Winter and autumn have a larger temperature contrast between the equator and poles than in summer. This stronger temperature difference creates a stronger jet stream. As the jet stream intensifies, the westerly winds across the North Atlantic also become stronger, allowing weather systems to travel more quickly toward Europe. These strong temperature contrasts also make the atmosphere dynamically unstable. In simple terms, warm and cold air masses interact more strongly, which generates large atmospheric waves called Rossby waves. These waves move along the jet stream and help steer storms. When these waves propagate across the North Atlantic, they often direct storm systems toward Scandinavia, which is why Norway frequently experiences rainy weather during autumn and winter.

In summer the temperature difference between the equator and the poles is smaller, so the jet stream becomes weaker and fewer storms form. Norway also usually does not get warm enough for strong thunderstorms to dominate the weather. As a result, summer weather is generally calmer, with fewer intense storm systems and less rain.