Köppen climate classification is a widely used vegetation-based empirical climate classification system developed by German botanist-climatologist Wladimir Köppen. It's based on the idea that climate is best defined by native vegetation. The formulas used in the classification correspond to those of the vegetation zones (biomes) that were being mapped for the first time in the late 19th century. It was first published in 1884 and was revised until 1940 with collaboration by German climatologist Rudolf Geiger.
Köppen-Geiger's classification is based on a subdivision of terrestrial climates into five major types, which are represented by the capital letters A - E. Each of these except for B is defined by temperature criteria. Type B designates climates in which the controlling factor on vegetation is dryness. The second subdivision (second letter) clarifies further the type of climate based on precipitation and temperature. Although Köppen-Geiger's classification did not account for the uniqueness of highland climate regions, the highland climate category, or H climate, is sometimes added to modified maps to cover the elevations higher than 1,500 meters.
The second legend for this dataset, describing the climate letter codes, is included in the dataset as a layer called Letter Code Legend. You can toggle the legend on as needed. You will find it by tapping on the "Layers" tab in the iOS SOS Remote App 4.0+.