The valley of the River Tagus is usually taken as the line that divides the country into two main blocks: the mountainous north and the relatively flat south. However, despite its small size (89,000 km2), Portugal reveals a great diversity of habitats and landscapes which can only with difficulty be fitted into regional divisions. Climate, relief, soil, and the hydrology all act together to differentiate neighbouring, and seemingly homogeneous, regions. Thus, Trás-os-Montes is called Além Douro Trasmontano by some and Trás-os-Montes e Alto-Douro by others, while the Beira Alta may be called Cordilheira Central, as well as Beira Alta or Beira Serra. Even the Alentejo can be divided up in various ways, and the only widespread agreement, with the exception of a few details, is on the boundaries of the Algarve.

Portugal has a wide coastal strip, which has obviously influenced the physical and human character of the country. The coastline, which is mostly straight, is 943 km long, of which three quarters runs in a mainly north-south direction, the remainder, the Algarve coast, running from east to west. The only large-scale indentations are the Aveiro lagoon, the estuaries of the Tagus and the Sado rivers, and the Formosa lagoon. The main rivers discharge into shallow estuaries of varying surface areas, with only the river Douro maintaining steep banks right to its mouth.

Most of the country is relatively close to the sea. Only one small section in the north-east is more than 200 km from the Atlantic and two-thirds of the population is concentrated in the coastal strip.

The coast is generally low-lying and sandy and extensive dune systems are found between the Aveiro Lagoon and the headland at Nazaré, and between the mouth of the river Sado and the cape at Sines. Ctiffs are the dominant feature in the south, particularly around cabo da Roca, in the area around Arrábida and Espichel, and along the coast between Vila Nova de Milfontes and the cape at Sagres.

The almost invariable roughness of the seas has formed sandy beaches, often backed by coastal dunes which extend a short distance inland.

Portugal’s climate is influenced by its position on the Atlantic and its proximity to the Mediterranean, as well as by the compact form of the Iberian Peninsula and the nature of its relief.

The Atlantic influence generates a mild climate with a narrow temperature range and high levels of atmospheric humidity, particularly in the Minho and Douro Litoral regions in the north-west.

These same conditions also prevail on numerous north - and northwest - facing slopes and on a significant proportion of the northern coastal strip.

The Mediterranean influence, giving rise to long summers, high average temperatures, and extreme aridity with little rain in winter, is mostly seen south of the River Tagus, but is also evident in the Terra Quente events of the Trás-os-Montes region and on the southern faces of the limestone hills.

Apart from in the mountainous areas, average annual temperatures do not exceed 17º-18º in the south nor go below 13º-14º in the north. The  Coastal areas are subject to less temperature variation than inland areas, the north-eastern parts of the latter showing a marked continental influence.

Rainfall is uneven, with marked differences between the north and south, varying between 1500 mm in the north-west to 300 mm in the south of the country. Its occurrence over the year is variable and at times extremely erratic; these variations increase from north to south, where in some places over 20% of the annual precipitation can fall in a single day.

The predominance of northerly winds contributes to the wet conditions found in the regions situated along the coastal strip, as well as on north-facing slopes. Easterly and south-easterly winds are at times accompanied by very rough seas.

Portugal’s territory is a veritable meteorological mosaic, and it is often difficult to draw boundaries between the different types of climate. Differences in relief between the north and the south, together with the fact that parts of the coastal relief run parallel to the coastal strip, weaken the influences of the Atlantic and the Mediterranean and go some way to explaining climatic variations within relatively small areas.