Rains – Environmental Encyclopedia

The water present in nature is found in rivers, seas, basins, and in the form of rain, which is one of the dynamic forms of the “Hydrological Cycle”. Rain is the precipitation of liquid drops of water.

These drops have diameters greater than 0.5 millimeters and can reach about 3 millimeters. Large droplets tend to flatten out and break into smaller droplets as they fall rapidly through the air. On the contrary, the precipitation of small drops, called drizzle, generally severely limits visibility, but generally does not produce significant accumulations of water.

The amount or volume of water that falls is expressed as the depth of water that is collected on a flat surface and is measured in a gauge of up to 0.25 millimeters.

Air masses acquire moisture by passing over hot water masses or moist surfaces. Humidity, or water vapor, is raised between air masses by turbulence and convection. This transport necessary to cool and condense the vapor is the result of various processes, and its study provides a key to understanding the distribution of precipitation in different parts of the world.

The phenomenon of elevation, associated with the convergence of the trade winds, produces a band of heavy rains near the equator. This band, called the Intertropical Convergence Zone, ITCZ, moves south or north depending on the season.

At higher latitudes, much of the uplift is also associated with moving cyclones which take the form of warm, moist air rising above a cooler air mass at an interface called a front. Altitude is associated, on a smaller scale, with the convection of heated air by a warm underlying surface that gives rise to showers and thunderstorms. The heaviest rains over short periods are usually due to these storms.

Air can also rise by being forced over a mountain barrier, so that the windward slope on the windward side receives more precipitation than the leeward slope on the other side.


average precipitation

The distribution of precipitation is very irregular, enjoying strong contrasts from one area to another. The annual distribution of precipitation on Earth reflects the influence of the distribution of land and sea and the height of the terrain. The world’s highest rainfall, some 10,920 mm per year, occurs at Cherrapunji in northeast India, where moisture-laden air from the Bay of Bengal is forced up the hills of Khasi from Asma State; up to 26,466 mm of rain fell in a year. Other rainfall records include around 1,168mm of rain in one day during a typhoon in Baguio, Philippines. 304.8 mm in one hour during a storm in Holt, USA and 62.7 mm in 5 minutes in Portobelo, Panama. In Spain, the highest rainfall is recorded in Galicia, on the Cantabrian coast and in the Serranía de Ronda.

artificial precipitation

Despite the presence of humidity and rising, sometimes the clouds do not produce precipitation. This circumstance has stimulated the study of precipitation processes, in particular how a raindrop forms from about a million tiny droplets inside clouds. Two precipitation processes are differentiated: the evaporation of water droplets at sub-freezing temperatures into small ice crystals which then fall onto warmer layers and melt, and the union of small droplets into larger drops which fall at higher speeds.

Efforts to effect or artificially stimulate these processes have led to extensive weather modification operations over the past 20 years. These efforts have had limited success, as regions with the greatest precipitation deficits are dominated by air masses without adequate humidity or altitude. However, promising results have been obtained and new methods of artificial rain are being actively studied. The influence of the relief is decisive in the distribution of precipitation, and with a peninsular periphery closed on all sides except the west, the rains find an almost invincible obstacle to penetrating the interior and the Levantine region.

Rains in the Iberian Peninsula

Rains in the Iberian Peninsula.

In general, the rainfall regime is very varied within the Peninsula, influencing in particular the relief, the orientation and the proximity or distance from the sea. According to the humidity index defined as the ratio between precipitation and potential evapotranspiration from Penman, the following areas can be distinguished in Spain: the arid regions, part of the Canary Islands and the desert area from Tabernes to Almería; Semi-arid regions, Ebro depression, Almería, Murcia, south of the Júcar basin, upstream of the Guadiana and the Canary Islands; Subhumid regions, the Duero basin, south of the interior basins of Catalonia, the Balearic Islands and the Guadalquivir, and humid regions, Galicia and the Cantabrian zone.

The rains are divided into two temporary periods: a maximum in autumn and a secondary one in spring, while the driest periods are those corresponding to winter and summer. The general exception is the west and south of the peninsula where the rainiest periods are autumn and winter.

average annual distribution

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