gives it a generally tepid, oceanic, mild to warm subtropical climate, with mild annual oscillations. ‘Azores High’ anticyclone The Azores archipelago is located in a transition and confrontation zone between air masses of tropical origin and masses of cooler air of polar origin. The climate of the archipelago is largely determined by variations in the atmospheric pressure field over the North Atlantic. These variations conditioned by the mass of the American Continent and the Atlantic water mass are overlapped by a semi-permanent subtropical Atlantic anticyclone, commonly known as the Azores High. This anticyclone experiences seasonal variations which can affect the archipelago in many ways. In winter, the Azores anticyclone is positioned further south, and allows for a descent of the Polar front, approaching it to the archipelago. In summer, on the other hand, the anticyclone's movement further north, leads to the departure of the polar front and its associated disturbances towards higher latitudes. Far enough away from the mainland coasts, the continental air masses that reach the archipelago are weakened by the maritime influence. The same can not be said for the higher altitudes (e.g. Mount Pico), where upper air masses of a continental origin and with a more direct pathway can reach the surface and present those areas with drier air and more extreme temperatures. At the same time, this free atmosphere circulating air transports aerosols to the archipelago, namely volcanic ash or fine sands from the Sahara desert, which sporadically affect the radiation and air quality. Daily maximum temperatures at low altitudes usually range between . The average annual rainfall generally increases from east to west, ranging from in Santa Maria to in Flores and reaching values above on the highlands of Pico. Köppen classification Under the Köppen climate classification, the eastern group (São Miguel and Santa Maria) is usually classified as Mediterranean while the central and western group (especially Flores and Corvo) is increasingly more humid subtropical and overall rainier because of the effects of the Gulf Stream. This stream has a large effect over the sea temperature which varies between in February and March, and in August and September, and increases earlier in the western group. Salvador Rivas-Martínez data presents several different bioclimatic zones for the Azores. Seasonal lag is extreme in the low-sun half of the year, with December being milder than April in terms of mean temperatures. During summer the lag is somewhat lower,