Summer heat

What we use

Land surface temperature measured by Copernicus Sentinel-3, the European Union satellites that pass over Italy twice a day. It is how hot the ground, the roof or the tarmac get seen from above: on an August day with 33 degrees of air temperature in the shade, tarmac reaches about fifty and grass about thirty. Each measurement covers a square one kilometre across. Copernicus data is open and free to use.

How we use it

We take every daytime pass in June, July and August of the latest summer and average them, leaving out cloudy days. Then we measure two things:

• the spot: how hot the surface around the address gets;
• the area: how the heat is spread across the twelve kilometres around, from the coolest point to the warmest.

The report puts the two numbers side by side, says how far above or below the local average this address sits, and draws a map of the heat with the address at the centre. It also adds how much ground is covered within 500 metres, because that is where the difference comes from: where there are trees, water and open ground the surface stays cooler and releases less heat at night.

The comparison is the useful part. "Thirty-eight degrees" on its own says nothing: at midday in summer tarmac is always around that, anywhere in Italy. What you feel living there is the difference between one part of a city and another, which reaches five or six degrees between a concrete district and one with parks.

When the report cannot answer, and why

• There is too little measured land around the address: this product does not measure the sea, and out at sea the comparison cannot be made.
• The last three summers left no usable measurement: this happens if the satellite is down or the product is missing for that period.
• The satellite service does not answer: the block says so and comes back in the next report.

Limits

• This is the temperature of the surface, what the satellite sees from above. What the thermometer reads in the shade comes from weather stations, and it is a different number.
• Each measurement covers one kilometre: it describes the neighbourhood, and two nearby addresses often share the same value.
• The comparison is with the twelve kilometres around, so it depends on what is around: in Bolzano the built valley floor is compared with the mountains, and the difference is eight degrees.
• The map colours adapt to the area of that report, and the degrees are written under the band: between two reports you compare the numbers, not the shades.

Official page: Copernicus Data Space Ecosystem.