In Aosta Valley, four main passes facilitate the transfer of air masses from the north and west to the inner valleys with changes due to the impact of the high mountains. The average altitude of the region is over 2000 m, but the morphology is characterized by a capillary articulation of side valleys. These aspects significantly influence the elements of the climate, in particular by creating a difference between solar radiation, which is greater for south-facing slopes than for north-facing ones, and determining an area of low precipitation in the central sector of the Aosta Valley.
In fact, the city of Aosta as an average of about 550 mm per year, whereas the areas close to the French border measure around 750 mm per year, while on the border with Piedmont the average precipitation is about 950 mm per year.
Being an Alpine region, the climate is determined by extreme spatial variability and the most relevant changes are related to altitude. In particular it occurs, with increasing altitude:
- The decrease in temperature, about 0.6°C every 100 m of height
- The exponential decrease in atmospheric pressure, about 1 hPa every 10 m up to about 3000 m
- The increase in air transparency and visibility
- The increase in the intensity of solar radiation
The cloudiness and precipitation in Aosta Valley is mainly due to large frontal systems, as usual in middle latitude regions. During summers, however, the precipitation is typically generated by medium and small-scale convective systems. In addition, precipitation increases with altitude mainly on the border areas, while it is not particularly affected by the altimetric variations in the central valley.
The topography at the local level (summit, ridge, slopes, valley floor) strongly influences the solar radiation received, the diurnal temperature regime, the wind speed and direction, the amount of precipitation and the snow supply. This local behaviour is reflected in the landscape, divided into altitudinal bands, each with a typical association of plants in balance with the climate.
The climate is, in general, semi-continental with many local peculiarities. The influence of the mountain environment is very clear: the Alps form a barrier, leaving the inland areas drier, with clearly evident differences between the areas at higher altitudes, which are more humid and cold, the valleys, which are less affected by precipitation, and the lower area of the Dora Baltea, which is characterized by the typical Mediterranean climate.
