Het regent pijpestelen — “It is raining pipe stems.” This is the Dutch equivalent of “It is raining cats and dogs” — used for heavy, torrential rain. The pijpesteel (pipe stem — the long, thin stem of a clay tobacco pipe) presumably describes rain falling in long, straight, heavy lines. The Dutch climate makes this idiom practically relevant — the Netherlands is famously wet, and regen (rain) vocabulary is linguistically rich.
Dutch weather vocabulary: het miezert (it is drizzling — a distinctly Dutch word for the fine persistent drizzle so common in the Netherlands), het stormt (it is storming), het hagelt (it is hailing), er is mist (it is foggy), het vriest (it is freezing), er staat een stevige wind (there is a strong wind — very common). The Dutch talk about weather constantly, partly because it changes constantly and partly because it is a universally safe social topic.
Weather-related idioms: Er zit meer in dan mooi weer alleen (There is more to it than just nice weather — something has more substance than appears), Na regen komt zonneschijn (After rain comes sunshine — equivalent to “every cloud has a silver lining”), Iets voor regen en wind bewaren (to keep something for rain and wind = to save something for a rainy day). Weather is embedded in Dutch idiom because weather is embedded in Dutch life.