A combination of atmospheric factors typical of summer, but intensified at the end of December, explains why much of central and southern Brazil entered a prolonged period of extreme heat.
Since the start of the week, large areas of the South-East, part of the South and the Center-West have recorded temperatures well above the expected norm for this time of year, in a scenario which combines persistence, records and health risks.
The phenomenon is classified by meteorologists as a heat wave, which is when temperatures are significantly above average for several consecutive days.
In the present case, the episode began to take shape on Monday (22) and intensified throughout the week, leading the National Institute of Meteorology (Inmet) to issue an “orange alert”, with danger valid until Friday evening (26).
The alert covers eight states and indicates that thermometers are expected to remain approximately 5°C above the climatological average, with a potential direct impact on the health of the population.
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.
In São Paulo, the capital recorded 35.9°C on December 25, the highest temperature ever observed for the month since official measurements began. The previous record, 35.6°C, was recorded in December 1998.
In Rio de Janeiro, thermometers reached 41°C, leading the city hall to activate heat level 3 on a scale of up to 5, used to indicate the severity and persistence of high temperatures.
But the phenomenon is not limited to capital cities or specific urban areas. Intense heat is spreading across large inland regions of the Southeast, South and Midwest, affecting agricultural areas, metropolitan areas and mid-sized cities.
In many of them, the heat is classified as strong or extreme, according to health authorities, especially when combined with low air humidity.
Why is this heatwave happening now?
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 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.
Which regions are most affected
The most intense heat is concentrated in much of the Southeast, but also progresses over areas of the South and Midwest.
São Paulo and Rio de Janeiro mainly, but Minas Gerais and Espírito Santo are among the most affected states, with temperatures well above the December average.
The phenomenon also reaches Santa Catarina and Paraná, to the south, as well as Mato Grosso do Sul and areas of the interior of Goiás, thus expanding the reach of the Center-South under the influence of the heatwave.
In the South-East, warming is stronger in regions far from the coast, where the influence of sea breezes is less.
In the Midwest, the trend is toward lingering heat with isolated rain showers, typical of very hot and humid days.
In the south, especially between Paraná and Santa Catarina, the most critical period was Christmas, with the expectation of gradual relief after the rains resumed.
In the North and Northeast, which are not under the direct influence of this heatwave, the most intense heat is concentrated inland, while the coastal areas remain relatively milder thanks to oceanic ventilation. There is nevertheless a risk of storms, particularly in the northern region.
Health risks and what to expect in the coming days
Inmet warns that heat waves increase the risk of dehydration, heat exhaustion and worsening cardiovascular and respiratory diseases. Feelings of fatigue, listlessness, dizziness and malaise can be signs that the body is having trouble handling excess heat, especially in the elderly, children and people with chronic illnesses.
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.
The trend, according to forecasts, is that the heat will persist at least until the end of the week, with the possibility of high temperatures until Sunday (28).
From there, gradual changes in the atmospheric pattern could allow the development of more organized rain systems, helping to reduce heat in part of the Mid-South. Until then, the country remains in a typical extreme summer scenario, increasingly common in an increasingly hot climate and more prone to intense events.
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