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Inmet issued a frost alert for 79 municipalities in Rio Grande do Sul, valid between 4 AM and 8 AM this Wednesday (29). The forecast indicates minimum temperatures of 3°C and a risk of crop loss in the Central-Western, Central-Eastern, Northwestern, and Southwestern Rio-Grandense regions, as well as the Metropolitan Region of Porto Alegre. The polar air mass, active since Monday, has already caused a minimum of -3.6°C in Bom Jardim da Serra, Santa Catarina.
The Inmet (National Institute of Meteorology) placed 79 municipalities in Rio Grande do Sul on frost alert this Wednesday (29), with minimum temperatures predicted to be 3°C between 4 AM and 8 AM. The warning includes regions that concentrate a significant part of the gaucho agricultural production, such as Central-Western, Central-Eastern, Northwestern, and Southwestern Rio-Grandense, in addition to the Metropolitan Region of Porto Alegre. The institute points to a slight risk of crop loss, especially for crops sensitive to cold such as vegetables, fruit trees, and young seedlings.
The polar air mass causing the cold has been active over Southern Brazil since Monday (27) and has already shown its intensity. In Bom Jardim da Serra, in the Santa Catarina mountains, thermometers registered a minimum of -3.6°C on Tuesday morning, the lowest temperature of the year so far. The white frost transformed the mountain landscapes and confirmed that this cold snap is the most significant of 2026 for the Southern region. For the 79 municipalities on alert, Wednesday promises to dawn with conditions favorable for ice formation on surfaces and vegetation.
What is frost and why does it form under these conditions
Frost is a meteorological phenomenon that occurs when water vapor present in the air passes directly into a solid state in a process called sublimation. The result is a layer of ice that deposits on surfaces, leaves, and crops, creating the white landscapes that characterize cold mornings in Southern Brazil. The so-called white frost, the most common type, transforms fields and rural areas into scenes that appear covered in snow.
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For frost to form, specific conditions are necessary: high humidity, clear skies without clouds, temperatures close to 0°C, and little wind. The absence of clouds favors the loss of surface heat by radiation, and calm wind prevents air mixing, allowing the layer closest to the ground to cool below zero even when the air temperature at two meters high still shows positive values. This is why Inmet issues an alert with minimums of 3°C: on the surface, the actual value can be much lower.
Which are the 79 municipalities on alert and which regions are affected
According to information released by the ndmais portal, the 79 municipalities covered by Inmet’s alert are concentrated in four regions of Rio Grande do Sul: Central-Western, Central-Eastern, Northwestern, and Southwestern Rio-Grandense, in addition to the Metropolitan Region of Porto Alegre. The geographical distribution shows that frost is not limited to high-altitude areas like the mountains, but advances to plain regions and valleys where agricultural production is intense.
The scope of the alert particularly concerns rural producers who cultivate vegetables, temperate climate fruits, and pastures. Crops such as lettuce, strawberries, tomatoes, and fruit tree seedlings are particularly vulnerable to frost, and the formation of ice on the leaves can cause irreversible damage in a few hours. For larger properties, the loss can represent weeks of work and investment compromised in a single morning.
The minimum of almost 4 negative degrees that has already hit Santa Catarina
Before the alert was issued for Rio Grande do Sul, the Santa Catarina mountains had already felt the force of the polar air mass. Bom Jardim da Serra registered -3.6°C on Tuesday morning (28), the lowest temperature of the year in all of Brazil. The frost covered fields, fences, and low-lying vegetation, and images from the region showed completely white landscapes at dawn.
Other mountain towns in Santa Catarina also recorded temperatures close to zero, including Urupema, São Joaquim, and Urubici, destinations that traditionally lead the state’s cold records. The intensity of the minimums confirms that the polar air mass that arrived this week is stronger than previous ones, and the forecast of frost for Rio Grande do Sul on Wednesday is a direct consequence of the advance of this cold system that already hit Santa Catarina hard in the previous dawn.
The risk to crops and what producers can do
Inmet classifies the risk of crop loss as mild, but experience shows that even light frosts can cause significant damage to sensitive crops. Leafy vegetables, such as lettuce and arugula, can be destroyed with just one night of frost, and young fruit tree seedlings lose their development capacity when exposed to temperatures below 2°C for prolonged periods.
Preventive measures include sprinkler irrigation during the early morning, which creates a layer of ice around the leaves that paradoxically protects them from even lower temperatures. Coverings with tarps, mulching, and the formation of smoke screens are alternatives for small producers, but depend on constant monitoring and action before the frost forms. For pastures, extreme cold can burn vegetation and reduce the availability of food for livestock.
What to expect from the cold in the coming days in Southern Brazil
The polar air mass causing the frosts is expected to lose intensity from Thursday (30), but minimums will remain low in Rio Grande do Sul and the Santa Catarina mountains at least until Friday (1st). From the weekend, a new cold front may reorganize the instability, bringing rain and a gradual rise in temperatures that will end the week’s frost cycle.
For those living in the 79 municipalities under alert, Inmet’s recommendation is to monitor daily updates and adopt protection measures for both crops and the health of the elderly and homeless, who are most vulnerable to extreme cold. Civil Defense can be contacted by phone 199, and Inmet maintains updated information on its website and app with details by municipality.
Are you in one of the 79 municipalities under frost alert or have you already woken up to ice in your backyard this week? Tell us in the comments how cold it is in your region and if the frost caused damage to crops or in your city’s daily life.
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