to a negative anomaly as the soil dried, whereas the surface heat flux anomaly continued to increase and likely contributed significantly to the positive 2-m temperature anomalies observed for August 2003. Despite its northern latitude, ranging from 60 and 70°N, Finland can experience high temperatures during summer (e.g. Kim et al., 2018) due to its relatively continental climate and the relative warmth (given its latitude) of the nearby North Atlantic Ocean. The current 2-m temperature record in Finland is 37.2°C and was set in Joensuu in eastern Finland in July 2010, during the Russian heatwave. In Summer 2018, many parts of Europe again experienced above-average temperatures for prolonged periods of time, and Finland was no exception. Anomalous warmth was recorded across almost all parts of Finland in May and July 2018. In terms of the national monthly mean temperature, May and July were record warm months, exceeding previous records by 0.5 and 0.4 degC respectively (FMI, 2018a,b). These observations of 2-m temperature, which are discussed further below, are the main motivation for this study. The first objective of this investigation is therefore to quantify the magnitude of the 2018 heatwave in Finland on a month-by-month basis using standard SYNOP observations. The second objective is to identify the large-scale circulation pattern which was present during the heatwave. The final objective is to use radiosonde soundings to determine the vertical extent of the heatwave over Finland. This last objective is motivated by the fact that most heatwave studies focus on 2-m temperature and that heatwaves, for example in 2003, can be strongly influenced by the lack of surface moisture (Fischer et al., 2007) and thus may be constrained within the boundary layer. Observations of 2-m temperature