Throughout thousands of years in human history, storms have been referred to as a destroyer or the wrath of Mother Nature, or even manifestations of divine power. The term “hurricane” itself originates from “Hurakan”, a destructive god of the Maya civilization. It is difficult to fully describe the power and devastation of a storm. Every year, our country faces a prolonged rainy season from May to December, with storms forming from the East Sea moving inland and directly affecting coastal provinces.
This year, the world has witnessed many extremely large and powerful storms, causing severe consequences in the areas they pass through. So how do storms form? Why do storms occur? The article below will provide answers to these questions.
What is a Storm?
A storm is a state of atmospheric disturbance and a type of extreme weather. Storms typically feature strong winds, heavy rain, and may include thunder, lightning, hail, and tornadoes. Additionally, the concept of storms also encompasses snowstorms, sandstorms, and dust storms. Storms (and tropical depressions) are powerful swirling systems characterized by low pressure at the center, strong winds, and a well-developed cloud system arranged in a spiral, accompanied by thunderstorms and heavy rainfall.
Satellite image of a storm.
The structure of a storm includes the eye, the eyewall, and the storm circulation. At the center of the storm is the eye, which is typically a clear area with light winds. However, the eye is not always clear and can be obscured by clouds. Stronger storms tend to have a more distinct eye compared to weaker ones. Surrounding the eye is the eyewall, where clouds form a towering wall (several kilometers high) and where winds blow the strongest. If you encounter the eye, you should seek shelter in a safe place. The eye can last for quite a long time (more than 1 hour) but can also exist for just a few minutes. Outside the eyewall is the storm circulation, where bands of clouds produce rain. These cloud bands play a crucial role in the evaporation-condensation cycle, providing energy to sustain the storm.
Wind strength refers to the sustained wind speed over a long period, which serves as the basis for evaluating the storm’s intensity. The damage caused by storms primarily results from gusts, which last only a few seconds to under a minute but have significantly higher speeds than the sustained wind. Therefore, weather reports always include information on gusts in addition to sustained wind speeds.
A storm is a tropical cyclone structured by a mass of warm, moist air with a strong updraft surrounding the eye, creating a system of spiraling clouds and rain into the storm’s center. This rotating wind region can have a diameter of hundreds of kilometers and forms over tropical waters in the northern hemisphere.
Depending on the formation region, storms have different names.
- Storms forming over the Atlantic: hurricanes
- Storms forming over the Pacific: typhoons
- Storms forming over the Indian Ocean: cyclones
In Vietnam, the term “storm” is used to describe tropical storms, a type of weather characteristic of tropical sea regions, often featuring strong winds accompanied by heavy rain.
According to international standards, storms are classified based on wind strength (based on the Beaufort scale and the Saffir-Simpson scale):
- Sustained winds below 63 km/h are called tropical depressions
- Sustained winds over 63 km/h (Category 8) are called tropical storms (“tropical cyclone” or “tropical storm”)
- Sustained winds over 118 km/h (Category 12) are called strong storms or typhoons
- Sustained winds over 241 km/h are called very strong storms or super typhoons
Every year, the world endures a rainy season, during which 40 to 50 tropical depressions form around the equator, developing into storms. In the Northern Hemisphere, the storm season runs from June 1 to November 30, while in the Southern Hemisphere, the storm season usually starts from January to March.
Hurricane Harvey from space.
How Do Storms Form?
Storms form over the ocean, originating from areas with temperature or pressure differences. When sea surface temperatures exceed a certain threshold, water vapor rises, intensifying the vertical and horizontal rotation. A tropical depression gradually forms. If the depression remains over warm waters, it will strengthen into a storm and can escalate into a powerful hurricane. Thus, storms only form over the ocean and weaken once they make landfall. A storm forms when sufficient conditions are met: heat, humidity, and the dynamics to create rotation.
Meteorologist Erik Palmen discovered that storms can only form over the ocean within latitudes of 5 to 20 degrees on either side of the equator, where temperatures are high (from 26-27 degrees Celsius or higher) to ensure a sufficient supply of massive amounts of water vapor evaporating from the sea surface to provide condensation energy for storm formation, and where the Coriolis force is strong enough to create rotation, facilitating storm development.
Storms cannot form in the 0 to 5-degree latitude zones on either side of the equator because the Coriolis force is too weak there to create rotation. The air mass within the rotating zone has a horizontal span of about 200 km and a length of about 1000 km, at an altitude of about 10-12 km above the ground.
Why Do Storms Occur?
Storms form in tropical regions because this natural phenomenon requires a very warm water current, with a minimum temperature of 26 degrees Celsius at a depth of at least 50 meters below the water surface.
Warm water creates significant evaporation, which is the fuel for storms. This highly humid air mass rises to 15 kilometers high. There, the air cools and condenses, causing the storm clouds to grow larger.
As the cold air descends, it draws in warm, humid air at a very high speed into the rising air column. This phenomenon occurs because the pressure here is lower than in surrounding areas, explaining why the clouds swirl around this chimney-like column.
Hurricane Irma sweeping through Sint Maarten.
Due to the Earth’s rotation, in the Northern Hemisphere, storms rotate counterclockwise, while in the Southern Hemisphere, they rotate clockwise, meaning storms always spin in the same direction.
The hotter the water, the higher the thermal machinery rises, and winds also intensify. At this point, the storm gains strength. Its power can sometimes reach levels equivalent to five nuclear bombs per second. However, as soon as it encounters a colder water current or makes landfall, the storm weakens due to a lack of warm, evaporating air.
Why Do Storms Mainly Occur in Summer and Autumn?
The primary active period for storms and tropical depressions is during the summer and autumn: From June to November (in the Northern Hemisphere) and from December to March of the following year (in the Southern Hemisphere), storms are most prevalent during summer and autumn, because during this time, all favorable conditions for storm formation and development are met: High sea surface temperatures (at least 26 degrees Celsius or higher), and a tropical atmosphere conducive to strong convection and large-scale rotational movements.
How do storms form and “die”?
