Rain storms are the cause of the most common disaster type – floods – and therefore incur the main economic damages.
Moreover, this is the reason for decreased crop yield on fields and a stimulatory factor of all slope processes (landslides, mudslides, landfalls, soil liquefaction). Rain storms have two regional connections.
Firstly, they are connected to the so-called frontal rains, located along the atmospheric front and are often followed by wind gusts.
Secondly, these are cyclonic rains. In case of tropical rains, they most often cause floods. Many occurrences that accompany a storm are no less frightening than the storm itself. Oftentimes, they exceed it in their destructive force. Frequently, rain showers that accompany a storm are much more hazardous than the wind storm itself. Electric forces (thunderstorms) are quite dangerous and destructive but they lag behind in their destructive force and the scope of this force’s impact.
Hydrometeorological occurrences that accompany hurricanes and storms are considered disastrous (highly dangerous), if they reach the following criteria in their intensity:
- wind speed (including gusts) of25 m/s and higher; for coasts of oceans, Arctic and Far Eastern seas’ aquatoria - 30 m/s and more; for aquatoria of Arctic seas - 25 m/s and more;
- turbulence – in oceans, the height of waves of over 8 meters and more; on seas – the height of waves that is extremely dangerous for sailing, fishing and shore facilities;
- heavy rain – precipitation amount of 50 mm and more in the course of 12 hours and less, in mountainous, mudslide and rainstorm regions - 30 mm and more in the course of 12 hours;
- prolonged rain–liquid atmospheric precipitation, falling down incessantly or almost incessantly in the course of several days that can cause high waters, floods and underflooding (GOSTP 22. 0. 03-95);
- large hail - the diameter of hail stones – 20 mm and more;
- heavy snowfall – precipitation amount of 20 mm and more in the course of 12 hours and less;
- blizzards lasting the entire day or night, prevailing wind speed of 15 m/s and more, and for coasts of Arctic and Far Eastern seas - 25 m/s and more;
- strong dust (sand) storms – prevailing wind speed of 15 m/s and more.
Rainstorms create floods, erosion, mudflows and landslides in the mountains; unseasonable and prolonged rains that are harmful to crops. Maximal values of precipitation intensity are higher in the summer and near oceans that provide moisture, as well as on windward mountain slopes, in the most humid regions of equatorial and tropical climatic belts. In these same conditions, fluctuations of maximal intensity values are relatively small. In inverse conditions, absolute intensity values are lower, but their fluctuations are higher. Therefore, floods and other consequences of rainstorms are possible in arid areas as well, and they are more harmful to the population, since here they are less prepared for them. Records of minute intensity belong to convective (thunder) rainstorms of the tropics in Central America—of up to 20 - 25 mm/min at an average intensity of over 10 - 20 mm/min. Convective showers often affect small territories (up to 200km2), do not last long (2-4 hours in the tropics, but more often 1 hour; in moderate climate–up to 30 min), are uneven, begin and end abruptly. The largest diameter of drops is achieved in such rainstorms (in the tropics– up to 7 - 8 mm, in middle latitudes–up to 4 – 5 mm). Frontal rainstorms last anywhere from several hours to 4 days, with intervals of up to 2-3 weeks. They cover territories with an area of up to hundreds of thousands square kilometers. During tropical cyclones, the intensity of rainstorms often exceeds 150 mm per day and reaches 500 – 800 mm/day (record established on the island of Réunion in May of 1952 - 1850 mm/day). More often, this rain shower lasts 5 to 10 hours. In 10-20 hours an annual precipitation norm may fall out. In regions where this norm is especially high (for example, in the Philippines - 2000 - 3000 mm), it can be reached by intensive rainstorms in 60 – 70 hours. In the subtropics of the Mediterranean Sea Atlantic cyclones can produce rainstorms with an intensity of up to 400 - 500 mm/day (record observed in Gens, Italy in 1882 – 810 mm/day). The highest intensity can be close to the daily one, but it more often composes 1/3 - ¼ of the daily norm. In Batumi, intensity of rainstorms exceeds 250 mm per day, which corresponds to several meters of snow in the nearby mountains of Adjariya. In a moderate climatic belt, intensity values are even lower: in England, Moldova, and Ukraine – 100 - 200 mm/hour), in central regions of the European part of Russia – 50 – 100 mm and in northern regions – up to 50 mm/day.
What is happening in the skies? Why do they suddenly start to unmercifully vent water flows on Earth? One of the causes of intensive, torrential, rains is an especially strong heating of moist ground in the hot summertime. The mass of moisture evaporating from the ground forms enormous rain clouds. The thickness of a cloud layer reaches 6 - 8 km, and sometimes even ten kilometers. Oversaturated and overloaded with water storm clouds emit rain showers and even hail. Powerful convective and ascending currents reign inside such clouds. Such rain showers are common in tropical latitudes. In our latitudes clouds are usually formed in a different way: during a frontal collision of different heated air masses, when the cold air wedges into warmer air, a more complex, rapidly progressing process develops along the entire line of the atmospheric front. Specialists call this process convection. Its physical aspect lies in the fact that there is a movement of large air masses accompanied by the transfer of heat and other physical factors. As a result cumulonimbus clouds that carry rain showers and thunderstorm are formed.