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Climate

Description of the Problem

The City of Rome enjoys favourable climatic conditions because of its geographic location on the Mediterranean Sea, which carries with it all the advantages of the maritime climate. The sky is generally clear, fog is uncommon, the temperature is mild, the moderate winds often have a positive influence on the temperature.

The pluviometric regime is typically Mediterranean. Rainfalls is concentrated in the fall and winter seasons, reaching a minimum in summer. The daily minimum and maximum values are always moderate and only for the few days of the year when either the minimum temperature is below 0 °C or the maximum temperature is higher than 36 °C. Snowfall is uncommon, but, when it does ocur, the blanket of snow seldom lasts longer than several days.

The effect of the wind is particularly favourable in spring and summer. In these seasons, the well-known "ponentino," the local name of the sea breeze, rising in the early afternoon, has a function of thermoregulation and can thus be considered an important environmental resource.

Some aspects of the weather conditions have an important influence on the pollution since they have a variable effect on the degree of concentration of the pollution factors. Particularly in winter, weather conditions create in the atmosphere the phenomenon of thermal inversion, preventing the mixing up among the different layers of the air, due to the convective currents, thus bringing about the stagnation of the pollution factors and an increase in the degree of concentration.

The presence/absence of wind and the main wind direction have a strong influence on the concentration of pollution factors. In fact, the occurrence of days without wind, or days characterised by South/South-East winds, always brings high peaks of pollution. By contrast, northern winds always bring low values of pollution.

Another important factor of air pollution is the relative humidity and the range of variation of humidity. In most of the above cases, when the minimum humidity is greater than 35 % and the range of variation is less or equal to 30 %, a high concentration of pollution factors can be seen. When the minimum humidity is less or equal to 35 % and the range of variation is greater than 30 %, the concentration of pollution factors is low. The most favourable condition for high concentration of pollution factors consists in a mix of high values of relative humidity and the absence of wind.

The temperature range does not have a strong influence on the concentration of pollution factors. In the case of high range of variation of the temperature, there is a lower probability of high concentration of pollution factors. This is due to the convective currents determined by the high temperature range, which results in mixing the air in the low layers of the atmosphere. In any case, even the probability of high concentration of pollution is significant.

Data Sources

In the urban area, five meteorological stations continuously collect climatic data, such as temperature, relative humidity, wind velocity and orientation, rainfall intensity, atmospheric pressure, etc., and organised them in a database form.

Such stations are:

• Urbe Airport;
• Ciampino Airport;
• Rome-Eur (operating for the last three years);
• Collegio Romano (located in the historic centre of Rome and collecting data since 1782);
• Monte Mario (for a limited period of forty years).

Located in the rural area around Rome are four meteorological stations, all managed by the Military Air Force (Aeronautica Militare):

• Fiumicino Airport;
• Vigna di Valle Airport;
• Guidonia Airport;
• Pratica di Mare Airport.

Since 1991, a solar spectrophotometric station at the Department of Physics of "La Sapienza University" has been measuring the total ozone and ultraviolet radiation, parameters which are in close correlation with the climate.

Methodologies

Climatic data are elaborated with statistical methodologies to calculate the synthetic parameters able to describe the weather conditions, such as frequencies, maximum values of phenomena observed in a given period, mean values, etc.

A study developed for the year 1991 by the Central Office of Agriecology (Ufficio Centrale di Ecologia Agraria, which manages the station of Collegio Romano) established a correlation between weather conditions and degree of concentration of pollution factors. Meteorological data about the most significant days in the year, from the point of view of the high or low concentration of the air pollution, were gathered and analysed. Such data are:

• daily temperature range;
• wind main direction and velocity during 24 hours;
• daily range, minimum degree of the relative humidity, occurrence of rainfall.

Results

A complete statistical study of all the meteorological data observed in a period of twenty years is available, both on a monthly and on a seasonal basis, only for the Fiumicino Airport, in the publication Frequenze e Medie delle Osservazioni Meteorologiche in Superficie, vol. 1, edited by the Air Force. Frequency distributions for some meteorological phenomena (wind velocity and direction, temperature and relative humidity) for the stations of Vigna di Valle, Guidonia, Urbe, Ciampino, Fiumicino and Pratica di Mare Airports, and Roma EUR are published in Caratteristiche Diffusive dei Bassi Strati Dell’atmosfera, vol. 10, Lazio, edited by ENEL and the Air Force.

Meteorological data, collected by the Collegio Romano station and mainly elaborated as mean monthly values, for the period from 1782 to 1978, are published in Meteorologia Romana, la Serie Storica delle Osservazioni al Collegio Romano, edited by V. Trevisan, 1980. Such data are partially updated by Mangianti F. and Beltrano M.C. in IL Collegio Romano, 100 Anni di Osservazioni Meteorologiche, 1990.

Uses

Using weather reports and predictions made from meteorological data to manage the interventions to control and lower the air pollution is the most important application of such data for the institutional powers concerning the City of Rome.

The car and bus traffic is considered in Rome to be responsible for about 85 % of the total air pollution. When a pollution threshold, established by law, is exceeded, the mayor can release an ordinance which temporarily forbids private cars, not equipped with catalytic converters, to circulate in the central hours of the day and in the central areas of the city. Such an ordinance is generally released when the predicted weather conditions are supposed to determine an increase in the degree of concentration of the pollution factors and is unnecessary when the predictions indicate a positive influence of weather conditions on the air pollution.