４−１−５

Blocking Anticyclones or Cut-oﬀ HighThe “blocking phenomenon” is observed often in the Aleutian Sea area in winter. It is aphenomenon by which a “cut-oﬀ high” [Note No.2] produced in the upper air is located at
the front of the extra-tropical cyclone and blocks the eastward migration of the cyclone.
If its migration is blocked, the cyclone may, however, maintain its intensity. This resultsin rough seas for a prolonged period. Therefore, one should pay attention to the “blockingphenomenon” when drawing up a navigation plan to such areas (Fig. 18).

DAY1DAY2

DAY3Fig. 18

20

Stationary low pressure, Japan Captainsʼ Association, DVD[Note No.2]
This is also referred to as cut-off high. A warm anticyclone in the upper

air which is generated by the separation of the warm air at low latitudes andthe high latitudes to the north, when the core located in the westerly beltof the upper air signiﬁcantly moves south-northward. Normally, the blocking
phenomenon occurs after a large stationary low pressure which seems toreact with the ground, appears. The migration of the high or low pressure (ascan be seen on a surface weather chart) normally moves to the east, carriedby the westerlies. However, this phenomenon tends to detour south-north,accompanied by a back ﬂow and so on, because the air is blocked at the westof the blocking anticyclone, and ascendant high pressure which penetrates thetroposphere in the middle latitude stays in the area for a prolonged period.
This is when cut-oﬀ high pressure most notably manifests in the upper-air (Fig.
19 From the Japan Meteorological Agency website).

Relationship between blocking anticyclone and cold air
Cold air is forced down southwardfrom the north polar region

Air is blocked by the blocking
anticyclone and stagnates

Cold airBlocking anticyclonePol
jet ar frostreamnt
Subtropicaljet stream

Two different jet
A high pressure cluster
streams split widely here, forms in the middle of themuch more so compared split jet streams
to an average year

Fig. 19 Blocking Anticyclone From the Japan Meteorological Agency website21

４−１−６ Low Pressure Rapidly Develops (Bomb Cyclone)(Figs. 20 and 21)
An extra-tropical cyclone which develops rapidly is referred to as a “bomb cyclone” in Japan.
According to an encyclopaedia of meteorological science, the bomb cyclone is deﬁned as“an extra-tropical cyclone that decreases its central pressure at more than 24hPa ×sin （φ）
within 24 hours (Note: φ refers to latitudes)”. For instance, if the position is latitude 40°
North, the air pressure will decrease more than 17.8hPa/24h per day.
Cyclones on the Japan Sea which have strong winds covering a wide area in early spring,
have low pressures which rapidly develop close to Northern Japan along with other low
pressures that rapidly develop in the east of Japan or oﬀ the coast near Chishima in winter,are known as bomb cyclones. Today, however, the Japan Meteorological Agency does not
use the term, because the word “bomb” is not appropriate, thus it is replaced by “rapidlydeveloping low pressure” instead.
Low

Low

Low

Low

TyphoonFrom typhoon to tropical
cyclone, sudden development ofcentral pressure is 944 hPa

29th at 12:00

30th at 12:00

Fig. 20 From the Japan Weather Association22

Jan. 2013 at 15:15

HiHiLowLowslowly

Low

-34hp
14th at 18:00
972hp
13th at 16:00
1002hp-30hp

Bomb Cyclone
HiFig. 21 From the Japan Meteorological Agency website

Regarding these types of cyclones, since they can cause rough weather and sea conditions
to be worse than expected, in order to safeguard one’s ship, close attention must be paid tothem.

４−２

Tropical Cyclones and Typhoons

４−２−１ Classiﬁcation and Naming of Extra-tropical Cyclones
Tropical cyclones refer to cyclones generated in tropical or subtropical waters, and thegeneration of which requires a continuous supply of water vapour energy. Therefore, tropical
cyclones are formed in sea areas where sea surface temperatures exceed 26 degrees Celsius.
Generally, this means tropical cyclones are formed in sea areas between 5 and 20 degrees
latitude, excluding equatorial waters (Figs. 22 and 23).

23

Typhoon No.21

Typhoon No.22

Fig. 22

Fig. 23

From the Japan Weather Association

From the Japan Meteorological Agency website

Extra-tropical cyclones are internationally classified into four categories according to themaximum wind speed as shown in Fig. 24.

24

International Tropical Storm Classiﬁcation
SymbolTDTSSTSTClassification

TropicalDepression

Tropicl Storm

SevereTropicalStorm

Typhoon(m/sec)〜 17.1

17.2 〜 24.4

24.5 〜 32.8

32.7 〜

Knots〜 33

34 〜 47

48 〜 63

64 〜

Beaufort Scale(Wind Speed)

〜7

8〜9

10 〜 1112 〜

East Pacific Ocean
and Caribbean Sea

Tropical
Depression

Tropicl Storm

Severe Tropical
Storm

Hurricane

Max Eind(m/sec)

Japan
Extra-tropical(Japan MeteorologicalCyclone
Agency)Indian Ocean andSouth Pacific Ocean

TropicalDepression

Fig. 24Taifu＊（Typhoon）

CycloneClassiﬁcation of Extra-tropical CyclonesAs can be seen in the chart, those that have the maximum wind speed of more than 64kts(32.7m/sec) and can be found in the western part of the North Pacific Ocean are called
Typhoons, while those that exist in the eastern part of the North Paciﬁc Ocean in the vicinityof the Caribbean Sea are referred to as Hurricanes.
*In Japan, those that have a maximum wind speed of more than 34kts (17.2m/sec) are called
Taifu (Typhoon). In the Indian Ocean and the western South Paciﬁc, they are referred to as“cyclones”. In general, they are called tropical cyclones (Fig.25).

25

Fig. 25 The Naming of Extra-tropical Cyclones Japan Captainsʼ Association,４−２−２

The Generation Mechanism of Extra-tropical Cyclones

Northeast and southeast

Extreme Easterly Wind

trade winds in the northernWesterly Windand southern hemispheres
blow into the equatorial

H i g h la

NE Tradewindarea and form a troughNorth PoleM e d iu mt i t u d e L o w p re s s u r e z o n elatitudes High pressure zone

(Figs. 26 and 27).
Equator Low pressure zone

EquatorMedium latit

SE Trade windudes High pressure zone

High latitude Low pressure zone

Westerly Wind
Extreme Easterly WindSouth PoleFig. 2626

Fig. 27 Japan Captainsʼ Association, DVD

This trough is known as an “equatorial trough” or “Intertropical Convergence Zone” (ITCZ).
This TCZ usually migrates westward while undulating north to south. When this undulation
becomes unstable and increases, it forms an eddy that generates a tropical cyclone (Fig. 28).

Fig. 28 Japan Captainsʼ Association, DVD27

The potential energy of extra-tropical cyclones can be generated by the diﬀerence between
the temperatures of those air masses. This energy source for a tropical cyclone is the latent
heat from a continuous supply of water vapour that is discharged when humid air rises, iscooled, and then condensed into droplets. (See Note:1 on P.7)
In other words, when humid air near a sea happens to start rising (Fig. 29), especially because
it tends to become an updraft as the cyclone converges in the ITCZ, once the water vapours
start condensing, cumulus and cumulonimbus clouds are produced (Fig. 30).

Humid air starts rising

Cumulonimbus is formed

Fig. 29 Japan Captainsʼ Association, DVDFig. 30 Japan Captainsʼ Association, DVD

Because of the emission of latent heat, the temperature increases, compared with the region
without clouds. Then, the rising warm air becomes less dense than the surrounding atmosphere,
as it continues to rise. This lowers the central atmospheric pressure of the tropical cyclone andkeeps the cyclone developing. In these unstable atmospheric conditions, several cumulonimbus
clouds of approximately 10 km in scale form horizontally (Fig. 31) and these eﬀects produce
tropical cyclones exceeding approximately 100 km in scale horizontally (Fig. 32).

28

Approximately 10km in scale
and formed horizontally

Approximately 100km in scale
and formed horizontallya tropical cyclone is generated

Fig. 31 Japan Captainsʼ Association, DVDFig. 32 Japan Captainsʼ Association, DVD

Tropical cyclones produced in tropical waters can develop into typhoons (shown in Table24), if the maximum wind speed within the area develops to exceed 34 kts (17.2m/sec) under
the Japanese classification. As to what kinds of cloud mass can develop into typhoons isstill yet to be revealed. However, since it is understood that the energy source of a typhooncomes from sea water surface vapour, it follows that the higher the temperature at an area
where the typhoon begins, the stronger the typhoon will be. In addition, for a strong typhoon
to develop, it needs to pass an area of sea where the seawater temperature is more than 28degrees centigrade.
On the contrary, if a typhoon reaches an area of sea that is less than 28℃ (or less than 26℃
to be exact) or ends up on land, it will start to weaken because of the typhoon’s energy
excretion. Later, it attenuates and the extra-tropical cyclone ﬁnally decays.

Fig. 33 Photograph Image (not Typhoon generated)

４−２−３

Structure of Extra-tropical Cyclones

Typhoons are huge atmospheric eddies with diameters ranging between hundreds tothousands of kilometres. Inside the eddy, strong updrafts form cumulonimbus. The “eye ofa typhoon” is formed by a downdraft at the centre of the typhoon, and the heaviest rain andstrongest wind occur below the “eye wall.” On the other hand, in the vicinity of the tropopause above the eye of the typhoon, an eddy of cirrus clouds is formed that ﬂows outward
clockwise due to the Coriolis’ force which is formed as a result of the earth’s rotation (Figs.
34 and 35) .
29

Fig. 34 Japan Captainsʼ Association, DVDkm
12Cumulonimbus
CumulusStrong upward airﬂow

Cumulonimbus Eye Wall

Typhoon eye

Height4Descending
airflow

8Cirrus（Silk cloud）ﬂowing outward

Bottomblowing

0100

200
Distance from center（km）

Fig. 35 Cross-sectional diagram of a typhoon

30300

km

“The spiral cumulonimbus band” extending from the centre of a typhoon is known as thespiral band. Within the spiral band, strong wind and heavy rain are generated by the strong
general air flow of the typhoon and strong downdrafts derived from huge cumulonimbus.
Attention should be paid to this weather condition and poor visibility (Figs. 36 and 37).

Cross sectional diagram of typhoon
Eye of typhoon

Downward ﬂow

Eyewall

Height:10 -15 km

Upward motion

Spiral band

Fig. 36 From the Japan Meteorological Agency website

Fig. 37 Japan Captainsʼ Association, DVD31