The climate of the Grand River valley is influenced by the heat of the sun falling on land slopes, the direction of the prevailing westerly winds and the closeness of the large water bodies of the Great Lakes. Prevailing winds in southwestern Ontario receive warmth and moisture from the Great Lakes. Alternating flows of warm, humid air from the Gulf of Mexico, and cold, dry, arctic air from the northwest cause changeable weather and thunderstorms, especially during spring and autumn.
From the plains bordering Lakes Erie and Huron, the rising land of the Huron and South Slopes climate zones receives moisture picked up by winds blowing over Lake Huron. This condenses as snow or rain on the lee-side land mass, creating 'snowbelt' areas with a higher than average rainfall and snow accumulation. This includes the land area lying on the west side of the Grand River watershed between the towns of Arthur and Stratford.
At the head of the Huron Slope, the higher altitude of the Dundalk Upland climate zone produces a cooler climate, similar to that of more northern regions in Ontario. Any moisture left in winds after they pass over the Slopes is dropped on this tableland as snow or rain, and winters are longer and colder here.
Close to the Lake Erie shore, winds passing over the lake are warmed in winter and cooled in summer, producing a warmer climate with a longer, frost-free period for plant growth. This area includes the flat lowland plains from the mouth of the Grand River northwards almost to Brantford.
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