The polar jet stream is strongest in the winter when surface temperature contrasts are greatest.

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The polar jet stream is a fast-flowing, narrow wind current that circles the Earth at the middle to upper troposphere. It is strongest in the winter when the temperature contrasts between the polar regions and the mid-latitudes are greatest. Warmer air at lower latitudes tends to rise and mix with colder air from the poles, creating a strong temperature gradient that drives the polar jet stream. In the summer, the temperature contrast is less pronounced, and the jet stream can weaken or move northward.

The Polar Jet Stream Is Strongest in the Winter When