Theory and Observations of Land and Sea Breezes*

Abstract

T HE LOCAL DIURNAL WINDS induced by temperature difference between land and sea are defined as land and sea breezes; the wind that blows from land to sea by night is the land breeze; the wind that blows from sea to land by day is the sea breeze. With the possible exception of some Arc-tic and sub-Arctic regions, land and sea breezes occur in coastal regions of continents , islands, or inland lakes. Their frequency on coasts depends essentially upon the latitude, season, cloudiness, and gradient wind. With hilly or mountainous terrain adjacent to the shore, the local winds are a combination of land and sea breezes with valley or slope winds and are thereby stronger and more frequent. In temperate latitudes land and sea breezes occur chiefly during the spring, fall, and summer with a maximum frequency in summer. On unseasonably warm days in Febru-ary, sea breezes have been observed in Massa-chusetts. In the tropics, land and sea breezes are almost a daily phenomenon during the dry season and are also frequent though weaker during the wet season. Fishermen and sportsmen use the winds to sail out to sea in the morning and to return in the afternoon. With clear skies the frequency of the phenomenon is greatest. At Burgas, Bulgaria, with scattered clouds (0 to 5 tenths), a frequency of 90 per cent was found, while for broken (6 to 8 tenths) and overcast (9 to 10 tenths) skies, frequencies were 39 per cent and 27 per cent respectively. Temperature difference between land and sea, and therefore the sea breeze tendency, are evident even during rainy weather. Sea breezes occur more frequently during light gradient-wind conditions. An off-land gradient wind is more favorable for their development than gradient winds parallel to the coast. Frequencies of sea breezes will vary considerably for different locations, depending on the climatic factors. Frequencies of days with sea breezes during the summer on the coast of Massachusetts are 30 to 40 per cent, on the Baltic coast of Germany about 20 per cent, on the Catalonian coast of Spain, 80 to 90 per cent. Frequencies will also vary considerably from year to year. At Lake Constance the number of lake-breeze days varied from 6 to 22 during May for seven years of record. Further inland from the coast the occurrence of sea breeze decreases markedly. Coasts with nearby smaller islands have a decreased frequency of the sea breeze. A coastal station under such conditions would have temperature ranges more similar to an inland station. THEORY Two distinct types of sea breezes exist: a sea breeze of gradual growth on calm or light-gradient-wind days and the frontal sea breeze on days with off-land gradient wind. The properties of each are quite different. The frontal sea breeze appears to be more frequent in temperate latitudes; of 50 cases of sea breezes at Danzig 43 were frontal.