This is the 16532nd most frequent Polish word.
wiosenna
"Spring" or "springtime" (adjective, feminine form).
In this sentence, 'wiosenną' is an adjective in the accusative form modifying 'pogodę', meaning 'spring'.
Każdego roku czekam na wiosenną pogodę.
Every year I wait for spring weather.
Here, 'wiosennym' is an adjective in the instrumental form modifying 'parku', suggesting that the park is related to spring.
Spacerowaliśmy po wiosennym parku pełnym kwiatów.
We strolled through the spring park full of flowers.
In this example, 'wiosenna' is an adjective in the nominative form which directly describes 'atmosfera', denoting that the atmosphere is characteristic of spring.