Why hail, how to prevent hail

Continuously from the end of last week until now, hail occurred in many places such as Lao Cai, Kon Tum, Song Tranh hydropower area . with an "unprecedented" level, causing great damage to people. . So why hail and how to predict as well as prevent hail?

Extraordinary hail in many places

In fact, hail can happen anywhere but mainly in the mountains. This is a dangerous weather phenomenon, difficult to predict and cause heavy damage.

On the afternoon of March 22, hail lasts about 20 minutes in Dien Binh commune and Dak To town, Dak To district (Kon Tum), the amount of stone falling is 50-70 tablets / m ² , the size of the stone is quite large, with pellets. reach 3cm. In March alone, Kon Tum had 4 hail.

Before that, in the evening of March 20, the Song Tranh hydropower area also suffered from a hail, tornadoes caused the water to flow into Song Tranh 2 hydropower reservoir. This area then suffered 4 tremors due to weather effects.

Around 15:30 pm on March 26 in many parts of Trung Khanh district, Cao Bang province had a rather heavy hail, ranging from 1 - 3cm, lasting about 10 minutes, affecting crops of 7 communes. in the district. Particularly in Than Giap commune, Mr. Luong Van Chu, 50 years old in Dong Tam hamlet, Than Giap commune, said that this was the biggest hail he had ever witnessed in the locality. The amount of stone is nearly 10cm thick, about 3 hours after 19 hours on the same day, all melted. After this hail, about 21 o'clock on the same day in the locality, there was another hail, however, the level of influence was not much due to the small stone size.

From about 1:50 am to 1:10 am March 27, a hail of extremely large stone sizes (10-14cm, 4-6cm average) and dense density have poured into Muong Khuong, Bac Ha and Si districts. Ma Cai (Lao Cai), causing serious damage to the people here. Meteorological expert assessed that this is the rain with the largest stone size ever.

Hail ravaged more than 12,000 houses in Lao Cai on March 27.

In Sa Pa, Mr. Nguyen Van Dien, the local people said, a hail lasting about 10 minutes from 18:50 minutes on March 27, ending at about 19 hours, the average diameter of stone grains from 1.5-2cm, large seeds reach 3cm. Hail comes with showers and strong gusts, wind speed is estimated at level 6-7, level 8 jerks, hail falls quite thick, after the rain ends the stone particles cover the front yard, in the garden and the way. .

And yet, the night of the morning of March 29, Bao Yen district (Lao Cai) also suffered a hail of intense intensity, accompanied by strong elements. Hail lasts about 20 minutes, starts at 1 hour and 40 minutes, ends about 2 hours, the stone is as big as a chicken egg, duck eggs, lots of big seeds with roaring fists .

Due to the unusually large size of hail particles, thousands of houses have been "hailed" by roofs. Hail penetrates many roofs of fibro cement, corrugated iron, tile ., only flat roof houses, concrete casting roofs are not penetrated.

Mr. Nguyen Duc Hoa, deputy head of the medium and long-term forecast department, the Central Hydrometeorological Center, said hail is a frequent phenomenon at the time of the season and in accordance with the law. At the present time, thunderstorms, hail can happen anywhere in the country, Hanoi area has also experienced hail. However, the phenomenon occurs most in localities in the northwest mountainous region. According to meteorological records, almost every year in localities like Lao Cai, Yen Bai and Ha Giang . hail with different intensity also occurred. However, the hail on March 27 in Lao Cai was determined to have an unprecedented magnitude.

Hail occurs continuously in many places and with the same intensity as before is also unusual. Mr. Nguyen Van Huy, the Hydrometeorological Center of Kon Tum province, said: "Kon Tum is located in an area with lots of hail. In recent years, hail is almost always in recent years. Hail in Kon Tum often appear in the late dry season, early rainy season, also when hail occurs in the late rainy season but often these hailstorms are not large.Normally hail in Kon Tum or appear in areas with hot weather such as Kon Tum City, Kon Ray, Dak To, Dak Ha and Sa Thay districts, . This year, in the last four hail events, there are 2 matches (occurring on the afternoon of March 13 and the afternoon of the day). March 24) again in Kon Plong district, which is the coolest area in the province, this is a rather unusual sign of the weather.

Why are hail?

According to Mr. Nguyen Duc Hoa's explanation, hail occurs due to instability in the air between a cold and hot climate. Specifically, about 4 - 5 days ago, Lao Cai took place quite hot weather, causing the ground to be hot. But on the morning of March 27, the northeast monsoon came down. The conflict between two hot and cold air masses stimulates convection to thrive. The steam rises high, condensing into small stones, these stones continue to coagulate and stick together to form larger stones and fall to the ground.

The book "The mysteries around us" explains more about hail phenomenon:

When clouds near the ground are picked up by air currents, the upper part of the cloud is usually below -20 degrees Celsius, causing a lot of clouds to turn into tiny ice particles. But the lower cloud layer is lower, due to many reasons that cannot be condensed into ice, turn into cold drops of water below 0 degrees Celsius. High-flow of non-stop air will bring a large amount of droplets This cold water is on the upper floor of the cloud. Shortly thereafter, they congealed with existing ice particles on the upper floor, making the volume of ice particles getting bigger and bigger, when the weight increased to a certain extent they would fall.

Hail has many different sizes and shapes, so too
accumulation of water vapor when moving in air. (Photo: SGTT)

When falling to the lower cloud layer, the outer surface of the tape is covered with a layer of water film, while being inflated by strong, weak water currents that are constantly rising and impacting. The longer the air flows, the more the "water jacket" layer of the bandage will continue to collide, leading to mutual entanglement, making the volume of the body band larger. At this point, air currents can no longer "juggle" the ice sheets, letting them fall to the ground, causing hail.

However, the more water the sky has, the louder the hailstone. The researchers say the power of air currents moving upward in the storm is the deciding factor for their size.

Where the storm appeared, the air moved up very quickly. When it reaches a certain height, this airflow cools, the water vapor in the water vapor evaporates condensing into a storm cloud. Finally, the setting is formed in the clouds, first like snow flakes, then like rain drops.

If the raindrops are caught again in the air moving up again, it will continue to move above the freezing point, and become a small ball of ice. This ice particle receives more tiny stones in the surrounding environment, and finally, when it is heavy enough, it falls down, and then gets caught in the turbulent activity of the air.

With each up and down trip like that, hailstone is added with new substances. When observing a cross-section of a raindrop, we will see layers like the tree's age ring, indicating how many "round- trip " trips have been made .

The areas of chaotic active air are the birthplace of the largest hailstones. Estimating the air moving upwards at a speed of about 160 km / h can create hail particles with a diameter of 12cm or more. A famous hail fell to Coffeyville, the 1979 Kansas weighed 750 grams, about 20cm in diameter.

Hail usually ends very quickly within 5-10 minutes and can last from 20-30 minutes.

Why hail only appears in the hot season?

In the late spring and early summer, sometimes in the morning the air is still cool, pleasant, hail comes in the afternoon when the temperature is quite high. While winter, even the coldest days never hail. Why is it "paradoxical" ?

Hailstones are still on the ground of Laos
Cai many hours after the rain stopped. (Photo: VnExpress)

According to the book "Ten thousand questions why" , hail and showers which are brothers together are formed from the cumulus cloud (also known as the cumulonimbus cloud), only different in the hail formations formed. in the condition of up and down air currents (convection) very intense. This condition is only available during the hot season, but rarely occurs in the cold season.

In hot and humid seasons, intense sunshine, very high levels of water vapor in the air. The low-floor atmosphere receives a lot of thermal energy that will heat up, forming a column of hot air under cold, very unstable. At this time, intense convection phenomenon arises, creating cumulonimbus clouds capable of causing hail. In the meantime, the air flow in the cloud is also strong enough to support large ice particles that form and grow in the clouds, causing them to continue to associate with small snowflakes or water droplets along the way, at the end. The same ice becomes a layer of clear and interlaced layers. When a large ice sheet reaches a level where the air flow is no longer enough to support it, it will fall to the ground, causing hail.

Wintering, because the sun shines on the ground, the heat collected here is very weak, does not cause strong convection. Meanwhile the air is dry, so even if there is convection, it is not easy to create large cumulonimbus clouds. Even if the cumulonimbus cloud is created, the convection current going up in it is not strong enough to maintain the formation and growth of ice particles, so the cold season has no hail.

Someone will ask: Conditions of formation of ice particles require very low temperatures, while the summer is very hot, how can there be ice? The problem is that in the hot summer, the temperature of the ground is up to 30 degrees Celsius, but the higher the air, the colder it gets. If at the bottom of the cloud, the temperature is 20 degrees Celsius in the cloud, the place is 4 km high, the temperature has dropped below 0 degrees C. A hail cloud can reach a height of over 10km. So in the air there are areas of temperature lower than freezing point, creating conditions for ice particles to form.

Predict and recognize hail

According to Mr. Nguyen Van Huy, Hydrometeorological Center of Kon Tum province, in fact hail often occurs only during strong storms and accompanied by heavy rainfall in a relatively short period of time. The shape and size of ice cubes freezing in hail are often very different, most commonly are spheres, cones, convex lenses, polyhedra and some other deformed shapes; diameter from about 0.5mm to a few dozen mm; weight from a few grams to a few hundred grams. Hail usually occurs only when there are thunderstorms, but hail is not in any storm. The frequency of hail in the thunderstorm is only about 10%.

Hail falls in the atmosphere with great speed. Falling velocity increases in proportion to the size, weight of the stone and ranges from 30 - 60m / s, particularly up to 90m / s. At such speeds, falling ice particles on objects or vegetation can leave traces and cause noise that is equivalent to a heavy train or truck, or a tracked vehicle passing through the bridge.

Mr. Huy also said that, unlike storms, floods or other natural disasters, preventing and limiting the effects of hail is effectively implemented or not, much depends on the understanding and efforts of every citizen. and family. With the current ability of hail prediction, especially predicting precisely where hail occurs before a relatively long time is very difficult. The Hydrometeorological Agency can only rely on the development of thunderclouds to give a warning 1-2 hours before hail occurs. However, this can only be done in places with meteorological observation stations or weather radar waves. In Kon Tum, due to the absence of weather radar, the whole province has only two meteorological gauges, so it is only possible to give early warning of the possibility of hail based on weather patterns, cloudy images and any fluctuations. often of weather factors observed at meteorological stations, but can not indicate the exact time and place of hail.

According to a warning from the meteorological agency, hail is an extreme weather phenomenon that is dangerous and only decreases gradually when the rainy season comes, which is about the end of May. on some features such as: strong daylight, dark clouds in the sky; at night there are thunderstorms, the wind is blowing all of a sudden, it is suddenly cold . If you see the sky is windy, the dark clouds cover the sky almost close to the eye, shaped like a breast, then windstorms float strong up, creating a continuous "buzzing, rumbling" sound, be wary of hail. If the next few sprinkles a shower, I feel that the temperature of the air, like the cold is very fast, is when hail has come. Everyone needs to find a safe place for me.

Is there any way to prevent the effects of hail?

As mentioned above, it is very difficult to predict hail and the exact area with hail. We also have no way to prevent hail because it is a weather phenomenon with unusual happenings of hot and cold airflows. People in areas where there is hail need to regularly monitor weather information to soon know the possibility of hail and always prepare shelter plans, avoid safety for people, animals and restrictions The effects of hail on items, utensils, machines, . if it happens.

Hail often occurs in mountainous and semi-mountainous areas, but most people in these areas still face difficulties, without conditions to build permanent houses, mainly roofs, roofs and corrugated iron roofs. and fibro cement. Most of the hail has small stone particle size so it only destroys crops, does not affect the people's houses much, only a few large and heavy stones fall into the poor quality roofs. However, the recent hail in Lao Cai has a large size of stones that most houses have broken roof, except for houses with concrete casting roofs. Thus, with crops, there are almost no measures against hail, and the most sustainable solution for the people's roof is solid concrete casting roof.

However, if you are worried about hailstorms that can fall into your garden and home, there are a few ways to minimize damage:

- With crops or crops that are easily crushed, you can build scaffolding along the beds, and should make triangular trusses to help reduce the impact of hailstones when colliding, rocks will fall to both sides tree beds without puncturing the cover, paying attention to making sure that the piles are against.

- With the roof, it is necessary to regularly check the condition of the roof and reinforce the roof. In critical areas, materials that can withstand impact should be used. Currently the market has a very durable Polycarbonate sheet material, high impact resistance, sound insulation, fire resistance and durability for many years in environmental conditions and harsh weather, not broken, light weight. and ultraviolet ray resistance (UV). Thick or multi-layer Polycarbonate sheets can even be used as bulletproof windows. It is possible to equip this material in the sections of light roofs, roofs, awnings, glass roofs, skylights, roofs . to avoid breakage when hail.

- The system of roof frame and purlin structure should also use good bearing, anti-corrosion, and reinforced materials. Construction materials and techniques can improve the rigidity of the roof frame and help reduce damage caused by hail.

- Making the roof slopes down to the sides, how to build this roof will reduce the impact from hail. Hail hitting the roof at an angle of 90 degrees will cause more damage than hail falling on the roof at an angle of deviation.

- With cars, there is a type of gas tarpaulin called Hail The Protector , a set of accessories including waterproof car tarpaulin with air blowing space to turn into protective air mattress, run by Car power supply or storage battery. The time to activate air bulge up to 30 to 60 minutes, the manufacturer will include a weather warning application installed on the smartphone, whereby the customer will have any preparation time in advance. any hail. When gas pump is not activated, tarpaulin can be used to cover rain and sun like normal, can be folded neatly in the trunk of the car. This product has the lowest price of $ 299, with all kinds of sizes suitable from sedans to SUVs, pickup trucks .

Hail The Protector gas tarpaulin helps protect cars when hail

- If you are going out on the street with hail, you should immediately stop looking for a hidden place, wear a helmet to avoid falling rocks, waiting for the stone on the road to dissolve to continue to avoid slipping.

- With heavy hail such as in Lao PDR on March 27, the above measures may not work, but to avoid human damage, people should find a place where they can "hide" under the table. , under the bed, find hard objects to cover his head.

- In addition to the above danger, hail can also bring other hazards such as carrying toxins, acid . If the cloud is formed from toxic waters, the environment is not clean, the dirt in Rainwater can damage human skin, cause allergies, so before using water contaminated with ice storms, people should take water samples, bring them to centers to check water quality.