Understanding the heat wave hitting São Paulo with successive records and storm warnings

• Learn more: Bus, metro and train fares will have new prices from January in São Paulo
• See also: December has worst reservoir level in Greater SP since 2015 water crisis

Experts explain that December is normally the month when the Brazilian southeast experiences the height of its rainy season. Persistent cloudiness and frequent rain showers act as a thermal moderator, preventing the sun from continuously warming the surface. However, this pattern was interrupted by a high-level cyclonic vortex (VCAN).

This system acts as an invisible “lid” on the atmosphere. At its edges it can generate rain, but at its center dry air descends from the upper atmosphere to the surface, dissipating clouds and preventing cold fronts from penetrating. The result is a thermal dome where solar radiation falls directly onto the asphalt and concrete, raising temperatures to desert levels.

The seriousness of the event is measured by its proximity to the city’s absolute record. Yesterday’s 37.2°C was only 0.6°C away from the highest temperature ever measured in São Paulo (37.8°C in October 2014), with the fundamental difference that in 2014 the record took place at the height of the dry season, and not during the period when the city should have been in the rain.

A critical factor closely watched by meteorologists is the so-called “high minimum.” Yesterday evening, around 11 p.m., thermometers in neighborhoods like Pinheiros and Centro indicated 25°C.

This phenomenon occurs because the warm air mass is so deep that radiative heat loss during the night is insufficient to cool the environment. For the human body, the lack of nighttime thermal relief increases the risk of hyperthermia and overloads the cardiovascular system, especially in the elderly and children.

Risk of strong storms

The scenario now enters a phase of great danger. As the atmospheric blockage weakens, moisture once again comes into conflict with the superheated air accumulated above the state. MetSul meteorologist Estael Sias details the transition:

— With warm air, with record heat, thunderstorms and rain tend to encounter warm air and form thunderstorms. So I think we will now have a rain regime, gradually over the next few days, returning to the regime expected at this time of year, which is the most frequent instability with almost daily rains in the São Paulo region.

This “available energy” in the atmosphere is the fuel for clouds with strong vertical development, the famous Cumulonimbus clouds. The forecast for the next 48 hours indicates a high risk of:

• Microexplosions and gales: Winds reaching 80 to 100 km/h.
• Hail: due to the strong thermal contrast between the ground and the upper layers of the atmosphere.
• Flash floods: very high volumes of rain over short intervals of time.

Indoor heat x-ray

While the capital monitors records, the interior of the state and the Ribeira Valley are experiencing critical temperatures. Monitoring of automatic stations indicates that cities are operating in a “state of caution” due to extreme heat:

1. Stone of Toledo: 42.1°C
2. Miracatu: 41.6°C
3. Itaoca: 40.0°C
4. Pariquera-Açu: 39.9°C
5. Record: 39.8°C
6. Sète Barras: 39.5°C
7. Juquiá: 39.5°C
8. Iporanga: 39.3°C
9. São Simão: 39.0°C
10. Barra do Turvo: 39.0°C
11. Capivari: 38.7°C
12. Holambra: 38.7°C
13. Pear trees: 38.7°C
14. Monte Alegre do Sul: 38.5°C
15. Valinhos: 38.4°C

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