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I notice on a website talking about the jet stream https://www.weather.gov/jetstream/jet

It says:

The winds blow from west to east in jet streams but the flow often shifts to the north and south. Jet streams follow the boundaries between hot and cold air. Since these hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the northern and southern hemisphere winters.

Why are these hot and cold air boundaries mostly pronounced in winter?

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I'll look at the climates of the town of Inuvik In Canada's Northwest Territory at a latitude of 68°21′42″N against that of Mexico City at a latitude of 19°26'N. In July, the daily mean temperatures are 14.1°C in Inuvik and 18.2°C in Mexico City — a difference of 4.1°C.

Compare that small difference with the daily mean temperatures in January of -26.9°C in Inuvik and 14.6°C in Mexico City – a huge difference of 41.5°. That factor of ten increase in the temperature difference in winter over summer drives a lot more north-south circulation in the winter compared to the summer, which in turn makes the jet streams stronger in winter compared to summer.

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Because in summer the Arctic and sub-Arctic are quite warm, some days even hot, so winds blowing north from the Med or North Africa will not produce so great a contrast as they would in the winter. The reason for the Arctic's hot summers is that for a while the sun is in the sky 24 hours a day.

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