Knowledge about rain
18/09/2014 15:35:02

 This article is to provide readers with basic knowledge about rain, such as heavy rain, acid rain & hails. This is one of climate factors influenced by climate change and impacting on natural disasters


What is heavy rain? 

Heavy rainfall happens as a result of some special types of weather, such as hurricanes, tropical depressions or tropical convergence zone, strong wind convergence over multiple zones, cold front, downburst … Especially when there is a combination among them at the same time, it is more dangerous as it causes heavy rain, strong winds, thunderstorms, hails in a long time and on a widespread area. Widespread rain could happen in one or more days, continuous or intermittent, of one or more rain events and regardless of rain forms.

Based on actual rainfall measured at the 24-hours monitoring stations of surface meteorology and rainfall stations in the meteorological network, rainfall levels are categorized by World Meteorological Organizations (WMO). Accordingly, heavy rain is divided into 3 levels: Fair rain: 16-50 mm/24 hours; Heavy rain: rainfall of 51mm - 100mm/24 hours; Very heavy rain: rainfall >100 mm/24 hours. Rainfall is calculated from 7pm previous day to 7pm next day. In studies of the rain effects, the rain level from 51-100 mm/24 hours has had negative impacts on human life.

What is widespread rain?
In fact the forecast area specified in our country can only be adjacent to one or two other areas and heavy rain forecast which always occurs on the surface area is relatively continuous. Therefore, the widespread rain regulations are defined as: Widespread rain is heavy rain process occurring in one or more of the forecast areas adjacent to the total monitoring stations: (1) Forecast area is considered to have widespread rain when half of total rainfall monitoring stations records heavy rainfall in that area; (2) Widespread rain occurs at 2 or 3 adjacent areas when total rainfall measured at stations exceed ½ or 1/3 of total rainfall measured at stations in those areas. Also, those stations must be adjacent to consider that event to be widespread rain at adjacent areas. And the areas must be based on the current division of small areas of forecast areas.

What is acid rain?
In reality the actual acid in liquid solutions is often measured by pH scale (logarithmic scale). Based on the acid corrosion, pH = 7 is identified as neutral. Typically pH = 5.6 (pH = 5.6 is pH level of water with saturated CO2) is considered as a basis for determining whether it is acid rain or not. It means that any rain measured with pH <5.6 is called an acid rain. Furthermore, acid rain, in the professional world, is called as “acid deposition”. These terms seems similar, in fact, they are different. “Acid deposition” means deposition of acid in the atmosphere to the Earth’s surface including both dry state (particulates) or wet state (acid rain), whereas “acid rain” merely mentions the deposition of acid in the atmosphere to the Earth surface in the wet state.

The formation of acid rain (Source:
Where does acid rain often occur?
In the rain there is always a certain amount of acid level (but its pH level does not reach the threshold of acid rain). But in the industrial sector, when rain happens, its pH level is always higher than other areas because the atmosphere of these areas is contaminated by gas & smoke emitted from plants. Rain combines with CO2 in the air to become carbonic acid with a low concentration. This weak acid can dissolve limestone. Rain also combines with other gases emission of plants. These emission gases can be carried by wind far away. They are then absorbed by moisture in the air, and become H2SO4 and HNO3. Rain again brings these acids to the areas far away from the contaminated areas. The acid rain accelerates corrosion process, such as rock corrosion. It also gradually pollutes lakes and waterways, thus causing dangers to living species there.
How is hail formed?
Hail consists of transparent or opaque ice particles (ice) with various sizes, such as as small as peas or as large as grapefruits. Hails are formed inside convection clouds (convention clouds are clouds with anvil shapes & usually cause thunderstorms) when a strong line-up brings rain droplets & ice crystals into the clouds where there is low temperature, and rain will be frozen into snow or ice particles if there is available nuclear condensation. Then ice particles carried through the clouds where there are millions of super-cold droplets touch the ice surface and immediately become frozen in that surface to become bigger particles. Now when ice particles or hails reach cloud peaks, they fall down from the edge of the cloud where there is weaker line-up. Hails keep falling down at areas where is stronger line-up and this process (cycle) repeats until there are larger hailstones. Process (cycle) will continue and hailstone grows until line-ups cannot push them up anymore, then they fall down from the clouds to the ground.
 Hail types and what are their effects on people, animals and crops?
Hail is rain having different sizes of ice particles, falling from the massive block of thundercloud and happening in strong thunderstorms & heavy rain with great intensity in the range of few minutes to several tens of minutes. But not in any thunderstorms hail happens, and the frequency of thunderstorms with hails is around 10%. (1) Small hails: in the form of transparent ice falling from clouds, most of hailstones have spherical or cone shapes, with their diameter to be equal or greater than 5 mm. (2) Hail: in the form of ice particles, maybe transparent, maybe opaque or both. Hailstones are often in spherical, cone or uneven shapes. Their diameter are from 5mm to 50mm. Hails could be scattered or combine together into uneven screen. The ice cubs weigh about 5 grams to several hundred grams. Their velocity is quite large and keeps increasing, leading to proportionally larger size and heavier weight of hailstones. Falling rate from 30-60 m/s, exceptionally up to 90m/s. With this velocity, when hailstones reach ground or vegetation zone, they can cause severe damages. Additionally, in the thunderstorms, hails are often accompanied by very strong winds, sometimes whirlwind, with destructive power. Besides, it is the hailstone could also cause house broken, destructed vegetation and even death. Therefore, hails are categorized as dangerous weather phenomena.
Hail devastates vegetable gardens in Da Lat 
Where does hail often happen and when is it formed?
In Vietnam, hail can occur in most localities throughout the country. The areas hail often occurs are mountainous areas or areas adjacent to the sea, bordered by mountains (semi-mountainous), and less frequently in plain areas. In the south, hails are often observed in the transition period from dry season to wet season, but mostly small hails. The main reason is that most regions in our territory are located in the semi-mountainous areas, and the Northern provinces are often affected by strong cold airs and south-western convergence winds. Hails are often formed in the transition months from cold season to hot season (April, May and June) or from hot season to cold season (September, October & November). The reason why hails often happen in this period is that there is a strong competition between air elements in different air masses with contrast features. It is this competition causing strong convection areas, resulting in strong showers and thunderstorms, or sometimes hails.

Hail is a dangerous weather phenomenon, so it is possible to break hail?


By studying falling hailstones, “ice embryo” has been found. In the clouds there are many water droplets by cooling process become ice on the ice embryo, thus ice embryo becomes larger and then hailstone fallen down. Therefore, in order to form the clouds causing hails, first there must be ice embryo and cold water droplets condense on the embryo. To prevent hails, one must think to reduce cold water droplets in the cloud. To do this, they apply the following solutions: (1) Reduce the supply of cold water droplets in the clouds, by using artillery to alleviate souring airflow or spreading particles absorbing moisture below clouds & hails, to make vapors vaporize early to condense into water, increase weight, thus they can hardly reach areas with cold temperature (2) Reduce cold particles in the clouds, by increasing ice particles or cooling drug, thus water droplets quickly become ice embryo and do not accumulate on existing little ice embryo. These ice embryos fall down to lower atmosphere and quickly become water crystal forming rain. (3) Increase ice particles, by taking advantage of rising air by exothermic condensation process, thus cold water droplets cannot meet ice embryo but fly up to the atmosphere with colder temperature (-300 degree Celsius), condense into small particles but yet become ice embryo, destroying materials for hails. Theoretically, those methods could be applied. However, which method is good, based on country conditions and on-going research results. There is a country spraying drug into clouds for them to be melting before it becomes ice particles or hailstone. They often use aircraft, missiles or artillery as tools for bringing these drugs into clouds.

What are recognition signals of hails to be happening?
As we already know, hails are transparent ice particles formed in convection clouds (clouds in thunderstorms). Therefore, recognition signals are similar to those of strong showers in thunderstorms. When you are in areas without forecast information of thunderstorms (including hails), you can recognize by your own to self-prevent. If there seems to be thunderstorm winds, dark clouds cover most of sky with breast-shapes, then stronger winds with “buzzing, roaring” sounds, you should be constantly alert to hails. After that, if there is sprinkled shower and you feel air getting quickly colder, hails could be coming. Maybe you could feel it by yourself in reality someday.
                                                                                              (Reference: National Hydrometeorological Center)

                                                                                                                                                           Mai Huong




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