Foggy first day in autumn weather

Porto Alegre and the metropolitan area experienced a foggy first day in a climatic autumn | Adrien Braga / Mitsul Archive

March has only begun and the fog has already appeared. Porto Alegre and the metropolitan area witnessed this Thursday (2) the first day of the so-called climatic or meteorological autumn in which fog was recorded. Autumn, although starting by astronomical standards only on the 20th, in climatology includes the quarter from March to May.

Data from the Brazilian Air Force at Salgado Filho Airport showed that the fog started today at 5:15 am, when the horizontal visibility dropped to less than 1,000 metres, the level that separates the fog from the fog. The restrictions began two hours ago when visibility dropped to 2,500 meters in the fog.

As is common at this time of year, even in early March, when fog forms, it tends to appear late in the morning and dissipate very quickly in the early morning. So much so that at the eight in the morning the airport no longer reports the phenomenon with a horizontal view of 1200 meters, hence the fog. Lowest visibility was 300m at 7am.

The nearby Kanawas air base also recorded reports of fog late in the morning and early Thursday morning. The first report, at 6 a.m., actually accused the phenomenon’s existence of a 400-meter view that stayed on at 7 a.m. and 8 p.m. updates.

Fog climate in Porto Alegre

Porto Alegre is perhaps one of the most foggy regions in Brazil and the reasons are geographical in nature. In addition to being in southern Brazil, where the movement of cold air favors this phenomenon more, the capital of Rio Grande do Sul is surrounded by several rivers and also has a lake (Guaíba) next to it, not to mention being north of Lagoa dos Patos.

The presence of rivers still makes the Porto Alegre area used for rice plantations, making many areas around the city swamps and swamps. Thus, what is not lacking is the surface moisture, which combined with nighttime coolness in the coldest months of the year ends up bringing many days with fog and mist in the city and in the metropolitan area.

A study by the Brazilian Air Force (FAB) of the replication of this phenomenon at Salgado Filho Airport, conducted between 1998 and 2006, by Luis Alexandre Fragoso de Almeida, showed that the months of the second half of winter are autumn, which is the highest. This phenomenon occurs at the airport.

The three months with the highest frequency of fog in the study period, according to research published by FAB, were June with 63 days, the highest with 56 days and July with 48. Then they appear April with 33 days, September with 18 and March with 12. The period is from November November to February has a very low rate of the phenomenon.

Causes of fog

When the air is cooled by the loss of nocturnal radiation, favorable conditions arise for the formation of mist banks along rivers, especially in valleys, as vapor mist over water sources. Such accidents are common in the fall, when the water is warmer and the air is already cold.

The nights get cooler, the temperature drops more, and the warm waters exchange moisture with the layer of cool air just above the surface. The frequency of this type of fog, in particular, is frequent with valleys cut by rivers.

The particles in the water source always evaporate, which leads to moisture getting into the air. The warmer the water, the greater the evaporation and the greater the presence of moisture in the air. If a cool, clear night is recorded, approaching the dew point temperature, fog begins to form and in this type of situation, as a rule, it is shallow, which allows you to see even the highest tall buildings.