Typhoons in the

Philippines

OVERVIEWIn the Philippines, tropical cyclones

(typhoons) are called bagyo. Tropical cyclones entering the Philippine area

of responsibility are given a local name by the

Philippine Atmospheric, Geophysical and Astronomical Services Administration

(PAGASA), which also raises public storm signal warnings as deemed

necessary.

Around 19 tropical cyclones or storms enter the Philippine Area Of Responsibility in a typical year and of these usually 6 to 9 make

landfall.

Why “Bagyo”?The term bagyo, a Filipino word meaning typhoon arose after a

1911 storm in the city of Baguio had a record rainfall of 46 inches

within a 24-hour period.

Variability in ActivityOn an annual time scale, activity

reaches a minimum in May, before increasing steadily through June,

and spiking from July through September, with August being the

most active month for tropical cyclones in the Philippines. Activity

falls off significantly in October.

The most active season, since 1945, for tropical cyclone strikes on the island archipelago was 1993 when nineteen tropical cyclones moved through the country (though there were 36 storms that were named by PAGASA). There was only one

tropical cyclone which moved through the Philippines in 1958.

The most frequently impacted areas of the Philippines by tropical cyclones are

northern Luzon and eastern Visayas. A ten year average of satellite determined

precipitation showed that at least 30 percent of the annual rainfall in the northern Philippines could be traced to

tropical cyclones, while the southern islands receive less than 10 percent of

their annual rainfall from tropical cyclones.

Public Storm Warnings and Signals

The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) releases tropical cyclone

warnings in the form of Public Storm Warning Signals. An area

having a storm signal may be under:

PSWS #1 - Tropical cyclone winds of 30 km/h (19 mph) to

60 km/h (37 mph) are expected within the next 36 hours.

(Note: If a tropical cyclone forms very close to the area,

then a shorter lead time is seen on the warning bulletin.)

PSWS #1 - Tropical cyclone winds of 30 km/h (19 mph) to 60 km/h (37 mph) are expected within the next 36 hours. (Note: If a tropical cyclone forms very close to

the area, then a shorter lead time is seen on the warning bulletin.)

PSWS #2 - Tropical cyclone winds of 60 km/h (37 mph) to 100 km/h (62 mph) are expected within the

next 24 hours.

PSWS #3 - Tropical cyclone winds of 100 km/h (62 mph) to

185 km/h (115 mph) are expected within the next 18 hours.

PSWS #4 - Tropical cyclone winds of greater than

185 km/h (115 mph) are expected within 12 hours.

Anatomy of a Typhoon

Eye – At this part, the Weather has no rain but sunny sky.

Inner Bands – The rain is harder and the wind is weaker.

Outer Bands – The rain is weaker but the wind is stronger.

Edge – both winds and rains has the same power.

Formation

Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its

own seasonal patterns. On a worldwide scale, May is the least active month, while

September is the most active month. November is the only month in which all

the tropical cyclone basins are active.

LocationsMost tropical cyclones form in a worldwide

band of thunderstorm activity near the equator, referred to as the Intertropical Front (ITF), the

Intertropical Convergence Zone (ITCZ), or the monsoon trough. Another important source of

atmospheric instability is found in tropical waves, which contribute to the

development of about 85% of intense tropical cyclones in the Atlantic ocean and become most of the tropical cyclones in the Eastern

Pacific.

The majority forms between 10 and 30 degrees of latitude away of the

equator, and 87% forms no farther away than 20 degrees north or south. Because the Coriolis effect initiates and maintains

their rotation, tropical cyclones rarely form or move within 5 degrees of the equator, where the effect is weakest. However, it is still possible for tropical systems to form within this boundary

Movements and Tracks

PathsMost tropical cyclones form on the side of the subtropical ridge closer to the equator, then

move poleward past the ridge axis before recurving north and northeast into the main belt of the Westerlies. When the subtropical

ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of

Japan and Korea tend to experience much fewer September–November tropical cyclone

impacts during El Niño and neutral years.

During El Niño years, the break in the subtropical ridge tends to lie near

130°E which would favor the Japanese archipelago. During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the

landfall threat to China and greater intensity to Philippines.

Typhoon paths follow three general directions.Straight track (or straight runner). A general

westward path affects the Philippines, southern China, Taiwan, and Vietnam.

A parabolic, recurving track Storms recurving affect eastern Philippines, eastern China,

Taiwan, Korea, and Japan.Northward track. From point of origin, the storm follows a northerly direction, only

affecting small islands.

Coriolis EffectThe Earth's rotation imparts an

acceleration known as the Coriolis effect, Coriolis acceleration, or colloquially,

Coriolis force. This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents. The poleward portion of a tropical cyclone contains easterly winds, and the Coriolis

effect pulls them slightly more poleward.

The westerly winds on the equatorward portion of the cyclone pull slightly towards the

equator, but, because the Coriolis effect weakens toward the equator, the net drag on

the cyclone is poleward. Thus, tropical cyclones in the Northern Hemisphere usually turn north

(before being blown east), and tropical cyclones in the Southern Hemisphere usually

turn south (before being blown east) when no other effects counteract the Coriolis effect.

LandfallOfficially, landfall is when a storm's center (the center of its circulation, not its edge)

crosses the coastline. Storm conditions may be experienced on the coast and inland hours before landfall; in fact, a

tropical cyclone can launch its strongest winds over land, yet not make landfall; if this occurs, then it is said that the storm

made a direct hit on the coast.

Fujiwara EffectWhen two cyclones approach one another, their centers will begin orbiting cyclonically

about a point between the two systems. The two vortices will be attracted to each other,

and eventually spiral into the center point and merge. When the two vortices are of unequal

size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit

around it. This phenomenon is called the Fujiwhara effect

DissipitationA tropical cyclone can cease to have tropical characteristics in several different ways. One

such way is if it moves over land, thus depriving it of the warm water it needs to power itself, quickly losing strength. Most strong storms lose their strength very rapidly after landfall

and become disorganized areas of low pressure within a day or two, or evolve into extratropical

cyclones. There is a chance a tropical cyclone could regenerate if it managed to get back over

open warm water.

ClassificationTropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third

group of more intense storms, whose name depends on the region.

A tropical depression is an organized system of clouds and thunderstorms with a defined,

closed surface circulation and maximum sustained winds of less than 34 knots (39 mph). It has no eye and does not typically have the organization or the spiral

shape of more powerful storms. However, it is already a low-pressure system, hence the name "depression" In the Philippines, the practice is to name tropical depressions from their own naming convention when the depressions are

within the country's area of responsibility.

A tropical storm is an organized system of strong thunderstorms with a defined

surface circulation and maximum sustained winds between 34 knots

(39 mph) and 64 knots (74 mph). At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually

present. Government weather services first assign names to systems that reach this intensity (thus the term named storm).

A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as

opposed to a depression or storm) is a system with sustained winds of at least 34 metres per second (66 kn) or 74 miles per

hour (119 km/h).A cyclone of this intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of circulation. The eye is often

visible in satellite images as a small, circular, cloud-free spot.

Surrounding the eye is the eyewall, an area about 16 kilometres (9.9 mi) to 80

kilometres (50 mi) wide in which the strongest thunderstorms and winds circulate around the storm's center.

Maximum sustained winds in the strongest tropical cyclones have been

estimated at about 85 metres per second (165 kn) or 195 miles per hour

(314 km/h).

Name SourcesThe list of names consists of entries from 17 East

Asian nations and the United States who have territories directly affected by typhoons. The

submitted names are arranged into five lists; and each list is cycled with each year. Unlike tropical cyclones in other parts of the world, typhoons

are not named after people. Instead, they generally refer to animals, flowers, astrological

signs, and a few personal names. However, PAGASA retains its own naming list, which does

consist of human names.

Therefore, a typhoon can possibly have two names. Storms that cross the date line from the central Pacific retain their original name, but the designation of hurricane becomes typhoon. In Japan, typhoons are also given a numerical

designation according to the sequence of their occurrence in the calendar year.