This is the 14544th most frequent Russian word.
пасмурно
Overcast; gloomy.
The word 'пасмурно' is used here as an adverb to describe the weather condition of being overcast.
Сегодня на улице пасмурно.
Today it is overcast outside.
In this sentence, 'пасмурно' again functions as an adverb, indicating the overcast weather during the past occasion.
Было пасмурно, но мы решили пойти на прогулку.
It was overcast, but we decided to go for a walk.
The word 'пасмурно' is utilized adverbially to generalize the overcast weather typical in the region during autumn.