How vegetation keeps cities cool - a tale of two capitals
On a summer day, cities can be 5-10°C hotter than surrounding countryside. Dark surfaces absorb sunlight. Concrete stores heat. Air conditioners pump hot air into streets.
This is the Urban Heat Island effect - and it's getting worse as cities grow and summers get hotter.
Surface temperature measured by satellite over Barcelona's urban core on a summer afternoon. Hot enough to burn bare feet.
Landsat satellites carry thermal sensors that measure surface temperature. Blue areas are cooler, red areas are hotter.
Range: 27°C to 52°C. The urban core glows red.
Range: 11°C to 42°C. More blue, more green spaces visible.
Trees and plants cool cities in two ways: shade blocks sunlight, and evapotranspiration releases water vapor that absorbs heat.
A single mature tree can provide the cooling equivalent of 10 room-sized air conditioners running for 20 hours.
Copenhagen has nearly three times the vegetation density of Barcelona - and surface temperatures 10°C cooler on average.
Within every city, the relationship holds: neighborhoods with more vegetation have lower surface temperatures. Parks are cool islands in a sea of heat.
This isn't just about comfort - extreme heat kills more people than any other weather event. Green infrastructure saves lives.
Trees along streets provide shade and cooling where people walk.
Vegetation on rooftops reduces building temperatures and runoff.
Parks create cool zones that benefit surrounding neighborhoods.
As climate change intensifies heat waves, urban vegetation becomes critical infrastructure. The data shows which neighborhoods need green investment most.
