Rain

Formation of the rain

The rain results from the evaporation of the moisture which exists in the Nature and more particularly of the great stretches of water (Lac S, Mer S…). This Steam rises and, in contact with colder air, condenses around cores of condensation (dust, pollens, aerosols…) and gives clouds. When the Condensation is too important and that the drop S of water then formed are too heavy (approximately 0,5 mms diameter), they fall, thus forming a rain, if the weather conditions lend themselves to it. Temperature variations on the course of the rain can thus cause other forms of precipitation: Snow, Hail, Grésil…

Quantitative measurements

The measurement of the rain, called Pluviometry, is done with a simple apparatus named Pluviomètre. This measurement corresponds to the height of water collected on a plane surface; it is expressed in millimetres, and sometimes in liters per square meters (1 l/m ² = 1 mm). In weather station, this measurement is made daily.

The rains are also characterized by their duration and their frequency throughout the year. These data are in particular used in order to dimension the networks of cleansing of the cities.

To compare the rainfall of different geographical areas, one uses an annual office plurality of the quantity of rain; one then expresses it in millimetres per annum (for example, approximately 2500 mm/an in wet Tropical forest, less than 200 mm/an in a ic zone Désert).

Qualitative measurements

The rainwater is initially comparable with distilled Eau but while falling it takes care of various biogenic salts and pollutants which make it less pure and sometimes not drinkable (Acid rains).

The dust collected by the rain can be sufficiently abundant to tint it, or to transform it into mud rain. The particles rich in iron oxide could give rise to the legends of rains of blood .

Culture

The attitude of the populations with respect to the rain differs according to the areas from the world. In the moderated areas, like Europe, the rain has a sad and negative connotation - “ It cries in my heart as it rains on the city”, wrote Paul Verlaine - whereas the sun is synonymous with joy. In margin of this traditionally negative vision of the rain, it is sometimes associated with positive values (appeasing, fertility, cleanliness) or esthetics.

In the dry areas, certain parts of the Africa, the India, the the Middle East, the rain is regarded as a blessing and is received with euphoria. It has a fundamental economic role, where the rivers are rare and the distribution of the Drinking water and the Irrigation are conditioned by precipitations.

Many cultures developed means of being protected from the rain (impermeable, umbrellas), and worked out systems of drain and evacuation (gutters, sewers). Where it is abundant, either by its frequency, or by its violence (Mousson), people instinctively prefer to put themselves at the shelter.

The rainwater profits naturally with the Agriculture. It can be also stored to face dry periods. Its acidity and the presence of dust make it frequently unsuitable to consumption, and require treatments.

The Urbanisation must take into account a management of the rain. The grounds made tight in the cities require the development of cleansing and drainage systems. By changing the proportion between streamed water and the water absorptive by the ground, the risk of Inondation is increased if the infrastructures under-are dimensioned.

Other uses of the term

In the coloured language, the rain can indicate an abundant precipitation of objects, even an abundance it even, like in the case of a rain of Or . The gold rain is as the Avatar as Zeus adopted to allure Danaé.

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