Why Does Hail Occur and How to Prevent Hail

Why does hail occur, and is there a way to predict and prevent it? The following article will address these questions for you.

What is hail?

Hail is a type of precipitation that falls as ice pellets or balls of varying shapes and sizes, caused by strong convection from thunderstorm clouds. The size can range from 5 mm to several centimeters, typically around a few centimeters in diameter, and they often have an irregular spherical shape. Hailstones generally fall alongside rain.

Hail is more common in mountainous regions or areas near the sea and mountains (sub-mountainous areas), while it occurs less frequently in flat regions. Therefore, in Vietnam, hail can occur in various regions and even during the summer. Specifically, in the northern mountainous areas of Vietnam, hail typically occurs from January to May each year, peaking from March to May, primarily due to the rapid influx of strong cold fronts.

Why does hail occur?

According to Mr. Nguyễn Đức Hòa, hail occurs due to instability in the atmosphere when cold and warm air masses meet. For instance, about 4 to 5 days prior, Lào Cai experienced intense heat, causing the ground to become very warm. However, on the morning of March 27, a northeastern monsoon swept in. The collision between the two air masses stimulated strong convection. Water vapor rises, condensing into small ice particles, which continue to freeze and combine to form larger hailstones that eventually fall to the ground.

The book “The Mysteries Around Us” provides a more detailed explanation of hail:

When clouds near the ground are lifted by air currents, the top part of the clouds is usually below -20 degrees Celsius, causing much of the water vapor within the clouds to turn into small ice particles. However, the lower cloud layer, due to various reasons, cannot condense into ice and instead forms droplets of water that are below 0 degrees Celsius. The ongoing upward air currents carry a large amount of these cold water droplets to the upper part of the cloud. They then freeze with the existing ice particles at the top, increasing the size of the ice particles. Once their weight reaches a certain threshold, they will fall down.

Hailstones come in various sizes and shapes due to the accumulation of water vapor while moving through the air. (Photo: SGTT).

When the hailstones fall to the lower cloud layer, their outer surface is coated with an additional layer of water, while they are continuously affected by varying air currents that rise and fall. The longer they are subjected to these air currents, the more the “water coat” on the hailstones collides and sticks together, causing their volume to increase further. Eventually, the air currents can no longer “juggle” the hailstones and let them fall to the ground, resulting in hailstorms.

However, it is not necessarily the case that the more moisture in the atmosphere, the larger the hailstones will be. Researchers indicate that the strength of the upward air currents in a storm is the determining factor for their size.

In areas where storms occur, air moves upwards very rapidly. Once a certain altitude is reached, this air cools, causing the water vapor in the water droplets to condense into a storm cloud. Ultimately, the freezing occurs within the clouds, first appearing like snowflakes and later resembling raindrops.

If these raindrops are caught in the upward-moving air currents again, they will continue to rise above the freezing level and become a small ball of ice. This ice particle picks up additional tiny ice particles from its surroundings, and once it becomes heavy enough, it falls down, only to be caught again in the chaotic air currents.

With each upward and downward journey, the hailstone is supplemented with new materials. When observing a cross-section of such a hailstone, one can see layers resembling the rings of a tree, indicating how many “round trips” it has made.

The chaotic air regions are where the largest hailstones are born. It is estimated that an upward air current moving at about 160 km/h can produce hailstones with diameters of 12 cm or more. A famous hailstone that fell in Coffeyville, Kansas in 1979 weighed 750 grams, with a diameter of approximately 20 cm.

Hail typically concludes very quickly, lasting between 5 to 10 minutes, but can also extend from 20 to 30 minutes.

Why does hail only appear in warm seasons?

At the end of spring and the beginning of summer, there are times in the morning when the air is still cool and pleasant, yet hail may occur in the afternoon when temperatures are quite high. In contrast, during winter, even on the coldest days, hail never occurs. Why is it “counterintuitive”?

Hailstones remain scattered on the ground in Lào Cai hours after the rain stops. (Photo: VnExpress)

According to the book “One Hundred Thousand Whys”, hail and heavy rain are fundamentally related as they both form from cumulonimbus clouds, with the distinction that hail forms under conditions of intense convection. Such conditions are typically present in warm seasons and rarely appear in cold seasons.

In the humid and hot summer, with intense sunlight, the moisture content in the air is very high. The atmosphere at lower levels receives a lot of thermal energy, heating up and creating an unstable column of air, with warm air beneath and cold air above. This results in strong convection, generating cumulonimbus clouds capable of producing hail. During this time, the upward air currents within the cloud are also very strong, sufficient to support the formation and growth of large ice particles within the cloud, allowing them to continue to combine with snowflakes or small water droplets on their way, ultimately becoming layered ice balls. When the ice ball grows to a size that the upward air current can no longer support, it will fall to the ground, causing a hailstorm.

In winter, due to the sun’s rays striking the ground at an angle, the heat received is quite weak, preventing strong convection. Furthermore, the air is dry, so even if convection occurs, it is not easy to form large cumulonimbus clouds. Even if such clouds are created, if the upward convection within them is not strong enough, it cannot sustain the formation and growth of ice particles; therefore, hail does not occur in cold winter months.

Some may ask: The conditions for forming ice particles require very low temperatures, while summer is hot. How can ice form? The issue is that in the hot summer, ground temperatures can reach up to 30 degrees Celsius, but the air becomes colder as it rises higher. For instance, at the bottom of the cloud, temperatures may still be 20 degrees Celsius, but at an altitude of 4 km, temperatures drop below 0 degrees Celsius. A hail cloud can extend to heights above 10 km. Thus, there are regions in the atmosphere where temperatures drop below freezing, creating conditions conducive to the formation of ice particles.

Forecasting and Recognizing Hail

According to Mr. Nguyễn Văn Huy, from the Kon Tum Meteorological Center, in reality, hail typically occurs only in severe thunderstorms and is accompanied by heavy rainfall for relatively short periods. The shape and size of ice pellets in hail vary greatly, with the most common forms being spherical, conical, convex lens-shaped, polyhedral, and several other irregular shapes; diameters range from about 0.5 mm to several centimeters; weights range from a few grams to several hundred grams. Hail usually occurs only during thunderstorms, but not every thunderstorm produces hail. The frequency of hail occurrence in a thunderstorm is only about 10%.

Hail falls through the atmosphere at very high speeds. The falling speed increases proportionally with the size and weight of the hailstone, ranging from 30 to 60 m/s, and can even reach 90 m/s. With such speeds, hailstones impacting objects or vegetation can leave marks and create noise comparable to that of a train or heavy truck passing over a bridge.

Mr. Huy also noted that, unlike typhoons, floods, or other natural disasters, the effectiveness of preventing and mitigating the damage caused by hail depends significantly on the understanding and efforts of each citizen and family. Given current capabilities, forecasting hail, especially predicting the exact area where it will occur in advance, is quite challenging. Meteorological agencies can currently only rely on the development of thunderstorm clouds to issue warnings 1-2 hours before hail strikes. However, this can only be done in areas with weather observation stations or weather radar. In Kon Tum, due to the lack of weather radar and only two meteorological stations in the province, early warnings about the likelihood of hail can only be made based on weather patterns, cloud images, and unusual fluctuations in weather factors observed at the meteorological stations, without being able to pinpoint the exact time and location of hail occurrence.

According to warnings from meteorological agencies, hail is an extremely dangerous extreme weather phenomenon and will gradually decrease as the rainy season approaches, around the end of May. Citizens can recognize the signs of impending hail based on several characteristics: strong thunderstorms during the day, dark clouds covering the sky; at night, thunder and lightning, a sudden stillness in the previously steady winds, and a sudden drop in temperature… If you notice strong winds and dark clouds almost completely blocking your view, resembling a mammary shape, followed by strong gusts of wind that create a continuous “whooshing, rumbling” sound, you should be alert for hail. If there are then a few drops of rain, and you feel the air temperature dropping rapidly, hail is on the way. People need to find a safe shelter immediately.

Is there a way to prevent hail damage?

As mentioned above, predicting hail and the exact areas that will be affected is very difficult. We also cannot prevent hail since it is a weather phenomenon characterized by the unusual interactions of hot and cold air masses. Residents in areas prone to hail should regularly monitor weather information to be aware of the potential for hail and always prepare safe shelter options for themselves, livestock, and to mitigate hail damage to goods, equipment, and machinery if it occurs.

Hail often occurs in mountainous and semi-mountainous areas, but most residents in these regions face difficulties and lack the means to build solid homes, primarily relying on thatched roofs, tile roofs, corrugated iron, and fibrocement. Most hailstorms produce small hailstones, causing only damage to crops without significantly affecting homes, except for a few larger and heavier stones that may fall on poorly constructed roofs. However, recent hailstorms in Lao Cai produced hailstones that were too large, resulting in roof damage to most homes, except those with solid concrete roofs. Thus, there are virtually no effective measures against hail for crops, while the most sustainable solution for residential roofs is solid concrete roofing.

Despite this, if you are concerned about hail potentially falling on your garden and home, there are several ways to minimize damage:

• For crops or plants that are easily damaged, you can set up a protective canopy along the rows, and using a triangular roof design will help reduce the impact of hailstones upon contact, allowing the stones to fall to the sides of the plants without penetrating the canopy; ensure that the supporting posts are sturdy.
• For roofs, regularly check the condition of your roof and reinforce it. In critical areas, use materials that can withstand impacts. Currently, there are materials such as Polycarbonate sheets that are very durable, impact-resistant, soundproof, fire-resistant, and last for years in harsh environmental and weather conditions. They do not shatter, are lightweight, and have excellent UV resistance. Thick or multi-layered Polycarbonate sheets can even be used as bulletproof windows. This material can be installed in skylights, awnings, greenhouse roofs, and carport roofs to prevent breakage during hail.
• The roof frame and trusses should also use materials with good load-bearing capacity, corrosion resistance, and be carefully reinforced. The choice of materials and construction techniques can enhance the rigidity of the roof structure and help reduce damage from hail.
• Constructing a roof that slopes down on both sides will reduce the impact force from hail. Hail striking the roof at a 90-degree angle can cause more damage than hail falling at an angle.
• For cars, there is a type of airbag cover called Hail The Protector, which is a set of accessories consisting of a waterproof car cover that inflates into an air cushion for protection, powered by the car’s electrical system or a backup battery. The activation time for the airbag to fully inflate is between 30 to 60 minutes. The manufacturer also provides a weather alert app that can be installed on smartphones, allowing customers to prepare before any hailstorm. When not activated, the cover can be used as a regular rain cover and can be neatly folded and stored in the car’s trunk. This product is priced starting at 299 USD, with sizes available to fit everything from sedans to SUVs and trucks.

Hail The Protector Airbag Cover for Car Protection Against Hail

• If you are on the road and encounter hail, you should immediately stop and seek shelter, wear a helmet to protect your head from falling hailstones, and wait until the hail on the road has melted before continuing to avoid slipping.
• For significant hailstorms like the one in Lao Cai on March 27, the above measures may not be effective, but to avoid injury, residents should seek shelter under tables, beds, or find hard objects to cover their heads.
• In addition to the above dangers, hail can also carry other hazards such as toxins, acids, etc. If the cloud is formed from toxic water sources or unclean environments, contaminants in hailwater could harm human skin and cause allergies. Therefore, before using any water that may be contaminated with hailwater, residents should take samples and bring them to testing centers to check for water quality.