What are the common winds?

Cyclone is a kind of rotating air vortex, which flies into the air with dust carried by the ground. This vortex is what we usually see as a whirlwind. It is a natural phenomenon caused by air flow, but why does the wind turn around?

whirlwind

We know that when the air flows around the uneven places such as trees, hills and buildings on the ground, or when the air rubs against the ground, it will quickly change direction, so it will produce a vortex that moves with the airflow, thus blowing a whirlwind. However, this kind of cyclone is very rare and very small.

The main reason for the formation of cyclones is that when a place is very hot by the sun, the air here will expand and some air will be squeezed up. After reaching high altitude, the temperature will gradually decrease, start to flow around and finally sink to the ground. At this time, the air in the heated area decreases, and the air pressure also decreases, while the surrounding temperature is low and the air density is high. In addition, the heated air falls from the air, so the air increases and the air pressure increases significantly. In this way, air will flow from the places with high air pressure around to the places with low air pressure in the center, just like water flowing downwards. However, because the air flows on the earth, and the earth is constantly rotating from west to east, the air will be influenced by the rotation of the earth in the process of flowing, and gradually shift to the right (the original north wind is deflected into northeast wind, the south wind into southwest wind, the west wind into northwest wind, and the east wind into southeast wind). So the cold air blowing from all directions rotates around the heated low pressure area, becoming an air vortex opposite to the clockwise rotation direction of the clock, forming a whirlwind.

In the center of this cyclone, due to the rising warm air and the continuous rotation of the surrounding air, it is easy to roll the dust, leaves and scraps of paper on the ground into the air and fly with the airflow. If the cyclone is very strong, sometimes some small animals on the ground, such as small snakes and bugs, will be swept into the air and floated away with the wind in the dust.

Generally, the height of a small cyclone is not too large. When it is rubbed by the ground or blocked by houses and trees, it gradually dissipates and becomes an ordinary wind.

Some people may also ask: since the ground is easy to produce cyclones when heated, summer is hotter than spring. Why are there fewer cyclones in summer and more cyclones in spring? This is because although the weather is very hot in summer, the ground vegetation is green, the land is wet, and the temperature difference is not big, so there are few cyclones in summer. But in spring, the leaves haven't grown, the grass has just sprouted, the farmland is bare, there is no shelter everywhere, and it is easy to get hot, which makes the air temperature on the ground change greatly and it is easy to blow a whirlwind.

Cyclones can fly into the air with dust and messy papers, and of course, they can also bring heat and water vapor from the ground to the air, so they cause vertical mixing of heat and water vapor in the air and make the vertical distribution of heat and water vapor even. The cyclone near the surface is very small, and the vertical exchange effect is not great, so a special microclimate is formed in the gas layer near the surface.

foehn

When the airflow passes through the ridge, dry and hot wind is easy to appear on the leeward side, which is called foehn. This kind of wind does not appear as often as mountain wind, but occurs when the air pressure on both sides of the mountain is different.

Schematic diagram of foehn

When one side of the mountain is high pressure and the other side is low pressure, air will move from the high pressure area to the low pressure area. On the way of air movement, it is blocked by mountains and forced to rise, the air pressure decreases, the air expands and the temperature decreases. Every time the air rises 100 meter, the temperature drops by 0.6℃. When the air rises to a certain height, it condenses with cold water vapor, forming rain and snow. When the air reaches the ridge, it becomes thin and dry, then it crosses the ridge and descends along the slope. In the process of falling, the air becomes dense again and the temperature rises. Every time the air drops 100 m, the temperature will rise 1℃. Therefore, when the air sinks to the foothills along the mountains, the temperature often rises sharply. Even if the air height on both sides of windward side and leeward side is the same, the air temperature on leeward side is always higher than that on windward side. Whenever a dry and hot burning wind blows on the leeward hillside, it often rains or snows on the windward hillside.

Burning wind has many hazards. It often dries up fruit trees and crops, reduces production, spreads fires in forests and villages and causes losses. /kloc-in the 0/9th century, there were several famous big fires on the northern slope of the Alps, all during the prevailing period of foehn. Foehn can melt a lot of snow in mountain areas and cause floods in the upper reaches of the valley; Sometimes it can lead to avalanches. If the terrain is suitable, strong wind burning can cause local wind disasters, blow away the roofs of rural houses in mountainous areas, blow down crops, uproot trees, endanger forests, and even cause accidents to ships in rivers, lakes and seas.

Burning wind has disadvantages, but it is also beneficial. Because it can accelerate the melting of snow in winter, cattle and sheep can eat grass outdoors without waiting until next spring. Foehn also enriches local heat resources, such as Swiss corn and grapes in the upper reaches of Rhone Valley, which are all matured by the heat of Foehn. These crops are difficult to mature in the neighboring areas that are not affected by the foehn.

land and sea breeze

In coastal areas, as long as the weather is good, the wind always blows from the sea to the land during the day; At night, the wind blows from land to sea. The wind blowing from the sea to the land is called sea breeze; The wind blowing from land to sea is called landwind. In meteorology, they are usually called land-sea breeze.

Schematic diagram of land-sea breeze formation

Schematic diagram of the origin of land-sea breeze

Sea-land wind, like monsoon, is a periodic wind formed by the influence of land-sea distribution. However, the land-sea breeze is a cycle of day and night, while the wind direction of the monsoon changes with the seasons, and the scope of the land-sea breeze is smaller than that of the monsoon. So, how is the land-sea wind formed?

During the day, the air on land heats up quickly, while the temperature on the sea surface changes little. In this way, where the temperature is low, the air is cold and sinking, and the air pressure near the sea surface is high; Where the temperature is high, the air is light and floating, and the air pressure on land is low. When the air on land rises to a certain height, the air pressure above it is higher than that at sea. Because the air pressure in the lower sea surface is higher than that in the land, and the air pressure in the upper land is higher than that in the ocean, the air always flows from the area with high air pressure to the area with low air pressure, so there is a small vertical circulation in the land-sea junction area. The air on land rises and flows from the sky to the ocean after reaching a certain height; Over the ocean, the air sinks, reaches the surface and turns to land. This kind of wind that flows from the sea surface to the lower land and is almost perpendicular to the coast is the sea breeze.

At night, the situation is just the opposite. On land, the air cools rapidly and the air pressure rises; The sea surface cools slowly (at the same time, the warm sea water in the depth and the cold sea water on the surface can communicate and mix), so it is still much warmer than the land surface. At this time, the sea surface is a relatively low pressure area. But after reaching a certain height, the sea surface pressure is higher than that of land. So the air in the lower layer flows from land to sea, and the air in the upper layer flows from sea to land. In this case, the flow direction of the whole vertical circulation also becomes completely opposite to the vertical circulation in the sea breeze ahead. In the lower layer of this complete vertical circulation, the airflow from land to sea, which is roughly perpendicular to the coast, is the land wind.

Generally speaking, sea breeze is stronger than land breeze. Due to the large temperature difference between land and sea during the day, the onshore gas layer is unstable, which is beneficial to the development of sea breeze. At night, the temperature difference between land and sea is small, the affected gas layer is thin, and the land wind is weak. The speed of sea breeze can reach 5 ~ 6m/s, while the land breeze is generally only1~ 2 m/s. The temperature difference in coastal areas is large, and the intensity of land-sea breeze is also large. With the distance from the coast, the land and sea winds gradually weaken.

The area with the strongest land-sea wind development is the area with the largest daily temperature difference and the largest land-sea temperature difference between day and night. Therefore, in tropical areas where the daily temperature changes greatly, winds on land and at sea can be seen all year round; The land-sea breeze is weak in the mid-latitude area, and most of it only appears in summer; At high latitudes, only on cloudless days in Wan Li in summer can we occasionally see extremely weak land and sea winds. In China's coastal areas such as Taiwan Province Province and Qingdao, the sea-land breeze is obvious, especially in the summer half year, the temperature difference between land and sea and the daily variation of temperature increase, so the sea-land breeze is strong and appears frequently. However, the sea and land breeze in winter is not as prominent as that in summer, and there are fewer opportunities.

Sea-land wind is a local wind system near the coast, which is caused by the significant difference in heat capacity between land and sea water.

During the day, the surface of land is heated by solar radiation much faster than that of the ocean, so the temperature on land is obviously higher than that on the ocean. The air on land is heated and the air pressure drops. At this time, the temperature in the ocean is relatively low and the air pressure is relatively high. The direction of the pressure gradient force is from the ocean to the land, thus forming a sea breeze blowing from the ocean to the land.

After sunset, the radiation cooling of the land surface is much faster than that of the ocean. At night, the temperature on land is lower than that on the ocean, and the air pressure on land is higher than that on the ocean. At this time, the direction of pressure gradient force is opposite to that of daytime, pointing from land to ocean, thus forming a land wind blowing from land to ocean.

In the lake-land boundary areas of larger lakes (such as Dongting Lake and Poyang Lake), a local wind system similar to land-sea breeze-lake-land breeze will also be produced.

This land-sea wind or lake-land wind is the result of sun exposure. So on cloudy days, there is no obvious land-sea wind because there is no obvious land-sea thermal difference.

Because the sea-land breeze is formed by local thermal differences, its influence is weak, the horizontal range is not more than tens of kilometers, and the vertical height is also within 1 ~ 2 kilometers. Under the influence of large-scale weather system, this local wind system is submerged in a more powerful large-scale wind system.

Dry and hot wind

In early summer, a kind of high temperature and low humidity wind often appears in some parts of our country, which is dry and hot wind, also called "hot wind", "fire wind" and "dry wind". It is a specific weather phenomenon with a short duration (usually about 3 days).

Due to the different natural characteristics of different places, the causes of dry and hot wind are also different. In the early summer of each year, the climate in China's inland areas is hot, the rainfall is scarce, the temperature rises strongly, and the air pressure drops rapidly, forming a strong continental thermal depression. Around this hot low pressure, the pressure gradient increases with the increase of air mass temperature, so the dry-hot airflow rotates around the hot low pressure to form dry-hot wind, which is dry-hot wind. Strong dry and hot wind will do harm to local wheat, cotton and melons.

Mongolia with dry climate, west of Hetao in China, Xinjiang and Gansu are the areas that often produce continental thermal depression. After the heat and low pressure leave the source, it will become drier and drier along the way through the dry and hot Gobi desert, and the dry and hot wind will become stronger. The climate of Tarim Basin, located in the center of Eurasia, is extremely dry, and the "burning wind" caused by the strong cold front crossing Tianshan and Pamirs often causes a wide range of dry and hot winds in the local area.

In the Huanghuai Plain, the main reason for the formation of dry-hot wind is based on the atmospheric drought in this area. The late spring and early summer is the season with the largest direct angle of the sun in the northern hemisphere, and it is also a sunny and rainy period before the rainy season in northern China. Under the control of dry air mass, it is sunny, dry and windy here, and the ground temperature rises rapidly (the average maximum temperature can reach 25~30℃), so there is less chance of rain caused by condensed clouds, and it is easy to form dry hot wind. This dry-hot wind is unfavorable to the later growth and development of wheat in this area.

In the Jianghuai Basin, the dry-hot wind is generated under the influence of the southwest airflow west of the Pacific subtropical high. The Pacific subtropical high is a deep warm and high pressure system, which consists of warm air from the ground to the sky. During spring and summer, this high pressure stayed over the Jianghuai Basin and then gradually moved northward. Because in the high pressure area, the wind direction is clockwise, so in the west of the subtropical high, the southwest wind is blowing. Located in the north and west of subtropical high, affected by this southwest wind, it produces dry and hot wind weather. In early summer, there is still a cold high pressure in the north, which continues to go south, weakening its power and degenerating; When it merged with the subtropical high, its power was strengthened, so that fine weather continued to be maintained and dry and hot winds became more obvious.

In the plain area of the middle and lower reaches of the Yangtze River, the weather is sunny and dry after the rainy season, and the dry and hot wind in the south is often accompanied by "summer drought", which is not conducive to the heading and flowering of double-cropping early rice (or middle rice).

The influence of dry and hot wind

The harm of dry-hot wind to crops is mainly due to high temperature, drought and strong wind forcing the evaporation of air and soil to increase, and the water in crops is rapidly consumed, thus destroying chlorophyll and other pigments, hindering the photosynthesis and synthesis of crops, and making plants wither rapidly from bottom to top. Especially dry-hot wind, often harms crops together with drought. The roots of crops can't absorb the proper water, but the dry and hot wind takes away a lot of water from the stems and leaves, which speeds up the withering and death of crops.

Dry-hot wind mostly occurs in early summer, which is the filling period of wheat in northern China. When it comes to dry and hot wind, the ears of wheat will not bulge, "dry up" ahead of time, the grains will shrink, the grain weight will drop, and the yield will be seriously reduced.

The harm degree of dry-hot wind is also related to the weather conditions a few days before the occurrence of dry-hot wind. If it suddenly clears up after the rain, it will be followed by hot and dry weather with high temperature and low humidity, which is even more harmful. If there is a little precipitation before the occurrence of dry-hot wind, it will help to reduce the harm of dry-hot wind. Judging from the sowing date, late wheat is susceptible. Therefore, the agricultural proverb says, "Early grain and late wheat are bad for ten years." Judging from the farming season, Xiaoman and Mangzhong are the same, and there is a saying in the agricultural proverb that "Xiaoman is dissatisfied and wheat is risky". In other words, the wheat is full and not filled with milk, so it is vulnerable to dry and hot winds.

monsoon

Monsoon is a kind of wind system with wide prevalence and obvious seasonal variation of wind direction. With the seasonal change of wind direction, the weather and climate also have obvious seasonal changes. The word "monsoon" comes from Arabic "mawsim", which means season. In ancient China, it was called trade wind, which means that the direction of this wind always changes with the seasons, and it is the messenger of the seasons. Now monsoon is often called trade wind, such as southeast trade wind, northwest trade wind and so on.

Monsoon is caused by land and sea distribution, atmospheric circulation, continental topography and other factors. It is a large-scale convection phenomenon with a period of 1 year. Asia is the most famous monsoon region in the world, and its monsoon characteristics are mainly manifested in the existence of two main monsoon circulations, namely, the northeast monsoon prevailing in winter and the southwest monsoon prevailing in summer, and their transformation has an explosive mutation process with a short transition period. Generally speaking, 1 1 is the winter monsoon period, June-September is the summer monsoon period, and April-May and 10 are the transition periods between summer monsoon and winter monsoon. However, the seasonal differences in different regions are different, so the division of monsoon is not completely consistent.

Monsoon is a widely prevailing wind system, and the wind direction changes significantly with the seasons. Like the wind belt, it belongs to the planetary circulation system, and its formation is caused by the temperature difference between the ocean and the land in winter and summer. Summer monsoon blows from the ocean to the mainland, and winter monsoon blows from the mainland to the ocean.

Monsoon has a wide range of activities, affecting the area of 1/4 on the earth and the life of 1/2 people. The western Pacific, South Asia, East Asia, Africa and northern Australia are all areas with obvious monsoon activities, especially the Indian monsoon and the East Asian monsoon. There is also a small monsoon region along the Pacific coast of Central America, while there is no obvious monsoon region in Europe and North America, only some monsoon trends and phenomena appear.

In winter, the temperature of the mainland is lower than that of the adjacent ocean. There is a cold high pressure on the mainland and a corresponding low pressure on the ocean, and the airflow blows from the mainland to the ocean in a large range, forming a winter wind. Winter winds prevail in the northern hemisphere, especially on the east coast of Asia. The northbound monsoon extends from the mid-latitude to the equatorial region. This monsoon originated from Siberian cold high, and in the process of its southward eruption, it produced strong northerly winds and northeast winds in East and South Asia. In Africa and the Bay of Bengal, there is also an obvious northeast wind blowing near the equator. Although there are winter winds in the East Pacific and South America, they are not as obvious as those in Asia.

In summer, the ocean temperature is relatively low, the mainland temperature is high, and the ocean appears high pressure or the original high pressure is strengthened, and the mainland appears thermal low pressure; At this time, southwest and southeast monsoons prevail in the northern hemisphere, especially in the Indian Ocean and South Asia. Most of the southwest monsoon originated in the South Indian Ocean, crossed the equator on the east coast of Africa, reached South Asia and East Asia, and even reached central China and Japan. The other part of the southeast wind mainly comes from the northwest Pacific, and the southerly or southeast wind affects the eastern coast of China.

Schematic diagram of monsoon formation

Summer monsoon generally goes through four stages: outbreak, activity, interruption and regression. The East Asian monsoon broke out the earliest, starting in early May, advancing from southeast to northwest, and stabilizing in late July. Usually retreat begins in mid-September, and the path is opposite to that when advancing. Under the counterattack of the northerly airflow, it gradually retreated from northwest to southeast.

The main reason for the formation of monsoon is the seasonal variation of thermal circulation between land and sea. In summer, the heating of the mainland is stronger than that of the ocean, and the pressure changes slowly with the height above the ocean. So at a certain height, there is a horizontal pressure gradient from the mainland to the ocean, the air from the mainland to the ocean forms a high pressure, the mainland forms a low pressure, and the air flows from the ocean to the mainland, forming an airflow opposite to the high altitude, which constitutes the summer monsoon circulation. In China, there are southeast monsoon and southwest monsoon. Summer monsoon is especially warm and humid.

But the degree of land and sea influence is related to latitude and season. In winter, the middle and high latitudes have a great influence on land and ocean. The cold high pressure center on land is at higher latitudes, while the low pressure is on the ocean. In summer, the influence of low latitude land and sea is great, the center of land thermal depression is southward, and the position of ocean subtropical high moves northward.

Of course, the seasonal movement of the planetary wind belt can also strengthen or weaken the monsoon, but it is not the basic factor. Whether the monsoon phenomenon is obvious or not is related to the size, shape and latitude of the mainland. The continental area is large, and the seasonal high and low pressure and pressure gradient caused by the thermal difference between land and sea are also large, and the monsoon is more obvious. North America is much smaller than Eurasia, with cold and high pressure in winter and low heat pressure in summer, so the monsoon is not obvious. The shape of the continent is a horizontal rectangle, and it is difficult for the warm air from western Europe to reach the eastern part of the continent, so the monsoon in the eastern part of the continent is obvious. The North American continent is a vertical rectangle, and the airflow entering the continent from the west coast can reach the east, so there is no obvious monsoon in the east of the continent. Whether from the thermal difference between land and sea or the monsoon movement of planetary wind belt, the low latitude of the mainland is conducive to the formation of monsoon. The latitude of Eurasia reaches a lower latitude, while North America is mainly distributed in the north of latitude 30, so the monsoon in Eurasia is more obvious than that in North America.

typhoon

Typhoon is an atmospheric vortex generated on the surface of tropical ocean, and it is a deep tropical weather system. It can extend to a height of more than 20 kilometers, and its horizontal range ranges from hundreds of kilometers to Qian Qian meters.

typhoon

A mature typhoon generally consists of spiral cloud belt, cloud wall area and typhoon eye. The outermost layer of spiral cloud belt is stratocumulus, followed by cumulonimbus and cumulonimbus. Spiral cloud belt is the transporter of water vapor and heat in typhoon system. The cloud wall area is a towering cumulonimbus cloud around the typhoon center, with a width of about 8 ~ 20 kilometers. Typhoon and rainstorm all appear in the cloud wall area, and the maximum wind speed appears in the place where the pressure gradient is the largest outside the cloud wall area. The biggest rainstorm occurred in the area where the cumulonimbus cloud developed most vigorously in the cloud wall area. The eye of typhoon is the center of typhoon. There is a downward airflow here, and there is not a cloud in the sky. You can see sunshine during the day and starlight at night, just like the eyes of a typhoon and a deep well on the cloud wall.

The cause of typhoon

A typhoon is like a machine. It must have enough energy to make it spin. The high temperature and high humidity conditions on the tropical ocean surface can provide the heat energy needed for the generation and development of typhoons. Experts at home and abroad have concluded that the sea surface temperature is higher than 26 ~ 27℃, which is the lowest condition for typhoon formation. The higher the sea surface temperature, the more favorable it is for the formation and development of typhoons.

Typhoon is a kind of constantly rotating atmospheric vortex. When a typhoon is formed at low altitude, there must be a convergence of cyclonic airflow, so that the convective movement develops and a large number of cumulonimbus clouds are formed. In the sea area near the equator, there is no typhoon near the equator because the geostrophic deflection is very small, which is not conducive to the formation of cyclone airflow. Only on the tropical ocean surface with latitude of 5 ~ 8 is it beneficial to the formation of typhoon.

In order to make the typhoon as a heat engine rotate, the upper layer needs an outward divergent airflow, just like installing an air extractor over the typhoon system to continuously suck the warm and humid air in the lower layer, thus strengthening the convective upward movement of the air, condensing more water vapor to release latent heat, providing more heat, and making the wind develop rapidly.

In order to keep the typhoon running continuously, it is necessary to keep the heat near the typhoon center from being blown away by the wind. This requires that the speed difference between the upper and lower parts of the typhoon system should be small, that is, the vertical shear of the wind should be small, so as to maintain the continuous supply of heat and promote the development of the typhoon.

We know that the equator is a low-pressure area, where the solar radiation is strong, the air is heated, and the southeast trade winds and northeast trade winds in the northern and southern hemispheres blow to the equator. However, the sun does not always shine directly at the equator, but only at the vernal equinox and autumnal equinox. After the vernal equinox, the position where the sun shines directly on the earth slowly moves north and crosses the equator. In the position of direct sunlight, the air is the hottest, the rising is the strongest, and the air pressure near the ground is the smallest, which is equivalent to the equatorial low pressure belt moving northward (along the position of direct sunlight on the earth). The southeast trade winds in the southern hemisphere also moved to the northern hemisphere. Due to the eccentric force generated by the earth's rotation, the southeast trade wind becomes a southwest wind in the northern hemisphere, which blows against the northeast trade wind in the northern hemisphere, thus forming an air vortex rotating counterclockwise. In eastern Asia, this kind of vortex is mostly produced on the ocean surface from the east of the Philippines to Guam with a latitude of 5 ~ 20, where the seawater temperature is high and the water vapor is sufficient, which has the conditions for vortex formation. In the initial formation stage, the diameter of the vortex is only about 100 km, and the vortex turns bigger and bigger. When it moves to 30 north latitude, its diameter can reach 600 ~ 1000 km, and the central wind is getting stronger and stronger, which may turn into a tropical storm or typhoon.

Various names of typhoons

There are typhoons all over the world in the tropical ocean, but the names of typhoons are different everywhere. Typhoons that occur in the western North Atlantic and the South China Sea are customarily called typhoons. Hurricanes occurred in the eastern North Pacific and the Atlantic Ocean. The Indian Ocean is called Indian Ocean Storm, and the Bay of Bengal and the Arabian Sea are called Bay of Bengal Storm and Arabian Sea Storm respectively.

Different stages of typhoon development have different names due to different intensities (according to the maximum wind force). They are collectively called tropical cyclones. When the maximum wind force near the center of tropical cyclone is 7 or below, it is called tropical depression; When the maximum wind force near the center reaches 8 ~ 9, it is called a tropical storm; When the maximum wind force near the center reaches 10 ~ 1 1, it is called a strong tropical storm; When the maximum wind force near the center reaches above 12 line, it is called typhoon.

China Central Meteorological Observatory and meteorological stations at all levels issued typhoon news and warnings in accordance with the above unified standards.

The moving path of typhoon

Typhoon, a cyclone vortex, is like a gyro played by children, spinning forward. The path of the typhoon refers to the moving direction of the typhoon as a whole. There are four factors that affect the typhoon moving path: ① The internal force of the typhoon rotating system always makes the typhoon move northward and westward; (2) The function of large-scale diversion flow, when the typhoon is located in the south of the subtropical high, it moves westward under the action of diversion flow in the east south of the subtropical high, and when it is located in the west of the subtropical high, it moves northward under the guidance of airflow in the south west of the subtropical high; (3) The interaction between typhoon and surrounding weather system. For example, when the typhoon approaches the westerly trough, it will be absorbed by the trough and then move northeast under the guidance of the southwest airflow in front of the trough; (4) Due to the influence of ocean surface temperature, typhoons tend to move to warmer ocean surfaces.

Although the above four effects affect the movement of typhoon, in fact, these four factors are constantly changing, and their interaction is more complicated and unpredictable. This is the reason why the forecast often fails when the typhoon path is complex and changeable.

hurricane

The word hurricane comes from the Caribbean devil hurrican, and some people say that Hurakan, the god of thunderstorms and cyclones, is one of the creation gods in Mayan mythology. The word typhoon comes from Tifeng, the son of Gaia, the mother of the earth, in Greek mythology. It is a monster with a hundred faucets. It is said that its children are terrible winds.

hurricane

Strong and deep tropical cyclones (maximum wind speed 32.7m/s, wind force 12 or more) are called hurricanes in the Atlantic Ocean and the eastern North Pacific Ocean.

It also refers to any strong wind accompanied by strong wind and any tropical cyclone, and any strong wind with a wind force of 12.

Both hurricanes and typhoons refer to tropical cyclones with wind speeds exceeding 33 m/s, but they have different names because of different regions. Strong tropical cyclones appearing in the northwest Pacific Ocean and the South China Sea are called "typhoons"; Hurricanes occur in the Atlantic Ocean, the Caribbean Sea, the Indian Ocean and the eastern North Pacific Ocean. Hurricanes can release amazing energy in one day. Hurricanes and tornadoes cannot be confused. The latter time is very short, and it is instantaneous, and the longest is no more than a few hours. In addition, tornadoes are usually accompanied by hurricanes. The biggest feature of tornado is that when it appears, it often has one or several funnel-shaped cloud columns similar to "trunk", accompanied by storm, lightning or hail. When a tornado passes through the water, it can absorb water and rise to form a water column, and then connect with the cloud, commonly known as "dragon with water". When passing through land, houses are often rolled down, and even people are sucked into the air.

Classification of hurricanes

1 level, with maximum sustained wind speed of 33 ~ 42m/s and 74 ~ 95m/h; 64~82 knots, 1 19~ 153 km/h; The storm surge is 4~5 feet (1 foot =0.3048 m), 1.2~ 1.5 m, and the lowest pressure in the center is 28.94 inches (1 inch =2.54 cm) of mercury, 980 mbar. The potential damage has not actually hurt the building, but it has not been fixed. Some coasts will be flooded and small docks will be damaged.

Typical hurricanes: Agnes-Danny-gaston-O 'filia.

Grade 2, with the highest sustained wind speed of 43~49 m/s and 96~ 1 10 m/h; 83~95 knots, 154~ 177 km/h; The storm surge is 6~8 feet, 1.8 ~ 2.4 meters, and the central minimum pressure is 28.50 ~ 28.965, 438+0 inch Hg and 965~979 millibar, which may damage some roof materials, doors and windows, and may damage vegetation. Floods may break through unprotected berths and threaten docks and ships.

Typical hurricanes: Hurricane Bob-Hurricane Bonnie-Hurricane Francis-Hurricane Juan.

Grade 3, with the highest sustained wind speed of 50 ~ 58m/s,11~130m/h; 96~ 1 13 knots, 178~209 km/h; The storm surge is 9~ 12 feet, 2.7~3.7 meters, and the central minimum pressure is 27.9 1~28.47 inches of mercury, 945~964 millibars, which may cause potential damage to some huts and buildings, and some may even be completely destroyed. Floods near the coast destroyed buildings, large and small, and flooded the inland.

Typical Hurricane: 1938 New England Hurricane Fran Hurricane isidore Jenny.

Grade 4, with the highest sustained wind speed of 59 ~ 69m/s,131~155m/h; 1 14~ 135, 2 10~249 km/h; Storm surge 13~ 18 feet, 4.0~5.5 meters, central minimum pressure of 27. 17~27.88 inches of mercury, 920~944 mbar, potentially damaging the roof of small buildings. Most areas near the sea were flooded, and inland floods occurred on a large scale.

Typical hurricane: 1900 Galveston hurricane Charlie hurricane Hugo hurricane Iris.

Grade 5, with the highest sustained wind speed ≥ 70m/s and ≥156m/h; ≥ 136 knots and ≥ 250 km/h; Storm surge ≥ 19 feet, ≥5.5 meters, with the lowest central air pressure.

Typical hurricanes: Hurricane Andrew-Hurricane Camille-Hurricane Gilbert-1935 Labor Day Hurricane-Typhoon Taipu-Hurricane Katrina.

Cause and effect

One reason why tropical oceans produce hurricanes is that warm seawater is its power "fuel". Therefore, some scientists began to study whether the warming earth will bring more powerful and harmful tropical storms. Most meteorologists believe that the earth seems to be getting hotter and hotter. They think that carbon dioxide and so-called greenhouse gases in the atmosphere are warming the earth. Researchers warn that people must seriously think about the problem of global climate change decades or even centuries later. However, it should be pointed out that a weather and climate event, such as a strong hurricane or an active hurricane season, does not mean global warming.

Urban wind

Cities, especially big cities, are affected by buildings, population concentration, factories and other heat sources, resulting in higher urban temperatures than suburbs. People call this urban warming the urban heat island effect. Many large and medium-sized cities in China have obvious urban heat island effect. This urban heat island effect is often hidden and not prominent under the influence of large-scale weather systems such as cold front and typhoon. When there is no obvious weather system influence, this urban heat island effect appears. Because the temperature above the city is higher than that around it, it will cause air convection. The warm air above the city will rise and fall in the suburbs, while the colder air in the suburbs will flow to the city to supplement the rising air in the city. In this way, a local small circulation between the city and the suburbs-urban wind is formed.

The wind speed of urban wind is not big, generally around 1 m/s, and the ground wind direction blows from suburbs to cities. This kind of urban wind, which is not strong enough to blow to the urban area, will have an important impact on air pollution. Pollutants (smoke, impurities and harmful gases) in the city often form a dome-shaped dust cover over the city with the rise of hot air, which blocks the rising airflow, turns it into a horizontal airflow from the city to the suburbs, and causes it to sink in the suburbs, thus bringing these pollutants to the ground near the suburbs. When there are pollutants discharged by factories in the wind sinking area of suburban cities, they will flow back to the city center with the urban wind, making the air in the city more turbid.

Therefore, in urban planning, it is necessary to study the distance of air flowing to the suburbs in the city, so that some polluting factories are arranged outside the sinking distance to prevent highly polluted air from flowing back to the city. On the other hand, the establishment of satellite cities should also be outside the urban wind circulation to avoid mutual pollution between central cities and satellites.