Climatic differences have a marked effect on the lives and work of the Portuguese people. Essentially there are northern and southern climatic zones, using the Rio Tejo as the boundary. Within these two zones there are climatic subregions that are uniquely Portuguese and others that are extensions of the climatic regions of Spain. The rainy north, an extension of the climatic zone of Spanish Galicia, is a heavily populated zone of small holdings where family labor produces a wide variety of crops. The semiarid, sparsely populated zone of latifundios in southern Portugal begins gradually midway through the country. The Algarve, a zone that is often as dry as Spanish Andalusia, is noted for its small farms and cultivation of tree crops.
The rainy north is generally defined as the interior of the Minho, the western portion of Trás-os-Montes, and the coastal plain extending from Porto to Lisbon. These regions have an average annual rainfall exceeding thirty-nine inches. The city of Braga, for example, almost consistently receives from fifty-nine to seventy inches of rainfall annually. The Coimbra area receives only about half as much as Braga -- from twenty to thirty-nine inches.
Temperatures in rainy, mountainous northern Portugal are considerably lower than those in the south, and northerners must have protective clothing during the winter months. Winter snows in the Serra da Estrêla and the Serra do Gerez frequently block roads, but warm, humid air currents off the Atlantic tend to modify the cold along the northern coast. Mean temperatures at Braga (627 feet above sea level) range from 48°F in January to 65°F in August. Viana do Castelo, on the coast, has virtually the same temperature range, but its precipitation level is usually nine inches in December whereas Braga's is twelve, because the moist air passing over the coast is cooled and precipitated as it reaches the mountains.
The northern interior, comprising the district of Bragança and the eastern portions of the districts of Vila Real, Guarda, and Castelo Branco, is cut off from the Atlantic rains by mountains. The plateau of Braganqa has a bleak, windswept aspect, and the land is mainly given over to grazing and rye growing. Vila Real, the center of the port wine region, often has summer temperatures in excess of 100°F. Normal precipitation in the northern interior at Mirandela (situated almost in the center of Trás-os-Montes) is barely one inch in July and little more than three inches in January. The usual temperature range in that area -- from 40°F in January to 75°F in August -- does not appear to be extreme, but the lack of rainfall and the consistent wind combine with the isolation of the region to make life bleak and harsh.
Southern Portugal has a Mediterranean climate with a low annual rainfall and many sunny days. The region closely resembles interior Spain, and the Algarve appears more like some parts of North Africa than Europe. The dry climate and warm temperatures favor olive growing and cattle grazing. The Algarve, which is separated from the Alentejo by the Serra de Monchique and the Serra do Caldeirão, is considered a distinct subregion within Portugal. Its only rainy months are in the late fall and winter, under the influence of the North Atlantic low-pressure system. Summer drought reflects the influence of the Azorean high-pressure system. At Caldas de Monchique (666 feet above sea level) the temperature is 50°F in January, and the rainfall is eight inches; but in July the rainfall is only one-tenth of an inch and the temperature 75°F. Algarvian coastal towns show similar temperature patterns. Because of the summer drought the region's farmers have traditionally depended on intensive irrigation.
The various climatic zones of the country are essentially influenced by a combination of factors: the North Atlantic low, the Azorean high, and the weather extremes of the Spanish Meseta. Winter and summer seasons are noticeably different in both the northern and the southern climatic zones. Along the coast the force of the prevailing winds has caused trees to grow at an angle. The prevailing winds at Lisbon are northerly, albeit with a westerly bias. In northern Portugal winter winds are southerly or easterly, and continuing through the summer they bring the dust of the Spanish Meseta and North Africa. An old Portuguese proverb states: "From Spain, neither a good wind nor a good marriage."
Summer weather conditions are generally favorable along the coast, but in the interior of the Alentejo and the Algarve there are frequent droughts. In the north the intense summer heat of the Douro valley (over 100°F) makes work almost impossible in the terraced vineyards. These conditions are caused by the consistent Azorean high, which extends into the Bay of Biscay. By winter the influence of the Azorean high diminishes in favor of the North Atlantic low, which brings humid sea air into contact with the land, causing heavy rainfall, particularly in northern Portugal. Drizzles and showers are frequent, and torrential rains occur occasionally. Because almost all of Portugal with the exception of the Algarve consistently receives more rainfall than Spain, the climate in general is milder and more pleasant.