WeatherPhilippines supports PAGASA in increasing awareness and educating Filipinos about natural calamities and disaster mitigation during the Typhoon and Flood Awareness Week. This year’s theme is “Science Based Information for Safer Nation against Typhoon and Flood.” In WeatherPhilippines, we believe that arming the people with sufficient weather knowledge is a key to empowering communities so they could start creating proactive disaster initiatives.
This week, we are featuring basic weather terminologies related to tropical cyclones in order to familiarize everyone with these types of weather disturbances that frequently strike the country.
Tropical Cyclone Basics
A Tropical Cyclone (known as “Bagyo” in the Philippines) is the global generic term for an intense circulating weather system over tropical seas and oceans. It is accompanied with very strong winds, heavy rains, and large ocean waves. Its wind circulation rotates counter-clockwise in the Northern Hemisphere (see Figs. 1 and 2). On the average, there are about twenty (20) Tropical Cyclones which enter the Philippine Area of Responsibility (PAR) every year, and about half of that makes landfall.
How Do Tropical Cyclones Form?
There are a couple of factors (see Fig. 3) that could contribute to the formation of a Tropical Cyclone here in the Philippine Sea and Western Pacific Ocean. These are:
- Warm Sea Surface Temperature (SST) of at least 26.5°C with a depth of 150 feet and high moisture or humidity present in the atmosphere. The heat from the sea is therefore the main energy source for Tropical Cyclones.
- Presence of the Intertropical Convergence Zone (ITCZ). The ITCZ plays an important role in the formation of Tropical Cyclones as it delivers convergence of Northeasterly and Southeasterly or Southwesterly Trade winds. Its convergence will trigger a rotation of low-level winds, which then develop into Tropical Cyclones – if other elements are present.
- Existence of Tropical Disturbances (also known as Low Pressure Areas or LPAs) within the ITCZ. When the ITCZ is very active, multiple tropical disturbances occur and it aids for development of Tropical Cyclones.
- Weak vertical wind shear or light winds in the upper atmosphere. If wind speeds in the upper atmosphere (20-50,000 feet ASL) are low (<20 km/hr), a tropical cyclone can develop rapidly usually within one to two days.
Whenever a typhoon makes landfall, loss of lives and the destruction of properties are mainly due to the hazards generated within its circulation.
Here is the list of hazards associated with typhoons:
- Storm Surge & Coastal Flooding
- Strong/High Winds
- Extreme Rainfall & Inland Flooding
- Eyewall Mesovortices/Miniswirls
- Landslides, Mudslides, or Mudflows
Common Misconceptions about Typhoons
In the Philippines, when people start hearing about an upcoming Tropical Cyclone most of them refer to it as “bagyo”, but there are four (4) different classifications of these large wind and rain systems. In every category, the risks differ from one another, and there could be instances when weaker ones such as Low Pressure Area (LPA), Tropical Depression, or Tropical Storm cause great havoc to lives and property given their capacity to draw torrential rains than high winds.
Further, in the light of the massive casualty and devastation in the aftermath of Super Typhoon Yolanda (Haiyan) in November 2013, Filipinos now have a benchmark on what a catastrophic typhoon can do. The tragic death of more than 8,000 people in the Visayas region became a constant reminder of the power of nature. In most cases, deaths were associated to coastal flooding generated by storm surges that came roaring onshore as the center of the Haiyan raced overland leaving tens of thousands, without homes and livelihood. This cataclysmic disaster affected millions across Visayas and Palawan and its impacts just cannot be ignored.
To date, the total cost of devastation run into several billions of Philippine Peso, while rebuilding and relocation programs continue to take shape on ground zero and other afflicted areas in the country.
Drowning is a major cause of death during a massive storm and in a country of many, many people living along vulnerable coastlines, the risk for mass casualty events like this cannot be ruled out in the future.
Disasters can strike at any moment, that is why it is important to understand the dynamics behind it which enables us to act in a timely manner and to prepare adequately for its impacts.
Storm Surge and Coastal Flooding
Storm Surge happens when ocean waves are being pushed towards the shore by the force of the wind and the intense low pressure of a tropical cyclone (see Figs.5 and 6).
A storm surge can produce Coastal Flooding along areas with shallow coastlines or beachfront areas. One good example is the storm surge generated by Typhoon Pedring on September 2011, which flooded Roxas Boulevard in Manila, damaging sea walls and structures facing the Manila Bay.
Storm surges can start flooding coastal areas already, 3 to 6 hours prior to landfall. In fast moving typhoons, storm surge can occur very suddenly with the passage of the eye alone. A Super Typhoon can generate storm surge height of more than 20 feet above sea level.
Tropical Cyclone Classification
There are four (4) classifications of tropical cyclone depending mainly on its strength or maximum sustained winds (based on 10-minute average):
- Tropical Depression – 45 to 61 km/hr
- Tropical Storm – 62 to 88 km/hr
- Severe Tropical Storm – 89-117 km/hr
- Typhoon – 118 to 219 km/hr
- Super Typhoon – 220 km/hr or higher
The classifications mentioned above are those located over the Western North Pacific Ocean (west of the International Dateline) and up to the Malay Peninsula/Gulf of Thailand.
Landslides, mudslides, or mudflows
This type of hazard is common along steep-slopes of mountains and volcanoes. Tropical Cyclones can release more than 20 billion cubic tons of rain within its circulation. When high amounts of rainfall fall along the mountain slopes, the saturated rocks, soil, and loose tree logs will start mixing up with water from the storm’s rainfall. Eventually, this could create landslides, mudslides, or mudflows along its way. This type of hazard also contributes high fatalities especially along riverbanks and slope areas.
Here in the Philippines, most deaths aside from flooding were mainly due to landslides, mudslides, and mudflows. An example of this was during the onslaught of Super Typhoon Reming (Durian) on November 30, 2006 – wherein the storm’s 400mm rainfall has triggered loose volcanic rocks cascading downward from the slopes of Mayon Volcano, and sent mudflows (lahars) rushing across portions of Albay including Legazpi City. About a thousand people perished in this tragic event.
Reducing Risks Through Emergency Preparedness
Natural calamities are like thieves that come in the middle of the night and catch you off-guard. Being prepared for such unfavorable events does not need to be expensive or tedious. Here are some ways we can do to prepare for disasters:
- Always make sure that your house has an emergency kit. Generally, it should contain necessary survival items such as first aid kit, flashlight, batteries, drinking water and empty containers for water storage, infant needs (if you have a small child), sanitary supplies, and instant meals enough to last for at least three days.
- Create a general plan of what to do in case a calamity strikes. This should include where to go and what to take with you when evacuating. Make sure that everyone in the family knows how to execute the plan, even small children.
- Join community emergency drills to be updated with the latest recommended survival tips.
- Keep all your important documents in one secure place or container that cannot be penetrated by water. Always take note where it is so you can grab it easily during times of disaster.
- Constantly update the emergency contact numbers. If possible, print and laminate it with all family members’ numbers and provide a copy to everyone.
Surviving When Disaster Strikes
Here are some survival strategies that you should keep in mind when faced with disaster:
- Determine hazard warnings assigned to your area. Constantly check different media channels (radio, television, and social media) for security advice and warnings.
- Avoid going near bodies of water during extreme weather and rain alerts.
- When authorities issue a directive for evacuation from your area, be quick about it and there should be no second thoughts. Immediately go to the identified evacuation areas with your emergency kits and other important belongings.
- Contact emergency numbers in case you need assistance in evacuation, or if a family member needs medical attention. Secure that mobile phones’ batteries are full just in case there are power interruptions.
While we cannot predict some weather systems such as thunderstorms, bulletins issued by PAGASA, as well as daily updates from private weather organizations such as WeatherPhilippines may be used to anticipate and prepare for possible impacts of heavy rainfall brought by various weather conditions. Disaster preparedness and education help a lot in mitigating damage to property and loss of lives. Ultimately, it pays to be #WeatherWiser.
(Contributed by: Gary Padgett as published on Typhoon2000.com)
David Michael V. Padua, Sr. Typhoon Specialist
NOAA/The COMET Program
Created by: WPF Meteorology and Sustainability Team