Don't understand anything about these abrupt changes in weather.

It is easy.

It generally blows from west to east. Norway is more or less the western slope of a north-south mountain ridge facing the Atlantic. Water is picked up over the Atlantic and when the clouds are forced upwards to cross the mountains it rains. So if there are clouds it rains. Thus it can hardly be any weathers other then sunshine or pouring rain/snow.

(Of course there are other weathers in Norway also, because this model is very simplified.)

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