during the winter months, one of the things to look forward to is a phenomenon called 'maré roxa', or 'purple tide'. the purple tide happens after a cold front leaves rio de janeiro, and marine currents coming from the south of brazil get to rio's coastline, paired with winds from the south-west - the result is a spectacularly clear, calm, warm crystalline water along the oceanic beaches, with temperatures between 0 and 25 degrees celsius (68 and 77 degrees fahrenheit).

heavy rains can affect the quality of all rio's beaches, sometimes making them unsuitable for swimming - on this link, the instituto estadual do ambiente (inea) provides the suitability of several beaches. constantly updated, the red flags mean that swimming is not advisable, green flags mean waters are clean, and yellow flags suggest using own judgment. any of rio's beaches may show polluted waters within 24 to 48 hours following heavy rains.