What is a cyclone?

In fact, there are many kinds of storms on the earth. Besides tropical cyclones, there are thunderstorms, extratropical cyclones, tornadoes, mesoscale convective systems (MCS) and so on. As tropical cyclone lovers, it is easy for us to distinguish tropical cyclones from many weather systems on satellite cloud images, but if two weather systems similar to tropical cyclones appear on satellite cloud images at the same time, one is in the western Pacific near Luzon Island and the other is in the western Pacific near Japan with a latitude of more than 30 degrees north, and everyone has a cloud cluster rotating around the center, then what criteria should we use to define which one is a tropical cyclone? Fixed or both? Therefore, the definition of tropical cyclone is strict.

The so-called tropical cyclone refers to an organized, non-frontal convective weather system that occurs on the surface of tropical oceans. The cloud system of this weather system has spiral characteristics at the same time, and the average wind speed near the center of the spiral cloud system also exceeds 17.2m * per second. Different parts of the world have different names for tropical cyclones, such as typhoons in the western Pacific, cyclones in the Indian Ocean, hurricanes in the North Atlantic and the East Pacific, and Willie-Willie in the southwest Pacific. Although the names are different, the nature of tropical cyclones is the same everywhere. In the northern hemisphere, tropical cyclones rotate counterclockwise, while in the southern hemisphere, they rotate clockwise. This disastrous weather system is usually accompanied by strong winds, heavy rains and huge waves, which will seriously threaten people's lives and property and have a considerable impact on people's livelihood, agriculture and economy. This is one of the serious natural disasters.

Hong Kong is located on the coast of South China, and it is affected by tropical cyclones from the western Pacific or the South China Sea every summer. Although Hong Kong has a high degree of urbanization, we are not afraid that tropical cyclones will pose a great threat to our lives when we walk into concrete buildings, but the damage caused by heavy rain and strong winds, even the loss caused by the suspension of economic operation when typhoon signal No.8 is above, can not be ignored.

According to the classification of the Hong Kong Observatory, tropical cyclones can be divided into four grades according to the highest wind force near their centers:

Tropical cyclone category

tropical depression

tropical storm

Severe tropical storm

typhoon

Maximum average wind force near the storm center10min.

62 kilometers per hour or less

63 to 87 kilometers per hour

88 to117km per hour

118km/h or more

In other parts of the world, their classification of tropical cyclones is slightly different, such as:

Taiwan Province Province:

kind

tropical depression

Mild typhoon

Moderate typhoon

Strong typhoon center pressure

& lt 1002

99 1~984

976~94 1

& lt940 central wind power generation (km/h)

38-62

63- 1 17

1 18- 183

& gt 183

In the North Atlantic, the intensity of hurricanes is classified by Saffir-Simpson scale:

Damage degree of hurricane-level central pressure (hPa)

(km/h) (nautical mile/h)

1

2

three

four

5 & gt980

965-980

945-965

920-945

& lt920

120- 153

154- 177

178-209

2 10-249

& gt250 64-82

83-95

96- 1 13

1 14- 135

& gt Minimum 135

medium

vast

serious

catastrophic

* Although we know that the wind speed of tropical depression is not so strong, it is difficult to define the wind speed of tropical depression (tropical cyclone) because there are obvious differences between Hong Kong and Taiwan Province Province. Therefore, we temporarily use the minimum wind speed of tropical storm 17.2m to define whether a low-pressure system meets the standard of tropical cyclone.

2. Where are tropical cyclones in the world? Are the intensity of tropical cyclones different in different areas?

In addition to the southeast Pacific and the South Atlantic, tropical cyclones can be found in many sea areas in the world, with the highest frequency of tropical cyclones in the western Pacific, about 30 per year. On average, more than 80 tropical cyclones are formed in the world every year.

The reason why there are so many tropical cyclones in the western Pacific near us is that the western Pacific is vast, so there are more areas where tropical cyclones are generated. Moreover, due to the earth's rotation, warm seawater heated by solar radiation in the Pacific Ocean accumulates in the western Pacific Ocean. Therefore, the western Pacific is also the warmest sea area in the world, so it is also called the "warm pool" in the earth's ocean, which makes the tropical cyclone generation frequency here the highest. Moreover, due to the high water temperature in the western Pacific, the intensity of typhoons generated in the western Pacific is generally stronger than that of tropical cyclones generated in other sea areas of the world; Moreover, the intensity of typhoons in the western Pacific will never be inferior to hurricanes in the Atlantic Ocean. We think that hurricanes in the Atlantic Ocean are stronger than typhoons in the western Pacific Ocean only because the media on the other side reported and exaggerated the news of hurricanes.

(Excerpted from the University of Illinois Meteorological Online Guide: Hurricanes)

Figure 1 Global Tropical Cyclone Distribution

3. What are the necessary conditions for the formation of tropical cyclones?

The formation of tropical cyclones is not accidental. We found that although there are often disturbing weather systems in the ocean, such as some thunderstorm clouds, only a few can develop into tropical cyclones. Generally speaking, to form a tropical cyclone, the following thermodynamic and dynamic conditions must be met.

As far as thermodynamic conditions are concerned,

1. On the vast ocean surface, the seawater temperature must exceed 26.5℃. Because the higher seawater temperature will make the atmosphere in unstable environmental conditions, which will make the disturbing weather develop.

2. There must be enough water vapor at the bottom of the atmosphere, which is conducive to the conditions of air rising:

There must be enough water vapor in the bottom and middle atmosphere to make the humidity high, because water vapor is the raw material of tropical cyclone clouds and rain belts; Moreover, the latent heat released when water vapor condenses will make the air temperature rise, become lighter, and then expand and rise. Therefore, the more water vapor, the more conducive to the upward movement of air. Otherwise, the strong convection and vertical movement of air will be difficult to develop.

As far as kinetic conditions are concerned,

1. The Coriolis force generated by the earth's rotation is not equal to zero.

In most cases, tropical cyclones will only form within a range of more than 5 degrees from the equator (north and south latitudes). However, within the equatorial range, the Coriolis force generated by the earth's rotation (see Endless Atmospheric Motion, Figure 5) is too small to form an air vortex. Even if the air vortex is generated, it will take a long time for the cyclone disturbance to reach the typhoon stage, which is not conducive to the formation of tropical cyclones.

2. The difference of wind direction or wind speed between the lower atmosphere and the upper atmosphere should not be too big (vertical wind shear should not be too big):

The wind speed of the upper and lower layers is too large, which will make the latent heat released when water vapor condenses not concentrate in the same air column, and make the latent heat released when water vapor condenses quickly overflow. In this way, the temperature of the air column will not increase obviously, and the air pressure will not decrease rapidly (ideally, the temperature will increase → the air will expand and become lighter → the air pressure will decrease). Moreover, too big difference in wind speed will blow away the tropical cyclone cloud system, making it difficult to organize the tropical cyclone cloud system and form or strengthen the tropical cyclone.

Fig. 2 Influence of vertical wind shear on tropical cyclone structure

3. If there is a vortex wind field at the bottom of the atmosphere, which makes the air vortex easy to form, or if there is a convergence area of air in the atmosphere, such as intertropical convergence zone, it will be more conducive to the formation of tropical cyclones.

Fig.3 The vortex wind field formed by the convergence of intertropical convergence zone and trade winds is the cradle of tropical cyclone formation.

With these basic conditions, of course, there must be some unstable and disturbing weather systems on the ocean surface that produce tropical cyclones. These disturbing weather mainly comes from intertropical convergence zone (ITCZ), followed by easterly waves, upper air cold vortex (TUTT for short), trough of low pressure, and even the vast cloud and rain belt split from tropical cyclone. The thunderstorm weather system from land to sea can also be the embryo formed by tropical cyclone.

4. How are tropical cyclones formed?

As mentioned above, the prerequisite for the formation of tropical cyclones is that there must be disturbed weather first, otherwise tropical cyclones will not appear.

As we know, the disturbed weather on the tropical ocean has vertical upward air movement. When the water vapor in the rising air condenses into a water point, the water vapor will release latent heat and heat the surrounding air. Then, the heated air will expand and become lighter, making the whole air mass rise, and then the air pressure will drop (Figure 4, left). When the ground air pressure drops, because the air pressure difference between the low-pressure system and the neighboring areas is more significant, more air will be pulled to the low-pressure center, and more water vapor will be brought to the low-pressure system while the air is pulled in, providing more heat energy for the low-pressure system. Furthermore, the air flowing into the low pressure will stir the seawater, make the seawater churn, release the heat stored in the warm seawater into the air, and further provide heat energy for the air flowing into the low pressure, thus further providing more energy for the low pressure (Figure 4, right). When the air enters the low pressure, it will not only be heated by seawater, but also be deflected by the earth's rotation (that is, the Colio effect), which will cause the air to swirl and eventually form a tropical cyclone.

It is through this positive feedback mechanism that tropical cyclones increase their intensity, just like snowballing effect. The strengthening process of tropical cyclones is called the second kind of conditional instability (CISK). It was not until the tropical cyclone exhausted all the conditions conducive to its own development that it stopped further strengthening.

Fig. 4 schematic diagram of the second kind of conditional instability (CISK)

5. What is the development process of tropical cyclones?

Generally speaking, the development of tropical cyclones can be divided into four stages:

I) formation stage

In the formation stage, the low pressure system over the tropical ocean began to have the characteristics of tropical cyclone spiral cloud system. At this time, the central air pressure is about 1000 hectopascals, and the central wind force is gale level or below. If we look at this tropical depression from the satellite cloud image at this time, we will usually find that its cloud system organization is not perfect and asymmetric, and it is not easy for us to determine the center position of this storm by the characteristics of spiral cloud system. The formation stage of tropical cyclones can be long or short, ranging from dozens of hours to days.

2) Development stage

At this stage, the central air pressure of tropical cyclone continues to decrease, the central wind force continues to increase, its circulation (coverage) also further increases, and the organization of its cloud and rain belt is gradually improving. Finally, the wind with a radius of 30 to 50 kilometers around the center is strengthened to hurricane level (≥ 1 18 km/h). At this time, the well-organized cloud and rain belt will roll inward from the periphery of the storm to the center of the storm, and an eye area may also appear at this time.

Iii) Maturity stage

This stage can generally last from one day to one week. At this time, a feasible eye area will appear in the center of the storm, the central air pressure will stop falling, and the central wind will not be further strengthened. At this time, its gale circle can occupy a radius of more than 300 kilometers at sea (but the actual gale range depends on the size of tropical cyclones). However, due to the movement of tropical cyclones, the scope of the gale circle is not necessarily symmetrical, and some places may be larger and some places may be smaller. Generally speaking, bad weather usually occurs on the path to the right of the center of the moving direction, which is the so-called "dangerous semicircle".

Iv) extinction stage

When a tropical cyclone lands or moves to a colder sea surface, it enters the extinction stage. In this case, because the supply of water vapor is greatly reduced, tropical cyclones can no longer rely on the latent heat released by water vapor (CISK mechanism) to maintain their intensity, so their intensity gradually weakens. In addition, if the tropical cyclone encounters the invasion of cold air or dry air, the CISK mechanism that maintains the intensity of tropical cyclone will also be affected, so the tropical cyclone will inevitably weaken.

Although the development of typical tropical cyclones can be roughly divided into the above four stages, only half of the tropical cyclones in the western Pacific will experience the above-mentioned complete tropical cyclone development stages every year, that is, from tropical depression to typhoon to tropical depression and then to dissipation. This is because tropical cyclones cannot meet all the favorable conditions for the development of tropical cyclones, such as lack of weak vertical wind shear and insufficient water vapor. Then, at its peak, its intensity will only remain at or below the intensity of strong tropical storms. However, some tropical cyclones can go through several stages of strengthening or weakening. For example, Typhoon Wayne, which affected Hong Kong three times in 1986, was one of them.

In addition, after landing, if tropical cyclones move to the warm and vast ocean surface, their intensity can be enhanced again and go through its development stage again; If a tropical cyclone moves to a higher latitude, it can also be transformed into an temperate cyclone, but in the process of transformation, it will lose its warm heart characteristics as a tropical cyclone.

Generally speaking, the life span of a tropical cyclone is about six days, and the longest life span of a recorded tropical cyclone is only about 3 1 day.

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Structure of tropical cyclone

6. What is the structure of a mature tropical cyclone?

The structure of tropical cyclones can be roughly divided into the following categories:

Low-altitude inflow layer

From the ground to the height of 3 kilometers, the airflow is mainly due to the inflow of air into tropical cyclones. When the air flows into the tropical cyclone, the air converges and rises, and then strong convective clouds are generated.

Intermediate rising layer

The height from 3 km to 8 km is called the middle rising layer. The air in this layer neither converges nor diverges, but mainly moves vertically upwards, so it is also called non-divergent layer. Therefore, large-scale air movement (such as subtropical ridge) will promote the air rise in this layer, just as vertical paper will be more easily blown by the wind than flat paper (as shown below). Therefore, we are used to using the 500 hPa high-altitude weather map (about 5500 meters above the ground) to predict the path of strong tropical cyclones. The track of weaker tropical cyclones is generally predicted by the upper-air weather map of 700 hPa.

A vertical piece of paper (right) will be blown by the wind more easily than a flat piece of paper.

Upper outflow layer

From the height of 8 kilometers to the top of the troposphere is the outflow layer of air. The air flowing out of the tropical cyclone can carry the high clouds of the tropical cyclone far away, so we can notice that when the tropical cyclone approaches, more cirrus clouds will appear in the sky (pictured). These clouds blown from the height of tropical cyclones are called "outflow cloud shields".

eye of wind

Because of the rotation and centrifugal force of tropical cyclone, there will be a small windless and cloudless area in the center of typhoon. This is the familiar "eye of the wind", with a diameter of about 30~40 kilometers. In the eye area, the wind is weak, dry and warm, and partly cloudy, but the sea waves are still large. The weak tropical cyclone is not strong enough to have such a clear eye area (see 15).

Fig. 5 Structure of mature tropical cyclone

7. Why is the closer to the tropical cyclone center, the stronger the wind?

Before we know why, maybe we should look at a simple example.

First, suppose you run along a circular jogging path (Figure 6, left), and its length is1000 m. If you run a lap on the runway at the speed of 10 km per hour, you need 6 minutes to finish the lap.

Now, you are also running along a circular jogging track. This jogging trail is included in the original jogging trail 1 km, which is only 500 meters long. Now you are also running at a speed of 10 km, which only takes 3 minutes.

Now, suppose you see two other people running laps on two different jogging tracks on the playground. Similarly, the two of them run laps at the speed of 10 km per hour. You will find that the runner in the inner ring ran a lap quickly, while the runner in the outer ring didn't finish it, so he ran slowly ... if you think so, you are wrong. However, they run at the same speed, and the inner runner seems to run faster because the circumference is shorter.

Similarly, while the tropical cyclone air flows into the center, the wind is affected by the pressure gradient force, Coriolis force and centrifugal force, so the wind does not blow directly into the tropical cyclone center from the periphery of the tropical cyclone, but slowly "turns" around the center and enters the center (Figure 6, right). At first, due to the large circumference of tropical cyclone, it takes a long time for an air particle to complete a circular motion; However, as the air particles get closer to the center, it takes less time to complete a week because the perimeter is gradually reduced. Therefore, even if there is no significant difference in the pressure gradient of tropical cyclones (keeping the speed of pulling air into the center unchanged), the closer the wind is to the center of tropical cyclones, the faster the air will flow-in fact, the speed of air movement is "conservative" and constant.

This law is called the law of conservation of angular momentum.

Figure 6 The closer to the center of a tropical cyclone, the faster the air flows (right), which is like running a big runway and a small runway at the same speed, and the inner ring runs faster than the outer ring (left).

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The movement of tropical cyclones

8. What factors affect the movement of tropical cyclones?

The movement of tropical cyclones is influenced by two main factors: internal force and external force.

In terms of internal force, it refers to the deflection force generated by the earth's rotation, which makes it have the inertia of moving to the northwest, and the influence of the mass (symmetry) of tropical cyclone itself on its movement.

External force refers to how the surrounding airflow affects the flow direction of tropical cyclones. Like a branch in a river, tropical cyclones are affected by the surrounding water flow. Moreover, the influence of this external force is more obvious than its own internal force.

Fig. 7 factors affecting the movement of tropical cyclones

Internal force factors:

In the process of tropical cyclone movement, if the influence of large-scale environmental field (large-scale airflow) is not considered, the movement of tropical cyclone is mainly affected by the following two kinds:

Beta effect:

The so-called "beta effect" simply refers to the effect of Coriolis force (earth rotation deviation) parameters changing with latitude. We know that the Coriolis force in the equatorial region is the weakest, and the closer to the poles, the stronger the Coriolis force acting on moving objects. Similarly, the movement of tropical cyclones will also be affected by this force. Therefore, the farther a tropical cyclone reaches the polar region, the greater the effect of Coriolis force on its motion. However, under the action of Coriolis force, the tropical cyclone itself will have an inertia moving to the northwest. However, the influence of this force on the movement of tropical cyclones is not significant and can generally be ignored.

Symmetry of tropical cyclones;

As we know, tropical cyclone itself is a convective weather system, so air will continue to enter this convective system. Generally speaking, the structure of tropical cyclones is generally symmetrical, so there should be no obvious difference in air intake in each quadrant. However, if the structure of tropical cyclone is asymmetric, that is, some quadrants have strong convective clouds (or large-scale convective clouds) and some quadrants have weak convection, then the airflow in different quadrants of tropical cyclone is also different. If the difference between inflow airflow and inflow airflow is obvious, it will affect the movement of tropical cyclone, and the inflow airflow can also play a role in promoting it. As for which direction to actually go, it depends on other factors.

External factors:

Large-scale atmospheric movement:

In Endless Atmospheric Movement (2), we have already introduced that there is a large-scale air movement on the earth. Compared with the air movement on this planet scale, tropical cyclone is only one of the mesoscale weather systems. Therefore, the movement of tropical cyclones will be dominated by large-scale atmospheric movements.

For example, when a tropical cyclone in the western Pacific is formed at low latitudes, it will first be dominated by easterly winds on the south side of the subtropical high ridge and move west to northwest. After that, if the tropical cyclone turns above 20 degrees north latitude and reaches the mid-latitude area, it will be affected by the mid-latitude westerly belt, which will speed up the movement of the tropical cyclone. Finally, the tropical cyclone moved northeast under the influence of the mid-latitude westerly wind field and β effect, ending its life.

Interaction of meso-to large-scale weather systems;

The movement of tropical cyclones will also be affected by other medium to large-scale weather systems. For example, tropical cyclones will have fujiwara effect with other tropical cyclones, making their movement unstable; The path of tropical cyclones will also be affected by the upper-air low-pressure system, mesoscale convective weather system (MCS) and northeast monsoon.

Other factors:

Terrain:

The influence of topography on tropical cyclone lies in its proximity to land. For example, when a tropical cyclone lands, the air flowing into the lower layer of the tropical cyclone will be affected by the terrain, so the wind flowing into the tropical cyclone will change in quality and quantity, and this change in air intake will affect the movement of the tropical cyclone. Moreover, large-scale islands (especially islands with high mountains running through the center) will also change the direction of large-scale winds. For example, the low-level easterly wind on the south side of the subtropical high ridge will deflect when it blows to a large island, and this large-scale wind deflection will change the moving direction of tropical cyclones when they approach the island. For example, this usually happens when a tropical cyclone passes through Luzon Island or Taiwan Province Province.

Sea water temperature:

Some studies show that tropical cyclones tend to move to wet areas (wet tongues) or warm ocean surfaces. However, the findings of this study have not been generally recognized, and only a few books regard seawater temperature and wet tongue as one of the factors affecting tropical cyclones. The purpose put forward here is for your reference only.

9. What are the moving paths of tropical cyclones in the western Pacific?

Due to the internal force of tropical cyclone circulation and the external force of atmospheric circulation, the path of tropical cyclone may not be very regular. In fact, according to previous meteorological records, we have never found two typhoons with exactly the same path. Although the paths of tropical cyclones vary greatly, they can be roughly divided into three categories:

1. Parabolic path: the path that moves to the northwest after moving out of the original place and orients to the north or northeast when it reaches about 20 ~ 25 degrees north latitude.

2. Westward route: from the original place to the west or northwest, through Taiwan Province Province or the Philippines into the South China Sea.

3. Irregular path: There is no rule to follow, some will turn around to the south, and sometimes two tropical cyclones will rotate with each other, that is, the so-called Fujiwara effect will appear.

Fig. 8 the general path of tropical cyclone movement in the western Pacific.

10. What is Fujiwara special effect?

The so-called "Fujiwara effect" means that when the center distance of two tropical cyclones is less than 1200 km (but the actual distance of Fujiwara effect depends on the size and intensity of the two tropical cyclones), due to the interaction of the vortex flow fields of the tropical cyclones themselves, the center paths of the two tropical cyclones tend to rotate counterclockwise and approach each other (in the case of the northern hemisphere). This phenomenon was first observed by Japanese meteorologist Dr. Fujiwara (Fujiwara Zuoping 1890- 1965) in the current experiment in 1923.

Although Fujiwara effect is defined as two tropical cyclones rotating around the same center, Fujiwara effect can be ever-changing, not necessarily two tropical cyclones rotating around the same center: it can be that one tropical cyclone completely dominates the moving direction of the other, or it can be that two tropical cyclones are staggered, or one follows the other, or even there is no Fujiwara effect between them. Therefore, whenever two tropical cyclones approach each other, it is usually difficult to predict the path of tropical cyclones.

Generally speaking, the interaction of the most common tropical cyclones can be divided into three categories:

One-way influence type:

When a generally strong tropical cyclone and a generally weak tropical cyclone approach each other, the strong tropical cyclone will dominate the path of the weak tropical cyclone and make the weak tropical cyclone rotate counterclockwise around it. For example, the impact of typhoon Tim 1994 on tropical storm Vanessa.

Unidirectional influence type

Type of interaction:

When two tropical cyclones are equal in intensity, they will rotate around a concentric center, and they will not get rid of each other until they are influenced by other weather systems or one of them is weakened. For example, Typhoon Wayne and Typhoon Na Wei on 1986.

Interaction type

Combination type:

A stronger tropical cyclone may absorb a smaller tropical cyclone and make it a part of its own circulation. It's like Maggie absorbed the low pressure area of the South China Sea at the beginning of 1999 (but only if the distance is close enough and the weak tropical cyclone is not affected by other weather systems).

Fujiwara effect is a proper term for the interaction of tropical cyclones in Asia. In the North Atlantic, the interaction of tropical cyclones is called "windmill cyclone".

As for the tropical cyclones that affected Hong Kong in the past, some of them occasionally had Fujiwara effect with other tropical cyclones, which made it difficult for the Hong Kong Observatory to accurately predict their tracks. For example, 1986 typhoon Wayne and 199 1 typhoon Nader asked the Hong Kong Observatory to issue tropical cyclone warnings three times.

1 1. Where does the tropical cyclone affecting Hong Kong come from?

The so-called tropical cyclone affecting Hong Kong here refers to the tropical cyclone path that will hang tropical cyclone warning signals in Hong Kong. Generally speaking, tropical cyclones that affect Hong Kong will affect Hong Kong in different ways. These tropical cyclones mainly enter the South China Sea from the western Pacific Ocean, pass through the central or northern Philippines, the Bahrain Strait or the southern part of Taiwan Province Province, and then move to the coast of Guangdong, affecting Hong Kong. Tropical cyclone tracks affecting Hong Kong can be roughly divided into the following six categories:

I) southeast-northwest type

This tropical cyclone first formed in the western Pacific Ocean in the eastern Philippines, and then crossed the Philippines or the Balindan Strait in a west-northwest or northwest path, entered the South China Sea and affected Hong Kong. Usually, tropical cyclones entering the South China Sea through this heavy path can make Hong Kong hang Typhoon No.8 or above, such as Typhoon Ellen at 1983 and Typhoon Sam at 1999.

Secondly, tropical cyclones moving in this way can also form in the southeast, south or even southwest of Hong Kong, and then blow to the west of Guangdong or push towards Hainan Island in a west-northwest or north-northwest way, such as the strong tropical storm Frankie of 1996. However, a tropical cyclone moving in this way is usually not as powerful as a tropical cyclone entering the South China Sea from the western Pacific Ocean, because its path is short and it will land, weaken and dissipate before becoming a typhoon.

Ii) east-west type

A tropical cyclone moving along this path enters the South China Sea from east to west, passes through northern Luzon or southern Taiwan Province Province from the western Pacific, and then skips south of Hong Kong, affecting Hong Kong. Tropical cyclones moving along this path can form in the South China Sea south of Hong Kong, and move to Hainan Island and northern Vietnam in the west or west-northwest direction, such as 1990 typhoon Becky, 1999 typhoon Maggie and 1995 strong tropical storm Luis.

3) North-South type

Tropical cyclones moving along this path usually form in the middle or south of the South China Sea, and then go straight to the coast of Guangdong in a stable northward way, affecting Hong Kong. For example, tropical storm Phil in 1992 and typhoon Victor in 1997.

Ⅳ) Southwest-Northeast type

Tropical cyclones affecting Hong Kong along this path are mainly formed in the South China Sea. After it is generated, it will move to the northeast, towards the eastern coast of Guangdong, or blow to Taiwan Province Province or the Pacific Ocean.

Examples of tropical cyclones moving along this path are typhoon Rhine in 1995 and tropical storm in July of 1999+00W.

V) first to the west or northwest, then to the north or northeast (turning type)

Tropical cyclones moving along this path will first enter the South China Sea from the western Pacific in a west or northwest way, and then move to the coastal areas of Guangdong in a north-northwest or north-northwest way in the southeast, south or southwest direction of Hong Kong, such as the severe tropical storm Helen of 1995, typhoon Bos of 1998 and typhoon Danny of 1999.

Vi) Irregular types

The paths of these tropical cyclones cannot be classified into the above five categories because they move slowly and have irregular paths; Or it is influenced by other weather systems, making its path strange and changeable [for example, when it interacts with other tropical cyclones (Fujiwara effect)]. Examples of irregular patterns are Typhoon Nader of 199 1 and Tropical Storm Mark of 1992.

Fig. 9 Tropical cyclone tracks affecting Hong Kong

12. Someone once said: "When a tropical cyclone moves near Dongsha Island, if it still moves to the west-northwest direction, then the tropical cyclone will continue to keep close to the moving direction of Hong Kong; And if it moves northward from the west, west-northwest or northwest when it arrives at Dongsha Island, it will continue to move northward and will not return to the west-northwest direction. " Is it true that this tropical cyclone will never change its direction after moving to Dongsha Island?

When you look up the records of tropical cyclones in the South China Sea in the past decade, you will find that this statement does have its credibility. Take a look at the examples in recent years: 1999 typhoon Sam, 1996 typhoon Sally, and 1995 typhoon Kent all entered the South China Sea by northwest-west path first, and then continued to approach the Guangdong coast by northwest-west path after reaching Dongsha Island, and then there was no big turn; As for 1999, York, which hoisted typhoon signal 10 in Hong Kong, began to move steadily to the northwest after circling once, affecting Hong Kong.

Now, let's take a look at the paths of Typhoon Boise, Typhoon Danny and Severe Tropical Storm Helen. At first, they all entered the South China Sea from the west or northwest, but in the waters near Dongsha Island, they changed to the northerly route, and there was no big change in direction after that.

If you still have some doubts, you might as well look at the following example.