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Climate

Description of the Problem

The city of Milan is situated in the central-western area of the Po Valley. The area is surrounded by Brianza hills on the northern side and plains on the other three sides. The topography is quite simple, with an average altitude of 100 m. The altitude sharply increases as it extends north beyond Brianza hills until reaching the Alps, over 3,000 m high and only 200 km from Milan.

Milan has a typical continental climate. Generally winters are quite cold and foggy and summers warm and humid; autumn and spring are characterised by highly changeable weather conditions with more frequent precipitation in September and October.

The Alps and Apennines that surround the Po Valley form the physical barriers that protect the area from the major circulations coming from Europe and the Mediterranean.

The presence of a large, densely urbanised centre has, as is well known, a considerable influence on microclimatic conditions, both in terms of temperature and wind fields. The examination of measured data in Milan points out a typical effect of metropolitan areas, the so-called "heat island" phenomenon, due to different anthropogenic causes such as multiple reflection between vertical walls of buildings, the turbulence caused by buildings, the production of energy generated by industry, heating plants, etc.

Moreover, the presence of the large, densely populated centre determines marked deviations in prevailing winds entering the city. The analysis of wind direction measured at different monitoring stations in the city shows that the degree of deviation from the original direction depends on the location of the station and on the developing direction, with a tendency to rotate in a counter-clockwise direction in relation to the city centre.

The Italian laws in the environmental field don’t include specific rules to protect climate on the local scale; in fact, its protection is indirectly committed to air quality legislation, which is generally focused on emissions and emissions control.

Data Sources

The surface meteorological stations in the metropolitan area of Milan are under the control of the surveying network of the Province of Milan, managed in collaboration with the IV Unità Operativa-PMIP of Milan. According to Italian laws, air quality surveillance is up to the provincial administrations that are responsible for managing and organising the air quality monitoring networks. Data are automatically transmitted from local microprocessors interfaced with measuring equipment at monitoring sites to the operative provincial centre at the PMIP, where they are validated twice a day by experts (the first time in the morning, the second one midday before sending the daily air quality summary bulletin). Then data are organised into a specific software system to be available for further analysis and are transmitted to the Department of Environmental Protection of the Lombardy Region, which collects data from all the provincial centres and organises them into its own EIS (Environment Information System), including specific tools for processing and displaying environmental data (see also the chapter on air quality in Milan).

The meteorological parameters measured by the network are defined by Italian law and are the following: wind velocity (WS), wind direction (WD), environmental temperature (T), precipitation (PP), relative humidity (RH), pressure (P), global and net solar radiation (SR, NR). The analysers relative to the first three parameters are located according to the regional rules at a standard height of 10-30 m. above ground, but in some locations measurements are carried on simultaneously at two heights (60-120 mt.). These parameters are measured in ten monitoring stations, which are located in the metropolitan area of Milan.

Vertical profiles of temperature and wind are available from thermosounding measurement carried on every six hours at the Linate Airport. To supplement information on microclimatic conditions, a sonic anemometer has been working since last year to investigate the Planetary Boundary Layer (PBL) behaviour.

Methods

In meteorological data analysis, different methods are used:

• to relate meteorological conditions with the occurrence of air pollution critical episodes and to investigate the rule in the dynamics of dispersion of wind transport phenomena;
• to estimate the dispersion capacity of a site on the basis of its typical or critical climatic situation;
• to survey continuously the principal meteorological variables for pointing out climatic alterations;
• to allow meteorological local forecasting to support planners for air quality control, chemical plants, and risks assessment;
• to prepare input data for mathematical models application.

For modelling studies, meteorological data are elaborated to investigate the most frequent or most critical conditions relative to the domain. This procedure is then used to choose representative meteorological scenarios to be simulated in a modelling analysis for a source impact evaluation and to prepare the required input data, such as mixing height, wind speed and direction, environmental temperature, stability classes, etc. To evaluate long-term impact, historical series’ of meteorological parameters available for the area of study are taken into account then, by means of statistical matrices, are used by the dispersion models. Moreover, surface measurements carried out by a monitoring network as well as vertical profiles measured at the Milan-Linate Airport thermo-soundings and topographical data are elaborated to generate input data for numerical meteorological models. These models are used for describing wind transport in local and mesoscale pollution phenomena and mixing atmospheric conditions by the computation of the main Planetary Boundary Layer parameters, such as mixing height, Monin-Obukhov length, friction, velocity, etc.

To describe thermodynamic local effects, statistical analysis of measured data are also carried out. For example, to investigate the deviations that winds undergo when they pass over the town, a simple statistical evaluation has been performed, based on correlation matrices among wind data measured in all the stations of the Province of Milan, in the city and outside. Further, to examine the heat island effect, temporal and spatial evolution interpolating methods are used together with specific studies to investigate the relationship between the temperature gradient and the distance from the city centre.

Results

Every day the satellite images and forecasting maps elaborated by the International Meteorological Service are examined at the PMIP of Milan to elaborate a comment included in the daily summary bulletin about the actual and forecasted air quality situation. The bulletin is sent to mass media and health and environmental authorities. Moreover, a monthly summarising report regarding the main features of the local meteorological situation of that month is elaborated, comparing them also with those of the historical series’ in order to illuminate any peculiarities.

Roughly every year a report relative to the Studies for the Air Quality Evaluation in the Province of Milan is printed by the Province of Milan, including a synoptic and local meteorological situation to support air quality data analysis. Monographs are published with reference to specific studies, such as, for example, the acid deposition analysis for the Milan area, in which pollution levels are related to rain pH and conductivity levels. Reports and monographs are available at the Province of Milan.

Furthermore, air quality bulletins relative to all the monitoring sites over the territory of the region and data statistical analysis are included in AURORA, mentioned above, and distributed by the Department of Environmental Protection of the Lombardy Region.

Uses

Reports and data are required by a wide variety of users: media (press and TV), schools, research and scientific centres, socio-economic institutions. Specific evaluations are generally carried out for local health offices, municipal, provincial, regional environmental authorities, etc.

Notes:

¹
Meteorological parameters:
WD: wind direction, WS: wind speed, T: temperature, AP: atmospheric pressure, RH: relative humidity, SR: solar radiation, PP: precipitation

²
The number is referred to all the monitoring stations located in the province of Milan territory, including those belonging to private network and to the provincial network