Crepuscular comes from the Latin crepusculum, which means twilight. Crepuscular rays can appear when the clouds (or terrain) that are responsible for the rays are below the horizon. The so-called crepuscular rays are solar rays cast on the purple red, while the sun is below the observer's horizon, and sometimes span the entire twilight sky towards the Earth's shadow band (the twilight wedge). If the purple red is obvious, look for these uncommon rays fanning out of the horizon.
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