Seven Degrees Between Staré Město and Ruzyně: IPR Publishes the Heat Data It Wants New Districts Designed Against

The Prague Institute of Planning and Development has published a storymap called Horko v Praze — Heat in Prague — setting out how much hotter parts of the city run than others, and how it arrives at the figure. IPR frames it as a planning instrument rather than a summer retrospective: the argument is that temperature can now be modelled at the scale of an individual street, and therefore designed for before a district is built.

The season it looks back on gives the headline numbers. Between May and September, IPR says, Prague had 29 tropical days and 12 tropical nights.

The gap opens at night

The institute's central figure is a difference rather than a maximum, and it belongs to the small hours.

"Built-up areas, if they are not well thought through, also prevent air from circulating. The heat has nowhere to go, and so in the first half of a tropical night the air temperature in Staré Město or Holešovice, for example, can be as much as 7 degrees higher than on the edge of Prague, in Ruzyně for instance,"

Ondřej Boháč, director of IPR Praha

The reason the gap widens after dark, IPR says, is that the urban fabric does not shed heat the way open ground does. Heat accumulated through the day in buildings and in hard surfaces — car parks, pavements, roads — is released back into the air at night, while the centre has less capacity to cool.

The daytime mechanism is the familiar one, and IPR lists it plainly: materials typical of built-up environments absorb solar radiation and store heat, and unlike trees and other vegetation they neither shade their surroundings nor cool them by evaporating water. Ordinary occupation adds more — car engines, air conditioning units and appliances concentrated in a small area.

Three methods, none of them sufficient alone

The part of the storymap that matters for planning is the methodology, because it is what decides how fine-grained the answer can be. IPR says it combines three, each with a different view and its own limits: measurements from meteorological stations, satellite data, and the PALM digital model, which links meteorological data to Prague's 3D model and computes air and surface temperatures directly in the streets.

The first gives accurate readings at a handful of points. The second covers the whole city but reads surfaces from above. The third is the one that produces a number for a particular street before that street exists — which is the claim the rest of the argument rests on.

What it is supposed to change

Boháč sets out the intended use in design terms rather than policy ones:

"Thanks to detailed data on Prague's heat island we can pick out the risky places when new districts are still being planned, and use adaptation measures well. Already in the design we can work with the orientation of buildings, vegetation, or the right materials and surface colours. It is also possible to evaluate the effect of building heights or the arrangement of residential blocks in a district on temperature and air flow,"

Ondřej Boháč

That last clause is the consequential one. Building height and block layout are not adaptation measures bolted on at the end; they are the substance of what a masterplan decides. A model that scores them for their thermal effect puts heat into the same conversation as density, views and daylight — where it has generally not been.

The storymap is published through IPR's ArcGIS platform. Read on its own, it is a record of a hot summer. Read as IPR presents it, it is an argument that the institute now has a number to bring to planning meetings, and intends to bring it.