Peter Dockrill for ScienceAlert:  Urban environments are often hotter than rural areas due to the way city structures trap and generate excess heat. This phenomenon, called urban heat islands, was first observed in the 1800s, but a new way of gauging it could help us make the most of this untapped source of geothermal energy.

Scientists in Germany and Switzerland have developed a means of estimating groundwater temperature hidden under the surface of our cities, based on surface temperatures and the density of buildings as measured by satellites.

A range of factors, including population density, vegetation levels, surface sealing, industrial structures, and transport all contribute to why cities are often hotter than the country.

But while scientists have long used satellites to measure heat on the surface, the relationship between ground temperatures and underground temperatures has not been fully explored.  Cont'd...