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Abstract (summary): Despite the potential hazards associated with thunderstorms, they have been underrepresented in climatology studies. Southern Ontario is Canadaâ s most active thunderstorm region, and the countryâ s most populous and industrialized region. To date there has been no analysis of past trends of thunderstorms in Southern Ontario, or any analysis of how thunderstorm frequency might change over the current century. This thesis consists of three research chapters flanked by an introduction (Chapter 1) and discussion (Chapter 5). In Chapter 2 manual thunderstorm observations from eight Environment Canada weather stations are evaluated for accuracy by comparing them to data from the Canadian Lightning Detection Network. The results indicate the manual observations are reliable for small distances around each weather station, as is expected given the normally localized nature of thunderstorms. In Chapter 3 the historical manual hourly thunderstorm observations are evaluated for trends over the past several decades. Daily precipitation and wind gust data are used as proxies to determine if there have been changes in thunderstorm intensity, and yearly thunderstorm occurrence is compared to the larger scale phenomena ENSO and NAO. No consistent significant trends were observed over this period in either thunderstorm occurrence or intensity and a correlation between thunderstorm frequency and ENSO and NAO was also not detected. In Chapter 4 thunderstorm occurrence was successfully related to convective available potential energy (CAPE), with the probability of observing a thunderstorm on a given day at each of the weather stations increasing with daily maximum CAPE. While there were no consistent significant trends in CAPE observed over the reference period, by statistically downscaling three general circulation models it was found that large and robust increases in CAPE are expected over the coming decades across all weather stations, which consequently will have the potential to result in an increase in thunderstorm frequency.