Rio has a tropical savanna climate are common during the summer, though rarely for long periods, while maximum temperatures above 27 °C (81 °F) can occur on a monthly basis.
Along the coast, the breeze, blowing onshore and offshore, moderates the temperature. Because of its geographic situation, the city is often reached by cold fronts advancing from Antarctica, especially during autumn and winter, causing frequent weather changes. In summer there can be strong rains, which have, on some occasions, provoked catastrophic floods and landslides. The mountainous areas register greater rainfall since they constitute a barrier to the humid wind that comes from the Atlantic. The city has had rare frosts in the past. Some areas within Rio de Janeiro state occasionally have falls of snow grains and ice pellets (popularly called granizo) and hail.
Drought is very rare, albeit bound to happen occasionally given the city’s strongly seasonal tropical climate. The Brazilian drought of 2014–2015, most severe in the Southeast Region and the worst in decades, affected the entire metropolitan region’s water supply (a diversion from the Paraíba do Sul River to the Guandu River is a major source for the state’s most populous mesoregion). There were plans to divert the Paraíba do Sul to the Sistema Cantareira (Cantareira system) during the water crisis of 2014 in order to help the critically drought-stricken Greater São Paulo area. However, availability of sufficient rainfall to supply tap water to both metropolitan areas in the future is merely speculative.
Roughly in the same suburbs (Nova Iguaçu and surrounding areas, including parts of Campo Grande and Bangu) that correspond to the location of the March 2012, February–March 2013 and January 2015 pseudo-hail (granizo) falls, there was a tornado-like phenomenon in January 2011, for the first time in the region’s recorded history, causing structural damage and long-lasting blackouts, but no fatalities. The World Meteorological Organization has advised that Brazil, especially its southeastern region, must be prepared for increasingly severe weather occurrences in the near future, since events such as the catastrophic January 2011 Rio de Janeiro floods and mudslides are not an isolated phenomenon. In early May 2013, winds registering above 90 km/h (56 mph) caused blackouts in 15 neighborhoods of the city and three surrounding municipalities, and killed one person. Rio saw similarly high winds (about 100 km/h (62 mph)) in January 2015. The average annual minimum temperature is 21 °C (70 °F), the average annual maximum temperature is 27 °C (81 °F), and the average annual temperature is 24 °C (75 °F). The average yearly precipitation is 1,069 mm (42.1 in).
Temperature also varies according to elevation, distance from the coast, and type of vegetation or land use. During the winter, cold fronts and dawn/morning sea breezes bring mild temperatures; cold fronts, the Intertropical Convergence Zone (in the form of winds from the Amazon Forest), the strongest sea-borne winds (often from an extratropical cyclone) and summer evapotranspiration bring showers or storms. Thus the monsoon-like climate has dry and mild winters and springs, and very wet and warm summers and autumns. As a result, temperatures over 40 °C (104 °F), that may happen about year-round but are much more common during the summer, often mean the actual temperature feeling is over 50 °C (122 °F), when there is little wind and the relative humidity percentage is high.
Rio de Janeiro is second only to Cuiabá as the hottest Brazilian state capital outside Northern and Northeastern Brazil; temperatures below 14 °C (57 °F) occur yearly, while those lower than 11 °C (52 °F) happen less often. The phrase, fazer frio (“making cold”, i.e. “the weather is getting cold”), usually refers to temperatures going below 21 °C (70 °F), which is possible year-round and is commonplace in mid-to-late autumn, winter and early spring nights.
Between 1961 and 1990, at the INMET (Brazilian National Institute of Meteorology) conventional station in the neighborhood of Saúde, the lowest temperature recorded was 10.1 °C (50.2 °F) in October 1977, and the highest temperature recorded was 39 °C (102.2 °F) in December 1963. The highest accumulated rainfall in 24 hours was 167.4 mm (6.6 in) in January 1962. However, the absolute minimum temperature ever recorded at the INMET Jacarepaguá station was 3.8 °C (38.8 °F) in July 1974, while the absolute maximum was 43.2 °C (110 °F) on 26 December 2012 in the neighborhood of the Santa Cruz station. The highest accumulated rainfall in 24 hours, 186.2 mm (7.3 in), was recorded at the Santa Teresa station in April 1967. The lowest temperature ever registered in the 21st century was 8.1 °C (46.6 °F) in Vila Militar, July 2011.