6.3 Cyclones and anticyclones Stor

6.3 Cyclones and anticyclones
Storms have been recorded throughout history, but the first comprehensive account of
a storm is given by Daniel Defoe (the ‘great storm’ of 26th November 1703). The first
observation on cyclonic systems is that of Benjamin Franklin (21st October 1743). He
noted that he was prevented from seeing a lunar eclipse in Philadelphia by a N.E. gale
but the eclipse was seen in Boston (300 km to the N.E.) Thus the storm itself must be
moving against the direction of its constituent winds. The development of the electric
telegraph allowed storms to be tracked. Admiral Fitzroy founded the U.K.
Meteorological Office. He noted that cold and hot air masses were both involved in
cyclonic behaviour (1840s). From 1910 to 1930, the Norwegian Meteorological
Office plotted warm and cold fronts – the first use of scientific aircraft. After 1960,
the use of meteorological satellites grew rapidly, enabling the plotting of cyclonic
motion and ‘fronts’.
Cyclonic systems can be found in low and middle latitudes. Middle latitude
cyclones are the systems responsible for the ‘bad weather’ in middle latitudes. They
are usually called depressions (not because of this but because they are low pressure
systems). Cyclones that arrive here form on the other side of the Atlantic
(where cold polar air over the North American continent meets warm tropical air from the Western
Atlantic – heated by the Gulf Stream). Similar effects occur in the North Pacific and
Northern Mediterranean (alpine air meeting air from North Africa). A depression
forms when a wave develops at the boundary between the two air masses. Air then
starts to flow across the isobars and a low pressure area develops with cyclonic
motion. This then moves in accordance with the winds in the warm sector. Since cold
air tends to move faster than warm air, the cold front tends to catch up with the warm
front and the warm air rises above the cold air.

Anticyclones form over Siberia, Canada and North Russia. These dominate the
Asian and North American climate in winter. They give rise to (mainly) dry weather
but can trap extensive low-lying clouds. Since winds are usually light, pollution levels
often rise. Most anticyclones last only four or five days, but occasionally they last
much longer. Such systems are called blocking highs. These block the movement of
cyclones in mid-latitudes, giving stationary fronts for days and even weeks.
Tropical cyclones are hurricanes (or typhoons). These are low pressure
systems of great intensity. Wind speeds of 100 m/s can be produced and heavy rainfall
always occurs. The cyclonic nature of air circulation in the lower atmosphere in a
hurricane or typhoon is easily seen by the nature and movement of clouds as seen
from a satellite. Rising air from the equator is humid and cools on rising giving a
girdle of clouds around the Earth. These give heavy rain in the equatorial regions. The
cold and warm air meets at the inter-tropical convergence zone (ITCZ) and minor
depressions along the edge. Some (and only some of these) develop into major storms.
The reason for the early stages is not clear, but once begun, the mechanisms that give
major hurricanes (or typhoons) are well understood:
• pressure starts to fall rapidly at the centre of the disturbance.
• Winds rise in a tight band, 30-60 km in radius (the central eye).
• As the storm grows it moves to the westerly in the trade winds (8- latitude) and
migrates to higher latitudes.
• The maturing storm expands while the central pressure stops falling. The route
depends on local surface conditions such as the surface temperature (and perhaps the salinity) of the sea (and is unpredictable). Hurricanes only form over warm
water (temperature greater than ).
• The storm grows to 300 km radius or greater and then begins to decay. Decay is
hastened by passing over cold water or land.
• The storm is pushed by mid-latitude westerlies in higher latitudes as it decays.

These storms are found in both hemispheres, most commonly in the North Pacific.
Most frequent in late summer (when the sea surface is warmest) but they can occur all
summer and autumn in the tropics. They are part of the process by which energy is
transported from the equator to the poles.

6.4 Global convection
The first model to describe large-scale global convection was proposed by George
Hadley in 1735. He noted that air in the lower latitudes is warmer than that in the
higher (polar) latitudes due to the greater solar flux reaching the equator. Tropical air
should move (rise) vertically and move northwards while the cool polar air should
move southwards. As the tropical air moves north, it loses energy by radiation before
descending to the ground, so replacing the southward-moving colder air. Similarly,
the cold air will gain heat from the ground (which is itself heated by radiation as
discussed earlier) and will therefore rise in the equatorial regions. Thus a circulation
system is formed transporting thermal energy from the equator to the poles. This is the
Hadley cell.
There are, however, significant differences between Hadley’s model and the
real air circulation patterns. It is true that there is a low pressure belt over the equator
and a high pressure region over the pole as the Hadley model would predict.
However, there is another circulating cell of air between N and N (and
similarly in the southern hemisphere) in which air rises in the colder regions (i.e. at
N) and descends in the warmer regions (i.e. in the opposite direction to the Hadley
mechanism). This is the Ferrel cell. Finally, there is a third cell between N (and S)
and the pole. This circulates in the same direction as the Hadley cell but is much
weaker. It is known as the Polar cell.

Kết quả (Việt) 1:

6.3 xoáy và anticyclones Cơn bão đã được ghi nhận trong suốt lịch sử, nhưng tài khoản toàn diện đầu tiên của một cơn bão được đưa ra bởi Daniel Defoe ('tuyệt vời bão' của 26 tháng 11 năm 1703). Người đầu tiên Các quan sát trên hệ thống lốc xoáy là Benjamin Franklin (21 tháng 10 năm 1743). Ông lưu ý rằng ông bị ngăn cản từ nhìn thấy một lunar eclipse tại Philadelphia bởi N.E. gale nhưng các thực đã được nhìn thấy ở Boston (300 km về N.E.) Do đó các cơn bão chính nó phải di chuyển với sự chỉ đạo của gió thành phần của nó. Sự phát triển của điện tử điện báo cho phép các cơn bão để được theo dõi. Admiral Fitzroy thành lập Vương Quốc Anh Văn phòng khí tượng. Ông lưu ý rằng lạnh và khí công chúng đã là cả hai tham gia vào lốc xoáy hành vi (thập niên 1840). Từ năm 1910 đến năm 1930, Na Uy khí tượng Văn phòng âm mưu ấm và frông lạnh-đầu tiên sử dụng khoa học máy bay. Sau năm 1960, sử dụng vệ tinh khí tượng phát triển nhanh chóng, cho phép các âm mưu của lốc xoáy chuyển động và 'mặt trận'. Lốc xoáy hệ thống có thể được tìm thấy ở vĩ độ thấp và trung bình. Giữa vĩ độ xoáy là hệ thống chịu trách nhiệm về 'thời tiết xấu' ở vĩ độ giữa. Họ thường được gọi là áp thấp (không bởi vì điều này, nhưng vì họ có áp suất thấp Hệ thống). Lốc xoáy ở đây đến hình thức phía bên kia của Đại Tây Dương (nơi không khí vùng cực lạnh trên lục địa Bắc Mỹ đáp ứng không khí nhiệt đới ấm áp từ phương Tây Atlantic – heated by the Gulf Stream). Similar effects occur in the North Pacific and Northern Mediterranean (alpine air meeting air from North Africa). A depression forms when a wave develops at the boundary between the two air masses. Air then starts to flow across the isobars and a low pressure area develops with cyclonic motion. This then moves in accordance with the winds in the warm sector. Since cold air tends to move faster than warm air, the cold front tends to catch up with the warm front and the warm air rises above the cold air. Anticyclones form over Siberia, Canada and North Russia. These dominate the Asian and North American climate in winter. They give rise to (mainly) dry weather but can trap extensive low-lying clouds. Since winds are usually light, pollution levels often rise. Most anticyclones last only four or five days, but occasionally they last much longer. Such systems are called blocking highs. These block the movement of cyclones in mid-latitudes, giving stationary fronts for days and even weeks. Tropical cyclones are hurricanes (or typhoons). These are low pressure systems of great intensity. Wind speeds of 100 m/s can be produced and heavy rainfall always occurs. The cyclonic nature of air circulation in the lower atmosphere in a hurricane or typhoon is easily seen by the nature and movement of clouds as seen from a satellite. Rising air from the equator is humid and cools on rising giving a girdle of clouds around the Earth. These give heavy rain in the equatorial regions. The cold and warm air meets at the inter-tropical convergence zone (ITCZ) and minor depressions along the edge. Some (and only some of these) develop into major storms. The reason for the early stages is not clear, but once begun, the mechanisms that give major hurricanes (or typhoons) are well understood: • pressure starts to fall rapidly at the centre of the disturbance. • Winds rise in a tight band, 30-60 km in radius (the central eye). • As the storm grows it moves to the westerly in the trade winds (8- latitude) and migrates to higher latitudes. • The maturing storm expands while the central pressure stops falling. The route depends on local surface conditions such as the surface temperature (and perhaps the salinity) of the sea (and is unpredictable). Hurricanes only form over warm water (temperature greater than ). • The storm grows to 300 km radius or greater and then begins to decay. Decay is hastened by passing over cold water or land. • The storm is pushed by mid-latitude westerlies in higher latitudes as it decays. These storms are found in both hemispheres, most commonly in the North Pacific. Most frequent in late summer (when the sea surface is warmest) but they can occur all summer and autumn in the tropics. They are part of the process by which energy is transported from the equator to the poles. 6.4 Global convection The first model to describe large-scale global convection was proposed by George Hadley in 1735. He noted that air in the lower latitudes is warmer than that in the higher (polar) latitudes due to the greater solar flux reaching the equator. Tropical air should move (rise) vertically and move northwards while the cool polar air should move southwards. As the tropical air moves north, it loses energy by radiation before descending to the ground, so replacing the southward-moving colder air. Similarly, the cold air will gain heat from the ground (which is itself heated by radiation as discussed earlier) and will therefore rise in the equatorial regions. Thus a circulation system is formed transporting thermal energy from the equator to the poles. This is the Hadley cell. There are, however, significant differences between Hadley’s model and the real air circulation patterns. It is true that there is a low pressure belt over the equator and a high pressure region over the pole as the Hadley model would predict. However, there is another circulating cell of air between N and N (and similarly in the southern hemisphere) in which air rises in the colder regions (i.e. at N) and descends in the warmer regions (i.e. in the opposite direction to the Hadley mechanism). This is the Ferrel cell. Finally, there is a third cell between N (and S) and the pole. This circulates in the same direction as the Hadley cell but is much weaker. It is known as the Polar cell.