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The alert – in effect in the regions of Mato Grosso do Sul, São Paulo, Goiás, Minas Gerais, Rio de Janeiro and Espírito Santo – means that thermometers could be 5ºC above the December average for five days or more in these regions.
The heatwave began on December 22 due to a combination of atmospheric factors typical of summer, but intensified by the end of the month.
The phenomenon is classified as such by meteorologists when temperatures are significantly above average for several consecutive days.
What draws attention to this episode is not just the specific heat of a specific afternoon, but its continuity.
Unlike isolated peaks, frequent in summer, the current atmospheric configuration favors successive days of high temperatures, including during the night and early in the morning, which makes recovery of the human body difficult and increases thermal discomfort.
According to consolidated data from Inmet on Sunday (28/12), among the 10 hottest cities in Brazil, four are in the “great danger” alert zone in southeastern Brazil. The other six are cities in the northeastern hinterland, a region traditionally warm at this time of year.
These are the highest temperatures in the country on Sunday:
- 1st – Três Rios (RJ) – 39.1ºC
- 2nd – Caicó (RN) – 38.8 ºC
- 3rd – Alegre (ES) – 38.5ºC
- 3rd – Coronel Pacheco (MG) – 38.5ºC
- 3rd – Sugarloaf Mountain (AL) – 38.5ºC
- 6th – Cambuci (RJ) – 38.4ºC
- 7th – Ibimirim (PE) – 38.3ºC
- 8th – Ibotirama (BA) – 38.2ºC
- 8th – Morada Nova (CE) 38.2ºC
- 8º – Salgueiro (PE) – 38.2ºC
125 km from Rio de Janeiro, Três Rios has around 80,000 inhabitants. The city is located in a valley area, which makes it difficult for winds to circulate and “traps” warm air. Caicó is in the heart of the semi-arid Rio Grande do Norte region, far from the sea, with lots of sun and little rain.
Inmet also published a partial list of the four cities that recorded the highest temperatures from this Monday (12/29) until this afternoon. They are:
- 1st – Sugarloaf Mountain (AL) – 39.2ºC
- 1st – Três Rios (RJ) – 39.2 ºC
- 3rd – Barra (BA) – 38.9ºC
- 4th – Coronel Pacheco (MG) – 38.7ºC
The cause of the heatwave
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The central explanation for the current heatwave is the action of a mass of hot and dry air which has settled over the center-south of the country. This system is reinforced by the South Atlantic Subtropical High, a large anticyclonic system that currently acts as an atmospheric blockade.
In practice, it prevents the advance of cold fronts and areas of more organized rain, keeping warm air “trapped” over the region for several days.
With fewer clouds and little circulation from instability systems, the ground receives more solar radiation during the day and loses less heat at night. The result is very hot afternoons and equally sweltering mornings, a classic heat wave pattern.
This type of blocking also helps explain why rains, when they do occur, are isolated and short-lived, insufficient to more broadly alleviate heat.
Another important point is the context of the beginning of summer. December is already, historically, a hot month in much of Brazil.
When an atmospheric blockage sets in during this period, it tends to reinforce conditions that would already be naturally favorable for warmth, further raising temperatures and increasing the chances of records being reached.
In general, meteorologists consider a heatwave to occur when temperatures are at least 5°C above average for a minimum period of five consecutive days.
Inmet adopts a similar criterion, but considers the increase relative to the monthly average, regardless of the exact number of days, as long as the deviation is significant.
In the current episode, both criteria are met in several regions: temperatures are constantly high and remain well above the climatological norm for December. This prolonged nature is the main factor of concern for health and civil protection authorities.
The recommendation from authorities is to increase hydration, avoid prolonged exposure to the sun during the hottest periods of the day and seek ventilated environments as much as possible. In the event of more intense symptoms, it is advisable to consult a doctor or call Civil Protection.
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