Tropical desert climate is distributed inTropic of CancerTo the interior of the continent or the west bank of the continent between 30 ° north and south latitude, and extending inland, such asAfricaThe north, the Arabian Peninsula in Asia and the Australian desert are typical examples.staySubtropical highOr under the control of trade wind belt, the prevailing tropical continental air mass is characterized by high annual average temperature, large annual temperature difference, larger daily temperature difference, scarce precipitation, and annual precipitation of less than 125mm.The climate zone is widely distributed in the desert, with few organisms, and only sporadic drought tolerant plants, such as cactus.There will be bushes on the edge of the desert.The vegetation type is tropical desert.[1]
Chinese name
tropical desert climate
Foreign name
(tropical) desert climate
Alias
Tropical arid and semi-arid climate、Tropical desert climate
Climatic characteristics
High temperature and drought throughout the year
Distribution rule
The west bank of the mainland near the Tropic of Cancer extends inland
Terrestrial natural zone
Tropical desert
Typical area
Africa, Oceania, West Asia
Main distribution areas
North Africa, Arabian Peninsula, Western Australia
Genesis
Alternate control of subtropical high pressure zone or trade wind zone
Desert climate is an extreme case of continental climate.It is most widely distributed in the subtropical desert. The basic reason is that there is little rain. It is difficult for plants to survive. The plant species and quantity are extremely scarce.The surface is bare, the air is very dry, and there is little water.
daysolar radiationStrong, the ground is heated rapidly, and the temperature can reach 60~80 ℃.The updraft is strong, but because the air is dry, there is little cloud formation and rain, only galesand dust。On the ground at night, the cooling is extremely strong, and it can even drop below 0 ℃.thus,Daily variation of temperatureVery large.It can be up to 50 ℃ or above.
The Sahara Desert is the largest desert in the world, and the tropical desert climate is also the most typical.It is extremely dry andextremely hotIn most areas, the average annual rainfall is below 50 mm, and in some places, it even keeps raining for many years, making it the driest climate on the earth.Extremely hot is a masterpiece of the desert. The absolute maximum temperature can exceed 50 ℃, and the ground temperature is even higher.
The hottest place in the world is in the Sahara Desert, no wonderGobiSpread eggs on the tableGo!ExploreThey are good at it.China also has "three stoves", but by contrast, it is nothing.In this harsh climate, the fittest survive. Naturally, there are few creatures that can survive. Only those drought tolerant dwarf plants and thirsty animals can become the masters of the desert kingdom.
characteristic
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A barren desert
The tropical desert climate has many characteristics, such as sunny days, strong sunshine, dryness, hot summer, large temperature difference between day and night, and many sandstorms.
The key is "dry".Because dry,Clouds and rainLess, more sunshine, more sunlight;Because it is dry, there is no cloud in the sky, which can not block some sunlight, and there is no water on the ground, which can not be evaporated to cool down. All the heat from the sun to the ground is used to heat the earth and air, so the temperature in summer is particularly high, and the temperature difference between day and night is particularly large;Because it is dry, there are few plants on the ground. When the wind blows, it blows sand.
The high temperature on the desert surface makes the temperature difference between the top and bottom of the gas layer close to the ground extremely great.The temperature distribution of cool and hot air above the atmosphere can reflect the distant scenery in front of and below the tourists, which is called Xiaxian in meteorologyMirage, commonly known asmirage。Therefore, travelers in the desert often see the blue lake in front of them. In fact, it is an inverted image of the sky in the distance, which makes them happy.On the black asphalt road in summer afternoon, you can often see a bright blue water on the road ahead. When you get closer, it disappears and moves to the front again.This is also a trick of mirage on the road.
The four seasons in the desert are also very different from those in other places at the same latitude.East China, because of the monsoonSexual climateSpring and autumn festivals are short.However, in desert areas, spring and autumn are shorter.Because the desert area is too dry, there is no water regulation.The temperature rises straightly in spring and drops straightly in autumn. The spring and autumn add up to only about two and a half months to three months.
When the spring and autumn festivals are short, the winter and summer festivals are particularly long, so some people describe that "in the arid region of Central Asia, there are only two seasons a year:SiberiaWinter andSahara's summer. "This is reasonable.The temperature change in desert climate is the mostextremeOf.Specifically, the tropical desert climate zone has the following climatic characteristics:
The characteristics of tropical desert climate are summarized as follows: it is dry and hot all the year round, especially in the hot season.
precipitation
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Desert Yin and Yang
North Africasahara desertIn Chinese, there was a record of no rain for many years;And inSouth AmericaIn Arica, a desert in northern Chile, there have been only three measurable showers in 17 consecutive years, and the total amount of the three showers is only 5.1 mm, with very little precipitation.
Also located inNorthern Desert of ChileOfIquiguiThere was no rain for four consecutive years, but a shower in the fifth year dropped by 150 mm, and a shower in the other year recorded 635 mm, which shows the great variability.Rainfall in tropical deserts is mostly explosive showers, which often cause severe soil erosion.
temperature
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Due to less clouds, strong sunshine, lack of vegetation coverage, and low air humidity, the daytime temperature rises extremely fast.There was a record of up to 58 ℃ in North Africa. Generally, the average monthly temperature in summer is between 30 ℃ and 35 ℃, and the high temperature lasts for a long time, such asthe arabian peninsulaThe average monthly temperature of Yading is above 30 ℃ for five months in a year.
The desert is cooler at night because there is no cloud all night long,Ground radiationStrong, fast heat dissipation, the minimum temperature at night is generally between 7 ℃ and 12 ℃, and there are days of thin frost.The annual temperature difference is generally about 10 ℃~20 ℃, while the daily temperature difference is larger, between 15 ℃~30 ℃.
In North Africatripoli On December 25, 1978, a meteorological station to the south recorded that the hottest temperature reached 37.2 ℃ in the daytime and dropped to - 0.6 ℃ in the evening. The daily temperature difference reached 37.8 ℃, which can really be described as "wearing fur coats in the morning and yarn in the afternoon".
The tropical desert climate is often cloudless, windy, sunny, high temperature and low relative humidity, so the evaporation power is very strong.The possible evaporation is more than 20 times of the precipitation, or even 100 times.The relative humidity in the air is very small, and about 2% of the relative humidity often occurs in the Sahara Desert of Egypt.
Main distribution
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A tropical desert with clouds and no rain
Tropical desert climate is mainly distributed inTropic of CancerThe adjacent interior of the continent or the west bank of the continent are mainly located in northern Africa, western Asia, central and western Australia, and the narrow region on the west side of South AmericaRegression lineNear the interior of the mainland and the west bank.In northern Africa, the Arabian Peninsula in Asia andAustralian DesertThe area is typical.
It is characterized by high annual average temperature, large annual temperature difference, larger daily temperature difference, and scarce precipitation. The annual precipitation is generally below 250 mm, and in many areas, it is only tens of millimeters, or even several millimeters. The precipitation variability is very large, and it often does not rain for several consecutive years.
staySubtropical high beltOr under the control of the trade wind belt, it is subject to the tropics all year roundContinental air massControl, drought and little rain, annual precipitation of about 100 mm, and in some places only tens of millimeters or less,potential evapotranspiration It is more than 20 times larger than the precipitation.The annual average temperature is higher than 18 ℃, and the summer is very hot. The average temperature in the hottest month is mostly 30 ℃ - 35 ℃, the absolute maximum temperature in some places is as high as 58 ℃, the maximum ground temperature can reach more than 80 ℃, and the average temperature in the coldest month is not less than 10 ℃.The daily temperature range can reach 35 ℃ - 40 ℃, and the minimum temperature at night can drop to about 0 ℃.With less clouds and strong sunshine, it has abundant solar energy resources.
The contradiction between heat and moisture is prominent.The distribution and formation of the world's great deserts are closely related to the tropical dry climate.natural vegetation It's a desert.
Africa's Sahara Desert, Kalahari DesertSouthwest AsiaThe Arabian DesertThar Desert, deserts in western and central AustraliaColoradoThe desert in the lower reaches of the river, California and Atacama Desert in South America have typical tropical desert climate.Tropical desert climate zoneSavanna climateThe north and south sides of the district.
These places have high air pressure and stable weather. The wind always blows from the land to the sea, but the humid air from the sea cannot enter the land. Therefore, the rainfall is very little and very dry. The rocks on the ground are weathered to form fine sand grains. The sand grains float with the wind and roll up to form sand dunes. The sand dunes are widely distributed and become vast deserts.Some placesRock weatheringThe speed is relatively slow, forming large pieces of gravel, which isDesert。The area of deserts on the earth is very large and tends to expand further.
Main vegetation
Tropical desert region
The climate in tropical desert areas is dry, hot and windy,Soil salt contentHigh.Plants can survive and grow only if they have strange ability to adapt to desert natural environment.Therefore, compared with plants in general areas, plants in desert are very different in appearance, internal structure and physiological function.
The main features are:
(1) Most perennialPsammophyteIt has strong root system to increase the absorption of water in sandy soil.Generally, the root depth and width are many times larger than the plant height and width（lateral root）It can extend far in all directions without layering, but it can spread evenly to avoid consuming too much water in the sand layer in one place.
Such as shrubYellow willowThe plant height is generally only about 2 meters, and its main root can be drilled tosandIt is 3 and a half meters deep, and the horizontal root can extend beyond 20~30 meters. Even if a layer of horizontal root is exposed by wind erosion, the whole plant will not die.The lateral roots of yellow willow planted for only one year can reach 11 meters.
However, some annual plants have very shallow roots. Occasionally, there is a little rain in spring, even if it is very little. As long as the surface is wet, they also make full use of it, grow, blossom and bear fruit vigorously, and complete their life cycle in a relatively short time, so as to escape the drought and high temperature summer.People call them“Ephemeral plant”。
(2) In order to reduce water consumption and transpiration area, the leaves of many plants shrink very small, or become stick shaped or spiny, or even leafless, and use twigs for photosynthesis.Haloxylon ammodendronThat is, leafless, from green branchesassimilationIs called“Leafless tree”。Some plants not only have small leaves, but also have small flowers, such as Tamarix（Red willow）That's it.Some plantsTranspiration, leafyEpidermal cellWall strength lignification,cuticleThickened, or the leaf surface hasWaxy layerAnd a large number of hairs are covered, and the stomata of leaf tissue are trapped and partially occluded.
(3) The surface of the branches and trunks of many psammophytes becomes white or grayish white. In order to resist the strong sunlight in summer and avoid being burned by the high temperature of the sand surface, such as Calligonum mongolicum.
(4) Many plants have strong sprouting ability, strong lateral branches, and can withstand wind and sand attack and sand burial.Tamarix (red willow) is like this. It can still live when buried in sandAdventitious rootThe sprouts grow more vigorously.In desert and Gobi areas, wind and sand activities are intense. Tamarix chinensis growing in low wetlands is often attacked by quicksand, which makes the shrubs accumulate sand continuously.After being buried in the sand, Tamarix can continue to grow due to the role of adventitious roots, so "the water rises and the boat rises", forming a tall shrub sand pile (sandbag).
(5) Many plants are succulent plants with high concentration of salt, which can absorb water from the soil with high salinity to maintain life, such asSuaeda salsa、Salt clawEtc.
The method of plant propagation in the desert is also very strange.Many annual or perennial plants have wings or hairs on their seeds,Seed maturityAfter that, they will fly with the wind and grow in the right place.
For example, the seeds of Tamarix ramosissima are small and have white crown hairs (tufts). They fall in the wind and are naturally sown. The germination rate of the seeds can reach more than 80%. Once the seeds fall on the low wetland, they can germinate in 2-3 days and grow rapidly.There are also plants likeFlower stickThe same pods have nodes, which fall intermittently when they are mature. Each node is bulging in a ball shape and light. When the wind blows, it rolls on the surface of the sand without being buried by the sand. When the conditions are suitable, it germinates and grows rapidly.AnotherArtemisia ordosicaThe seeds of“Artemisia oleifera gum”It becomes sticky and rolls on the sand dunes with the wind. When a lot of sand is stuck on the whole body, it germinates.
wild animal
There are few wild animals in tropical desert. In Africa, there are camels, big eared foxes, sahara geckos, desert vipers, horny rattlesnake plovers, desert night cats, desert hares, desert hedgehogs, Maolitari toads, and in Asia, there are wild horseslepus yarkandensis 、Mongolian gazelle、wild boar、Eryx、Prairie Viper, AmericaRattlesnake, American Poisonlizard、Sensitive fox, desert tortoisePrairie wolf, butterflies.
Causes
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atmospheric environment
The ship of the desert
First, acceptSubtropical high beltcontrol。Subtropical zoneHigh pressure is a power high pressure, which is due toequatorThe air over the subtropical zone flows continuously and accumulatesDowndraft, resulting in the increase of low air pressure, which belongs to warm high pressure.The main reason for the sunny and dry weather in the subtropical region is the prevailing downdraft.
Second, driedtrade windControl of.The trade wind is fromSubtropical high pressure zoneblow towardEquatorial low pressure zoneThe directional wind ofnortheast trades；Leaving to the left in the southern hemisphereSoutheast trade wind。The west bank of the mainland in the subtropical area of the Northern Hemisphere is located in the northeast trade windleeward, the east coast of the mainland is on the windward side of the northeast trade wind;Similarly, the west bank of the mainland in the subtropical region of the southern hemisphere is on the leeward side of the southeast trade wind, and the east bank of the mainland is on the windward side of the southeast trade wind.In this way, the precipitation on the west coast of the continent in the subtropical regions of the southern and northern hemispheres is scarce, the climate is dry, and there is desert distribution;However, the eastern coast of the mainland has more precipitation, which is not desert.
Third, bycold currentImpact.The ocean current has a great impact on the climate, and the cold current has a cooling and dehumidifying effect on the area it flows through,Warm currentIt has the function of warming and humidifying the area it flows through.From the World Climate and Current Map, we can see that the west coast of the mainland in the subtropical region is affected by the cold current, so the precipitation is scarce and the climate is dry;The east bank of the mainland is affected by the warm current, so there is more precipitation.As for the causes of the tropical desert climate in the interior of the continent in the subtropical regionSubtropical highOr dry trade wind, it is also related to the fact that it is far away from the ocean and is less affected by the ocean.
Temperature and humidity
Tropical desert climate landscape
The tropical desert climate appears in the subtropical high belt andTrade wind beltThe west bank of the mainland and the interior of the mainland.It is subject to subtropical high all the year roundDowndraftFrom the perspective of the whole earth, the formation of dry climate regions (arid regions) is mainly related to latitude, atmospheric circulation and other factors.Between 15~35 ° north and south latitudeSubtropical high belt(also known as the return high pressure zone) is the area where trade winds blow all the year round.The air in the high-pressure zone has a sinking effect, which is formed when the air sinksAdiabatic heating, so that the relative humidity is reduced and the air is very dry.The trade wind is blowing from the subtropical high beltEquatorial Low The steady wind direction ofequatorThe process of continuous heating;The hotter the air is, the more water is consumed, which makes it a very dry dry wind.In this way, in the control area of the subtropical high pressure zone, the atmosphere is very stable, the humidity is low, and there is little cloud but little rain, making it an arid area with little rainfall on the earth.Most of the great deserts in the world are distributed here, such as the Sahara Desert in North AfricaArabian Desert, Atacama Desert in South America, etc.Therefore, there are“Return to desert zone”It is called.
The formation of tropical desert climate varies slightly from place to place.The tropical desert climate on the west side of South America is narrow and long, because ① Andes is located on the west bank of the South American continent, making the tropical desert climate narrow. ②There areHumboldt Current After cooling and dehumidification. ③Located in southeast trade windLeeward slope, less precipitation.
PakistanThe cause of tropical desert climate is almost unique in the world.Its cause is mainly affected by the following two aspects: one is the serious destruction of ancient forests and other vegetation.Reason 2:Iranian PlateauEdge mountain blocking,Southwest monsoonIt is difficult to reach or the southwest monsoon is difficult to penetrate.
mirage
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Mirage is a unique landscape in desert climate area.Looking out at the sea shore, or navigating on the sea, sometimes in calm conditions, you will suddenly see strange phenomena such as ships, islands, towers, and city skylines in the air. When the wind is strong, this scene will disappear immediately.This is a kind of illusion in the atmosphere, called mirage, or simply mirage.This phenomenon occurs not only on the sea surface, but also in the air layer of lakes, rivers, deserts, Gobi and other places.Shen Kuo of the Song Dynasty of China once recorded this illusion in Mengxi Bitan:“DengzhouThere are clouds in the sea from time to time, such as palace terraces, battlements, figures, chariots and horses, which can be seen from the past.Dengzhou is now Shandong ProvincePenglai County, onthe bohai seaSouth bank, in the Bohai SeaMiaodao IslandsIt is tens of kilometers away from Penglai. What is recorded here is exactly the illusion of Miaodao Islands seen by Penglai.
In ancient times, because it was impossible to explain the cause of this phenomenon scientifically, various myths and legends were given, and the natural illusion was regarded as a mirage in the sea.In fact, the formation of mirage is caused by the intense abnormal refraction of light in the atmosphere under special meteorological conditions.Refraction of lightIt means in differentAir densityIn this case, the speed of light changes and the direction of travel twists and turns.When you insert a straight rod into the water obliquely, it seems to break from the side!This is caused by light refraction.In summer, the daytime sea temperature is low, the lower air is affected by the water temperature, the upper air is cold, and the upper air is cold and warmTemperature inversion phenomenonThe density of the lower air appears to be particularly large, while the density of the upper air appears to be particularly small.When the light from the distant scenery is gradually refracted from the dense gas layer into the thinatmosphereAnd occurs in the upper layertotal reflection, and return to the dense air in the lower layer.After such a curved route, it will appear in the eyes of the observermirageThe phenomenon ofImageIt is much higher than the real thing.This phenomenon is also calledMirage。 In the desert, because the sand and stone are scorched by the sun in the daytime, the air close to the sand layer rises extremely fast, forming the temperature distribution of the hot lower layer and the cold upper layer, resulting in the phenomenon that the density of the air at the lower layer is much smaller than that at the upper layer.At this time, the light of the front scene will change from the air with high density to the air with low densityrefractionTo formLower Mirage。Seen from afar, it looks like a reflection in the water.People who travel long distances in the desert are very hot and thirsty. When they see the mirage, they often mistakenly think that they have reached the cool lake. However, when a gust of wind and sand rolls over, it is still an endless desert. This scene was originally just an illusion.
There is a kind of "gravel circle" in the desert.It's a big stoneThermal expansion and cold contractionAfter several times of fragmentation, a circular circle of crushed stones was formed on the ground, much like the works arranged artificially, which was actually formed naturally.
