However, frost sometimes occurs in the city. Occurrence of radiation fog is common, causing several delays in early flights. Unlike other large Brazilian cities much farther north, notably Brasília, São Paulo, and Rio de Janeiro who observe a pronounced summer maximum in precipitation amounts, the city of Porto Alegre experiences a prominent winter maximum in precipitation values and cloud cover, for the summer season is primarily hot and dry; though evidently high humidity levels often give a distinct mugginess to the air and negatively impacts air quality. Annually Porto Alegre receives an average-total precipitation amount of . This average makes the city slightly wetter than Rio at yet, to a considerable extent, drier than both São Paulo at and Brasilia at . The highest recorded temperature was  while the lowest recorded temperature was . Porto Alegre lies in a transitional area between the subtropical forest and the Southern Brazilian grasslands (pampa). A number of conservation programmes have been established to protect native trees. Many of the city's avenues have been planted with different tree species. One striking example of this is Teresópolis Avenue, where bottle trees have been planted. The city is covered in green vegetation and Lapacho and Jacaranda are the main species that can be found locally. The trees from the hills are protected. Two environmental conservation areas can be found in this city: "Delta do Jacuí" (Jacuí Delta) State Park and Lami Biological Reserve. The urban area has many parks and plazas, making Porto Alegre one of the greenest provincial capitals in Brazil. The first city squares date from the second half of the 18th century and were originally large public spaces used as food markets. The city has of green space, occupying 31 percent of the city's area. This is an average of 17.6 m² per person. More than one million trees line the public streets and SMAM plants an average of 30,000 seedlings each year. The four main parks are: Parque Farroupilha, a park; Jardim Botânico (The Botanical Garden of Porto Alegre), with some 725 species of vegetation on about of land; and Parque Marinha do Brasil (The Brazilian Navy's Park), a vast park of more than which offers a wide variety of sports fields and tracks. The city's cycleway is called the Caminho dos Parques, which at over long links the Moinhos do Vento, Farroupilha and Guaíba shore parks.