Heat kills more people than any other weather event. It is not so much the day's high that is the danger as the succession of hot days and nights that no longer cool down: the body does not recover, and mortality rises with the number of consecutive hot nights.
Why this indicator matters
A night is called tropical when the low does not fall below 20 °C. Cities are the first victims: concrete and asphalt store heat by day and release it at night, so the centre can stay several degrees above the surrounding countryside.
A poorly designed home holds on to yesterday's heat. The enemy is wasted time: you need to open up at the exact moment the outside becomes cooler than the inside, to push out the accumulated heat, then close again as soon as the outside warms up, to keep the heat out.
How to read it
Over the next seven days, the box counts the days at 30 °C or above and the nights at 20 °C or above, and speaks up only if one of the next three days crosses one of these thresholds. It gives the expected high.
It then suggests a ventilation slot: the evening hour, from 17:00, when the outside air drops below 26 °C and stays there for at least four hours, and the morning hour when it climbs back above. If the air never drops below 26 °C, the sentence says so: ventilating would be pointless.
The thresholds used on this site
Every sentence in the “Today” box comes from a numerical rule. Here are the rules for this indicator, exactly as they are applied in the code.
- Hot day: high of 30 °C or more; very hot: 35 °C or more.
- Tropical night: low of 20 °C or more; from 3 nights out of 7 days, the indicator switches to “caution”.
- The indicator speaks up if one of the next 3 days crosses either threshold.
- Reference indoor temperature: 26 °C (a warm, non-air-conditioned home).
- Open up from 17:00, if the outside air stays below the reference for at least 4 hours.
A worked example: Toronto
In Toronto, with the forecasts of 2026-11-02 08:13, this indicator stays silent: none of the conditions above is met at the moment. The forecast high over the next three days is 20.3 °C.
Its limits
The real indoor temperature depends on insulation, orientation, shutters and which floor you are on; the 26 °C threshold is a reference, not your thermometer. The calculation does not allow for the urban heat island, which can add several degrees at night in a city centre. It does not replace public health guidance: older people, children and the sick need to be watched.
Frequently asked questions
Why ventilate at night rather than by day?
Because the outside air is then cooler than the air inside. Opening up in the middle of the afternoon lets in air that is hotter than the air in the home.
At what temperature do we talk about a heatwave?
Official thresholds vary by region and duration. On this site we use simple benchmarks: 30 °C for a hot day, 35 °C for a very hot day, and a low of 20 °C for a tropical night.
What can I do when even the night stays hot?
Keep shutters and windows closed by day, drink regularly, spend time in the coolest room, and wet your skin and clothes. A fan helps sweat evaporate, but not above about 35 °C.