Impact and Assessment of Tropical Cyclones

From a physical hazard to an impact

A tropical cyclone brings powerful winds, intense rainfall and potentially a storm surge: a rise in sea level driven by the storm. These are hazards. Their impacts are the consequences for people and environments. A flooded road, for example, is physical damage; its closure can also prevent injured people reaching hospital.

It helps to follow the chain of consequences. Primary impacts result directly from the storm, such as winds tearing roofs from houses or floodwater destroying crops. Secondary impacts develop from that initial damage: families lose shelter, farmers lose income and food supplies become disrupted. Secondary impacts can continue long after the wind and rain have stopped.

How people's lives and livelihoods are affected

Strong winds can damage homes, businesses and electricity lines. People may be injured by falling debris, while damaged power supplies disrupt homes and essential services. Intense rainfall, storm surges and coastal flooding can drown people and damage buildings. Landslides can bury homes and block roads, cutting communities off from assistance.

These effects overlap, but three categories help organise them:

• Social impacts affect people's lives: deaths, injuries, displacement, interrupted education and disrupted healthcare.
• Economic impacts affect money and livelihoods: damaged property, lost crops, business closures, unemployment and rebuilding costs.
• Environmental impacts affect nature: damaged habitats, lost vegetation, erosion and polluted water.

One event can produce all three. If flooding destroys farmland, the loss of crops is an economic impact, a resulting food shortage is a social impact, and damaged soils or polluted water are environmental impacts. Floodwater that carries sewage into drinking-water supplies can also increase the risk of waterborne disease.

Environmental damage can prolong human losses

Winds can strip leaves from trees or blow trees down, damaging habitats. Flooding can carry sewage and oil into rivers and coastal ecosystems. Storm surges can erode beaches and wash away vegetation; saltwater flooding can damage crops.

These are not separate from people's wellbeing. Damage to farmland and fishing environments threatens food supplies and livelihoods. Beach erosion can leave the coast more exposed to later erosion. Assessing a cyclone therefore means considering both its immediate destruction and the consequences that persist afterwards.

Haiyan: assessing the scale and duration of damage

Typhoon Haiyan struck the Philippines on 8 November 2013. Its winds and storm surge devastated the coastal city of Tacloban on the island of Leyte. Across the affected areas, more than 6,000 people died, around 1.1 million homes were lost, and more than 4 million people were displaced.

These figures describe different dimensions of the disaster. Deaths show the irreversible human cost; housing losses show physical destruction; displacement shows how many people had to leave their homes. Two months later, about 21,000 families remained in evacuation centres, demonstrating that the social impact continued beyond the storm itself.

Major rice- and sugar-producing areas were damaged, linking agricultural losses to disrupted livelihoods and food supplies. Forests and trees were lost, while oil and sewage polluted local ecosystems.

To judge severity, choose a clear basis: loss of life, number of people affected, geographical extent, duration or difficulty of recovery. Financial losses alone are not enough. Hurricane Sandy affected the USA in October 2012 and caused estimated property damage of around US$65 billion. That large monetary total measures damaged assets, not the same thing as Haiyan's displacement or death toll. A balanced assessment considers several measures rather than declaring one storm worse from a single statistic.

Using social media as local evidence

Social media can provide rapid accounts of damage and people's needs. During Haiyan, Twitter carried information about storm damage and relief activities. Relief workers shared photographs from affected areas, while other users expressed fear, support or political opinions. These types of post are useful for different purposes: a photograph may document local damage, whereas an expression of sympathy is not evidence of its extent.

For any post, establish who produced it, when it was posted, where the reported event happened and whether it is a first-hand account. Trace reposted material back to its original author and compare the claim with independent reporting or official information.

For example, a post reporting that a road is blocked would suggest an access problem at that location. It would not establish that all roads across the region were blocked. Repeated sharing of that same post would still represent one report, not many independent observations.

Haiyan attracted posts from around the world. This means online activity can reflect international attention as well as local suffering. A decline in posting does not necessarily mean that people have recovered.

Reading satellite evidence of environmental change

Satellite images provide a broad view of places that may be difficult to reach on the ground. To assess damage, compare images of the same area before and after the storm, checking their dates, colour key and level of detail. Match recognisable features such as the coastline before looking for changes.

NASA's assessment of Tacloban used ASTER satellite images dated 3 April 2004 and 15 November 2013. In this type of false-colour image, vegetation appears red, built-up areas white or silver, bare ground tan, and water or shadows black. Colours therefore need interpreting through the key: red does not mean fire.

Consider a comparison in which a coastal patch changes from red to tan. The observation is that vegetation cover has been replaced by exposed ground. Storm-surge damage is a possible explanation, but the colour change alone does not prove the cause. Similarly, a new dark patch might indicate floodwater, but shadows must be ruled out.

NASA identified coastal vegetation loss and mud-covered ground around Tacloban as evidence of storm-surge damage. However, the nine-year gap between the images complicates interpretation: land clearance and urban growth could also explain some changes. The images were too broad in scale to identify individual destroyed city blocks. Ground photographs and other information were needed to strengthen conclusions.

This method transfers to other satellite comparisons, but always use the relevant image's own key. Satellite evidence can show the distribution of damage; it cannot directly count deaths or explain every change.

Combining evidence into a judgement

Socio-economic data turn local observations into measures of scale. Use death and injury totals, displaced population, damaged housing, employment losses and financial costs. Check whether each figure covers a city, a region or the whole country, and whether it is an early estimate or a later total.

Combine the sources because each answers a different question. Social media can reveal local experiences and urgent needs; satellites show the spatial extent of physical change; statistics quantify human and economic losses. Haiyan's housing and displacement figures demonstrate a major social disaster, while evidence of vegetation loss and pollution demonstrates environmental damage. Continuing displacement shows why its impact cannot be assessed only at the moment of landfall.