Tóm Tắt

Bão biển nhiệt đới là danh từ được dịch từ tiếng Anh "tropical cyclone" hoặc "tropical storm".  Theo định nghĩa quốc tế, bão biển nhiệt đới phải có gió mạnh hơn 63 km/giờ (cấp 8, 34 knots). Nếu gió yếu hơn 63 km/giờ, gọi là áp thấp nhiệt đới (tropical depression). Nếu gíó mạnh hơn 118 km/giờ (cấp 12, 64 knots), bão được gọi là bão to với cuồng phong (typhoon). Ngoài ra còn có bão rất to hay siêu bão (super typhoon) với gió mạnh hơn 241 km/giờ. Danh từ "typhoon" được dùng trong vùng Biển Đông và Tây Bắc Thái Bình Dương; "hurricane" trong vùng Đại Tây Dương; và "tropical cyclone" trong vùng Ấn Độ Dương.

Ở nước ta, mùa bão thường bắt đầu từ miền Bắc vào tháng 5,6 và di chuyển vào miền Nam cho đến hết tháng 12. Ba tháng 9,10 và 11 thường có nhiều cơn bão nhất. Trung bình, hàng năm có từ 4 đến 6 cơn bão vào bờ biển nước ta.

Mỗi khi có bão với gió mạnh hơn 63 km/giờ, bão được đặt tên bởi Cơ Quan Khí Tuợng Nhật Bản (Japanese Meteorological Agency) ở Tokyo. Kể từ năm 2000, tên bão được lấy từ danh sách dưới đây:

1) What is a hurricane, typhoon, or tropical cyclone?

     The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone".  A tropical cyclone is a low-pressure system which derives its energy primarily from evaporation from the sea in the presence of high winds and lowered surface pressure and the associated condensation in convective clouds concentrated near its center.

     Tropical cyclones with maximum sustained surface winds of less than 17 m/s (63 km/h, 34 kt) are called "tropical depressions".  Once the tropical cyclone reaches winds of at least 17 m/s (63 km/h)  they are typically called a "tropical storm" and assigned a name.  If winds reach 33 m/s (118 km/h, 64 kt), then they are called:  a "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E);  a "typhoon" (the South China Sea and the Northwest Pacific Ocean west of the dateline); a "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E); a "severe cyclonic storm" (the North Indian Ocean); and a "tropical cyclone" (the Southwest Indian Ocean).

    To facilitate identification and tracking, the storms are generally given alternating masculine and feminine names, or numbers which identify the year and annual sequence. Tropical cyclones are the most destructive of seasonally recurring rapid onset natural hazards.  Between 80 to 100 tropical cyclones occur around the world each year.  Devastation by violent winds, torrential rainfall and accompanying phenomena including storm surges and floods can lead to massive community disruption.

In Vietnam, tropical storms start hitting the north coast in May/June in the North, and generally, as the year proceeds, they move farther  to the South until December when the typhoon season is over. According to a recent study by the United Nations Disaster Management Unit, September, October and November are the "peak" months with the most storms. On average, four to six storms are expected to strike the coastal areas in a typical year.
 
 

     The Northwest Pacific basin tropical cyclones were given women's names officially starting in 1945 and men's names were also included beginning in 1979. Beginning on 1 January 2000, tropical cyclones in the Northwest Pacific basin will be named from a new and very different list of names. The new names are Asian names and were contributed by all the nations and territories that are members of the WMO's Typhoon Committee. The new names will be allotted to developing tropical storms by the Tokyo Typhoon Centre of the Japanese Meteorological Agency which is the RSMC for the basin.

    These newly selected names have two major differences from the rest of the world's tropical cyclone name rosters. One, the names by and large are not personal names. There are a few men's and women's names, but the majority are names of flowers, animals, birds, trees, or even foods, etc, while some are descriptive adjectives. Secondly, the names will not be allotted in alphabetical order, but are arranged by contributing nation with the countries being alphabetized. Fourteen nations or territories contributed ten names each for a total of 140 names. The names contributed by Viet Nam include Saomai,  Lekima, Bavi, Conson, Sonca, Trami, Halong, Vamco,  Songda and Saola.
 

     To undergo tropical cyclogenesis, there are several favorable precursor environmental conditions that must be in place:

1.  Warm ocean waters (of at least 26.5 C [80 F]) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m [150 ft]).  Warm waters are necessary to fuel the heat engine of the tropical cyclone.

2.  An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection.  It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.

3.  Relatively moist layers near the mid-troposphere (5 km [3 mi]). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.

4.  A minimum distance of at least 500 km [300 mi] from the equator.  For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur.  Without the Coriolis force, the low pressure of the disturbance cannot be maintained.

5.  A pre-existing near-surface disturbance with sufficient vorticity and convergence.  Tropical cyclones cannot be generated spontaneously.  To develop, they require a weakly organized system with sizable spin and low level inflow.

6.  Low values (less than about 10 m/s [20 mph]) of vertical wind shear between the surface and the upper troposphere.  Vertical wind shear is the magnitude of wind change with height.  Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center.

     Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop.  Recent work  has identified that large thunderstorm systems (called mesoscale convective complexes [MCC]) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC.  These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi], are strongest in the mid-troposphere (5 km [3 mi]) and have no appreciable signature at the surface. It  is  hypothesized that genesis of the tropical cyclones occurs in two stages:  stage 1 occurs when the MCC produces a mesoscale vortex and stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds.

5) What is the Beaufort scale?

The Beaufort scale is an empirical measure for describing wind intensity based mainly on observed sea conditions.

For winds:  1 mile per hour (mph) = 0.864 knots (kt)
            1 mph = 1.609 kilometers per hour (kph)
            1 mph = 0.4470 meters per second (m/s)
            1 kt = 1.853 kph
            1 kt = 0.5148 m/s
            1 m/s = 3.600 kph

For pressures:  1 inch of mercury = 33.86 mb = 33.86 hPa

For distances:  1 ft = 0.3048 m
 

Last modified: July 4, 2006