A pyranometer (also called a solarimeter) is an instrument that measures the amount of sunshine reaching the ground. Pyranometer comes from the Greek words “pyr,” meaning fire, and “ano,” meaning sky. The instruments record the amount of sunlight in terms of watts per square meter, and they calculate this in one of two ways. A less accurate method used in cheaper models of pyranometers detects light using small, silicon-based photodetectors. This method is not considered to be ideal because such photodetectors do not capture the full spectrum of sunlight very well. The other type of pyranometer uses a thermopile, which is a collection of highly sensitive thermocouples that detect temperature changes across junctions. Pyranometers can be used to evaluate weather conditions and, when readings are taken over extended periods of time, changes in climate.