Tennessee River in the United States

The topographic structure of North America is characterized by three north-south longitudinal zones, which is of great significance to the differentiation of continental climate. The most significant influence is the Cordillera mountain system in the west, which consists of triple mountains and a series of mountain plateaus and basins. It is not only quite high, but also very wide, and the coastal areas lack bays that go deep into the mainland. Therefore, on the one hand, the Cordillera mountain system has become an important obstacle to the eastward invasion of the polar Pacific air mass, limiting the mild and humid maritime climate to the west coast north of 40 north latitude, and the intermountain plateau and intermountain basin in the leeward position become semi-arid and arid climate; On the other hand, the polar Canadian air mass and the tropical Gulf of Mexico and Atlantic air mass can only move in the central and eastern parts of the mainland because the Cordillera Mountains cannot invade the west. The Rocky Mountain in the east of Cordillera Mountains is also an important climate dividing line between the east and the west of the mainland, which not only leads to a great difference in precipitation between the east and the west, but also has a certain impact on the temperature. Generally speaking, the annual precipitation in the western part of the Rocky Mountain is below 500 mm, with little rain or drought in summer, and the temperature in winter is higher than that in the eastern part of the same latitude, except for the coastal areas and windward slopes north of 40 north latitude. Except for the northern part of high latitude and parts of the great plains near the Rocky Mountains, the annual precipitation in the east of the Rocky Mountains is above 500 mm, and the proportion of precipitation in summer increases. The climate data of Spokane and Montreal can illustrate this problem. Of course, this difference is also related to factors such as atmospheric circulation and ocean currents.

To the east of the Rocky Mountain is the central plain, which is low and flat, and there is no east-west mountain barrier. It is open to the north and south, and the Hudson Bay, the Great Lakes, the Mississippi River system and the Gulf of Mexico are interconnected. Such terrain conditions are conducive to the smooth movement of air masses with different properties in the north and south. In winter, the dry and cold polar Canadian air mass can go straight to the south, resulting in cold wave weather and a sudden drop in local temperature; In summer, the warm and humid air masses in the tropical Gulf of Mexico and the Atlantic Ocean can freely go north to the coast of Hudson Bay, bringing sultry and rainy weather. The Central Plains has become a place where cold and warm air masses meet and compete with each other, and cyclone activities are frequent, especially in winter. Therefore, the weather in the central plain is changeable, which is the area with the biggest seasonal variation of temperature and precipitation in North America and strong mainland.

The Appalachian mountains in the east are not high or wide, and the continuity of the mountains is poor, which does not constitute a significant boundary in climate, but it still has a great impact on climate in some areas. For example, on the northwest slope of the Appalachian Mountains, the polar Canadian air mass passes through the Great Lakes region, which is slightly denatured in winter and often forms heavy snow; Due to the high altitude, the tropical Gulf of Mexico and Atlantic air mass swelled in the south of the mountain, forming topographic precipitation, with annual precipitation exceeding 65,438+500mm, making it one of the rainy areas in North America.

253. The pressure distribution pattern in North America is similar to that in Eurasia. Why doesn't the east coast of North America have a typical monsoon climate like the east coast of Asia?

Monsoon is the dominant wind direction in a large area, which changes significantly with the seasons. There are different opinions about the so-called significant changes. At present, the popular views are: (1) 1, and the prevailing wind direction changes at least120 in July; (2) The average frequency of prevailing wind direction in1month and July exceeds 40%; (3) At least in 1 month and July, the average composite wind speed exceeds 3m/s ... This seasonal wind blows from the mainland to the ocean in winter and from the ocean to the mainland in summer. With the change of wind direction, the characteristics of weather and climate also change. For example, along the Arctic Ocean in Siberia, the direction of Xia Feng is opposite in winter, and there is a monsoon phenomenon. However, because the climate difference between winter and summer in North America is not as significant as that in East Asia, Southeast Asia and South Asia, it is not included in the monsoon climate type.

The pressure distribution pattern in North America is similar to that in Eurasia, with four active centers, namely, two sub-polar low centers-Iceland low in the North Atlantic and Aleutian low in the North Pacific; Two subtropical high centers-azores high in the North Atlantic and Hawaiian high in the North Pacific. In addition, the mainland also has a barometric activity center, which is high in winter and low in summer, alternating between winter and summer. The intensity and central position of the above four atmospheric activity centers also change with the seasons.

In the North American continent, because the thermal difference between sea and land in winter and summer is not as significant as that in Asia, the intensity of high and low pressure is relatively weak. In winter, the forces of subtropical low pressure in the two oceans are strengthened, and the center of subtropical high pressure in the two oceans moves southward, and its intensity is weakened. On the mainland, there is a high pressure, the center of which is located in the northwest, which is connected with the subtropical high of the two oceans, and its northern edge extends northward in a tongue shape, separating the Aleutian low from the Icelandic air pressure system. However, the continental high in North America is far less powerful than the Mongolian-Siberian high in Asia, and it is often disturbed by cyclone activities. In fact, low pressure and high pressure alternate constantly, and the weather is unstable. The above air pressure configuration mode determines the winter wind direction in North America. Sea breeze mainly blows along the Pacific coast, in which the westerly wind is north of 40 north latitude, the westerly wind and northwest wind are south of 40 north latitude, and the northeast wind turns south of 30 north latitude; Throughout the continent, the land breeze is basically blowing in winter, so the west wind and northwest wind prevail along the Atlantic coast, both of which are near sea breeze. There is also a local high pressure in the southeast of the mainland (east of the lower reaches of the Mississippi River), so between the Appalachian Mountains, the Mississippi River and the Great Lakes, sometimes southerly winds and southwesterly winds blow; The north wind blows along the coast of the Gulf of Mexico; The southern Florida Peninsula is in the trade wind zone, and the northeast wind prevails. In summer, the mainland heats up, forming a continental low-pressure center in the southwest, the subtropical high ruptures, the center position expands northward, and its power increases. At this time, the center of azores high moved to the vicinity of 35 north latitude, and its control range could be extended to the southeast of the mainland; The Hawaiian high is 40 north latitude, which controls the weather on the whole west coast. However, Iceland and Aleutian retreated northward at low pressure, and their forces weakened. Under this high and low pressure layout, the wind direction distribution is roughly opposite to that in winter. Along the Pacific coast, the southwest wind blows to the north of 50 north latitude, from the west wind to the northwest wind between 50 and 40 north latitude, and from the south of 40 north latitude to the north wind and northeast wind; The whole continent is dominated by sea breeze, with southeast winds prevailing along the Atlantic Ocean and the Gulf of Mexico, and south winds blowing on the great plains.

Although the winter and summer monsoon directions in eastern North America change with the seasons, they are far less significant and stable than those in East Asia. One of the reasons is that the North American continent is smaller than Asia, and more importantly, there is a lack of topographic barriers across the east and west of the continent to separate the cold and dry air from the humid air from the Gulf of Mexico. Air masses with different properties cross each other, and cyclone activities are frequent, which destroys the monsoon circulation.

254. What is the impact of ocean currents on the climate in North America?

Current affecting North America, including Alaska Warm Current and California Cold Current in the Pacific Ocean; There are Labrador cold current and Gulf warm current in the Atlantic Ocean.

In the Pacific Ocean, the Japanese warm current (Kuroshio) reaches 35 N and flows eastward to 160 E.. Under the action of the west wind, it continues to flow eastward, which is called the North Pacific Warm Current. When it reaches the mouth of Columbia River on the west coast of North America, it divides into two branches: the north branch flows counterclockwise, which is called Alaska Warm Current, and the south branch is called California Cold Current. The Alaska Warm Current, which brings a lot of heat to the high-latitude sea area, makes the Pacific coast of the westerly belt north of 40 north latitude deeply affected by its temperature and humidity, but its power and influence are far less than that of the North Atlantic Warm Current in Europe. This is because the Kuroshio is weaker than the Gulf Stream. In addition, the mountains in the western part of the North American continent are close to the coast, and Alaska extends westward, forcing the Alaska warm current to turn westward. On the other hand, the western Europe is flat, the coast is tortuous, and the Atlantic Ocean and the Arctic Ocean are unblocked, which makes the warm current of the North Atlantic flow northward, thus expanding its influence. The cold current in California mainly affects the climate along the Pacific coast south of 40 north latitude, which makes the temperature in this area drop, especially in summer, which is foggy. It is estimated that 60 kilocalories are lost per square centimeter every year, which is about half of the total solar radiation obtained, and the precipitation in this area is also reduced.

In the Atlantic Ocean, the Labrador cold current carries the Arctic Ocean ice from Baffin Bay to the south, and the surface water temperature is lower than 10℃, which makes the coastal area of Labrador Peninsula cool in summer. Gulf Stream, also known as Gulf Stream, is the most powerful warm current in the world. Its surface water temperature is above 26℃, and its flow rate is 82 million cubic meters per second, which is 60 times that of all rivers on the earth. However, there is often a cold water belt between this warm current and the North American coast. In winter, the temperature difference between the Gulf Stream near Cape Hatteras and the coastal waters is 8℃, and that between new york and Boston is 12 ~ 15℃. In addition, the wind direction on the east coast of North America is offshore in winter, which makes it more difficult to benefit from the warm current. In summer, due to the increase of coastal seawater temperature, the difference between the coastal seawater temperature and the flowing water temperature in the Gulf of Mexico is not obvious. At this time, the southeast wind landed through the Gulf Stream, and the air mass became more humid and unstable, which had an important impact on the climate in eastern North America. The warm current from the Gulf of Mexico reaches the outside of Newfoundland, meets the Labrador cold current, melts the ice carried by the cold current, and provides conditions for cyclone activity and fog formation. Therefore, the Gulf Stream is biased to the northeast and flows to the northwest waters of Europe.

255. What are the basic features of the climate in North America? Why is the temperate continental climate dominant here?

The basic characteristics of the climate in North America are:

First of all, temperate continental climate is dominant. North America has a wide latitude extension, but most of it is located in the north temperate zone; At the same time, the topographical structure is characterized by three longitudinal zones, and the western mountains and eastern highlands, especially the tall Cordillera Mountains, run through the north and south, forming an important obstacle to the operation of marine air masses. Therefore, the climate in North America mainly belongs to temperate zone and sub-frigid zone, especially the continental climate in sub-frigid zone is dominant. The general feature of continental climate here is cold winter, with the lowest average temperature of 1 month. Summer is warm and hot, and July is the hottest month. The annual temperature is relatively high; The annual precipitation is moderate, mainly in summer rain. However, compared with the more continental Asia, the temperate continental climate of this continent is far less extreme than that of Asia. The annual temperature is not as high as that in Asia at the same latitude, and the concentration of rain in summer and drought in winter are not as good as that in Asia.

The second is the diversity of climate types. There are many types of climate in North America, from tropical to cold zone, which are also prominent on all continents. But North America has its own characteristics, such as the distribution of polar ice sheet climate is wider than that of Asia; Asia lacks the temperate maritime climate on the west coast, while the east coast of North America does not have the typical monsoon climate like the east coast of Asia, and it is replaced by the temperate continental humid climate and subtropical humid climate. In addition, this continent has no equatorial rainforest climate, tropical marine climate and tropical dry and wet season climate, and the distribution of tropical arid and semi-arid climate types is not as extensive as that of Asia.

Third, the climate type and structure are unique. Various climate types in North America also have their own characteristics in distribution and arrangement. First of all, the three distribution types in the northern part of the continent, namely, the polar ice sheet climate, the polar long cold climate and the continental climate in the sub-frigid zone, are all arranged in the north and south, and they extend from east to west in addition to the polar ice sheet climate, which is basically similar to the situation in the northern part of Eurasia and has obvious zonal structural characteristics. Since then, the east and the west have compared the distribution and arrangement patterns of climate types: roughly east of west longitude 100, from north to south, it is temperate continental humid climate and subtropical humid climate in turn. Generally speaking, they also alternate from north to south and extend from east to west, which basically embodies the zonal structural characteristics. To the west of west longitude 100, including the great plains and Cordillera mountain area, the drought intensified from east to west due to the deep inland, and the climate type was arranged in the east-west direction, extending from north to south, showing the characteristics of non-zonal zonal structure. On the west coast of the mainland, the order of climate types is temperate maritime climate, subtropical summer arid climate, tropical arid climate and semi-arid climate. Although they alternate regularly from north to south, they all extend from north to south. Therefore, this is a comprehensive embodiment of banded and non-banded structural characteristics.

256. What are the characteristics of the distribution of winter and summer isotherms in North America? What is the reason?

The distribution of January isotherms has the following two characteristics: (1) isotherms all bend to the south. The northwest isotherm is northwest-southeast, along the Pacific coast, almost parallel to the coast, related to the westerly belt and the warm current passing along the coast; Inland, the degree of cold is strengthened, and the isotherm turns to the southeast, and to the Mississippi River basin, the isotherm turns to the east; Further east, the isotherm turns to the northeast and slants to the coast, reflecting that the impact of the Atlantic winter on the climate of the North American continent is obviously weaker than that of the Pacific Ocean. (2) The isotherm distribution is dense. The average temperature in central Greenland 1 month is as low as -44℃, and that in southern Florida Peninsula is 20℃, indicating that the temperature gradient in winter is steep, and the temperature difference between north and south reaches 64℃.

The mainland generally warms up in summer, and even in high latitudes, the temperature is obviously higher than that in winter because of the long sunshine time. Reflected in the isotherm distribution in July, it has the following characteristics: (1) The isotherm distribution is sparse. It shows that the temperature gradient in summer is not as big as that in winter. The lowest average temperature in July in central Greenland is about-12℃, and the highest temperature in desert areas in southwest Greenland is about 32℃. The temperature difference between north and south is about 44℃. (2) The isotherm obviously bends to the northwest. On the Pacific coast, the north-south direction of the isotherm is parallel to the coast, but it enters the mountainous plateau and basin area eastward, and the isotherm forms an arc protruding northward, indicating that there is little difference between the north and the south of the Pacific coast in summer. For example, affected by the cold current, the California coast is foggy and sunny, and the temperature is almost the same as that of the Canadian coast. At the same time, it also shows that the temperature along the Pacific coast is much lower than that in mountainous plateaus and basins of the same latitude. The latter is located in arid and semi-arid areas with abundant sunshine and strong heating. (3) To the east of the Rocky Mountain, the isotherm generally runs southeast, and bends slightly to the south in Hudson Bay and the Great Lakes, which is due to the adjustment of water areas. From the east to the offshore, the isotherm slightly protrudes to the north, indicating that the Atlantic coast is affected by the ocean in summer. The temperature in Labrador and Newfoundland is lower than that in the inland of the same latitude, but it is higher than that in the Pacific coast due to the influence of the cold current.

257. What are the rules of precipitation distribution in North America?

The distribution of annual precipitation in the North American continent is roughly bounded by the Rocky Mountains, with great differences between the east and the west. To the west of the Rocky Mountains, the annual precipitation is generally below 500mm, but there is a rain belt along the Pacific coast north of 40℃ north latitude. Among them, the coastal areas of Canada and Alaska, north of 50℃ north latitude, face westerly winds and active cyclones all the year round, and the uplift of coastal mountains forms the most abundant precipitation zone in North America, with an annual precipitation of more than 3,000 mm. For example, the annual precipitation in Rupert Harbor is 2,399mm, while that in Yakuta is 3,264mm ... The distribution of annual precipitation along the entire Pacific coast shows a decreasing trend from north to south. If Olympia is 1, 288 mm, San Francisco is reduced to 5 10 mm, and San Diego is only 250 mm Since then, the precipitation in the south has increased again.

To the east of Rocky Mountain, the general distribution law of annual precipitation is gradually increasing from northwest to southeast. Except for the Great Plains near the Rocky Mountains and the coastal areas of the Arctic Ocean, the Arctic Islands and Greenland, it is generally above 000 mm, of which the central plain is 500 ~ 1 0,000 mm, and the precipitation in the eastern Appalachian Mountains and the southeastern United States is above 1 0,500 mm.

258. What is the uniqueness of the continental climate in the sub-frigid zone of North America compared with that in Asia?

The continental climate of North America's sub-cold zone, like the same type in Asia, is distributed in the south of the polar long cold climate zone, and its northern boundary is equivalent to the July 10℃ isotherm. The southern boundary is roughly on the line of 50 north latitude, but in Asia, the eastern part of this boundary is slightly north, and it is bounded by the line from Waixing 'an Mountains to the coast of Okhotsk. They are the most widely distributed climatic zones in the whole continent, especially in Asia, with the largest area of about 7 million square kilometers, which is the largest climatic type zone in the world.

The continental climate of North America's sub-cold zone is unique: (1) it is cold in winter, and the average monthly temperature of/kloc-0 is below -20℃; Summer is mild, and the average temperature in July is about 65,438+06 ~ 65,438+08℃, which is close to other latitudes, so the annual temperature range is very high, but the continental degree is not as extreme as that in Asia. (2) Humidity. The annual precipitation in this area is between 250 ~ 000mm in the west and north, and between 500 ~ 1 1,000mm in the east. Its spatial distribution is just the opposite of the same type in Asia. It is more in the west of Asia, about 300 ~ 600 mm, reduced to 300 ~ 400 mm in the east, and the least in the northeast, only 200 ~ 300 mm In the seasonal distribution of precipitation, summer rain is the main one, but the concentration is not as good as that of the same type of climate in Asia. Therefore, on the climate map of Ke Ben, the continental climate of the sub-frigid zone in North America belongs to F type, while Asia belongs to W type. Because of weak evaporation and high relative humidity, this is the characteristic of the two continents.

259. What are the similarities and differences between the temperate continental humid climate and subtropical humid climate in North America and the temperate monsoon climate and subtropical monsoon climate in Asia?

Compared with the corresponding Asian temperate monsoon climate and subtropical monsoon climate, the temperate continental humid climate and subtropical humid climate in North America have both similarities and many differences. The basic characteristics of continental humid climate in temperate zone of North America are: cold and little rain in winter, mild and humid in summer, and the seasonal changes of temperature and precipitation are not as obvious as those in temperate monsoon climate of East Asia. The temperate monsoon climate in Asia is cold and dry in winter, and the severe cold period and degree exceed the continental humid climate in temperate zone of North America. For example, in Harbin, the monthly average temperature is below 0℃ for five months, in which 1 month average temperature is about 10℃ lower than that in Montreal at the same latitude, and the average temperature in July is about 2℃ higher than that in Montreal. The precipitation in Harbin is less than that in Montreal, and the seasonal distribution is more concentrated in summer. The precipitation from June to August accounts for 67% of the whole year, while the precipitation from winter 10 to March only accounts for 12%. The seasonal distribution of precipitation in Montreal is relatively uniform.

The subtropical humid climate in the southeast of the North American continent and the subtropical monsoon climate in the corresponding areas of East Asia belong to the subtropical east coast climate, both of which have the characteristics of warm winter, and the monthly average temperature of/kloc-0 is above 0℃. However, if a strong cold wave invades the south, the temperature can also drop below freezing point, which often brings devastating freezing damage to plants in Florida and southern Texas. In China's subtropical monsoon climate zone, the absolute minimum temperature can also drop below-10 ~-20℃, and the lowest value-18℃ appeared in Hankou on June 30, 2007, due to the influence of cold air from the south. Compared with other parts of the world at the same latitude, the humid subtropical monsoon climate in Asia is characterized by outstanding low temperature in winter. Summer is warm and hot, and the average temperature in July is 24 ~ 28℃. In China's subtropical monsoon climate zone, especially in Central China, it is generally hot in summer, and the average temperature in July is around 28℃, even exceeding 29℃ in some areas.

There is abundant precipitation, with annual precipitation of 1, 000 ~ 2,000 mm, mainly summer rain, and the seasonal distribution is relatively uniform, which is also the same feature of subtropical climate in North America and East Asia. In winter, there are many cyclones, cloudy and rainy in both places. In addition, there are many tornadoes in the subtropical humid climate of North America in summer, and the southeast coast is often hit by hurricanes in summer and autumn to produce heavy rain; The humid subtropical monsoon climate in East Asia, the plum rains at the turn of spring and summer and the combined winds in summer and autumn are also special and important weather phenomena.

260. What are the outstanding features of the Mediterranean climate in North America?

The Mediterranean climate belongs to the west coast of the subtropical latitude continent, with the characteristics of rainy winter, dry summer, mild winter, cool coastal summer and hot and dry inland.

Due to the barrier of the high Cordillera Mountains in the west, the Mediterranean climate zone in North America is only distributed in the narrow strip along the Pacific coast at 30 ~ 40 north latitude. Winter here is controlled by the westerly belt, and cyclones are active. Winter rain generally accounts for more than 50% of the annual precipitation. In summer, the weather is dry and stable because it is located at the eastern edge of the subtropical high in the North Pacific Ocean and controlled by the northeast trade wind. The proportion of summer rain is less than 3%, and the drier the summer goes to South Vietnam. The annual rainfall is moderate, ranging from 300- 1 000mm, decreasing from north to south. Affected by California cold current, the summer temperature in coastal areas is not high, and the average temperature in July is 16 ~ 20℃, which belongs to the hot summer type. January is 8 ~ 12℃, so the annual change is very small. However, across the east of the coastal mountains and into the California Valley, due to the influence of the cold current, the average temperature in the hottest month in all cities in the valley is above 22℃, and the average daily maximum temperature is 30 ~ 35℃. On July 27th, it was 65,438+09,365,438 in Sacramento and the maximum daily temperature in the western United States. The west of Sacramento is not far from the Pacific Ocean, but the average temperature in the hottest month (July) is about 7℃ higher than that in San Francisco, so the temperature in the whole year is higher than that in cool summer. Due to the existence of the Mediterranean, the Mediterranean climate in southern Europe is more distinctive and wider than that in North America.