The Laugavegur trail in Iceland is usually cool and wet in September. Expect daytime temperatures around 11-13°C and nighttime temperatures between 6-9°C. It rains almost every day, with about 3.2 mm to 9.0 mm of precipitation.
The weather changes often, with rain followed by sunny or partly cloudy periods. You might see moderate to heavy rain. Cloudy skies are also common, making the trail feel cool and damp.
In short, the temperature stays mild during the day and cooler at night, but the weather can change a lot. It will mostly be wet, so be prepared for rain every day.
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