Geophysical Research Letters | Schemm et al. 
Evidence is presented that the frequency of extremely strong fronts, which occur mainly in summer, has increased over Europe in ERA-Interim reanalyses data (1979–2014). Fronts are defined using a common detection scheme based on gradients of equivalent potential temperature (θe) at 850 hPa. The frequency increase is due to increasing atmospheric humidity, which in turn is reported as statistically significant over Europe in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). There is no trend in the frequency of extremely strong fronts in North America where humidity trends are close to zero. Because frontal precipitation increases with frontal strength, measured by the θe gradient, the increase in the number of extremely strong fronts may help explain regional patterns of longer-term trends in strong precipitation events.
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