Nelson's Column, Trafalgar Square, London, UK, thermogram. The colours show variations in temperature. The scale runs from white (warmest), through yellow, orange, red and purple to black (coldest). Thermography records the temperature of surfaces by detecting long-wavelength infrared radiation. This image was captured in daytime. Buildings reflect and produce a lot of heat, meaning that London is consistently warmer than its surrounding countryside. Nelson's Column was built between 1840 and 1843 to commemorate Admiral Horatio Nelson, who died defeating the Spanish Navy at the Battle of Trafalgar. In the background is the London Eye.