This is the 14248th most frequent Polish word.
promieniu
"Promieniu" means "radius" or "ray" in a specific grammatical case (locative or dative) in Polish.
In this context, 'promieniu' denotes the locative description of the zone influenced by the rain.
Znajdowaliśmy się w promieniu padającego deszczu.
We were within the radius of falling rain.
'Promieniu' is the locative singular form of the noun 'promień' (radius), used here to describe a spatial extent.
W promieniu pięciu kilometrów nie ma żadnych sklepów.
Within a radius of five kilometers, there are no shops.
Here, 'promieniu' in the locative case describes the area affected by the spreading light.