Introduction, Monitoring and Measurement of Tropical Cyclones

Why a tropical cyclone is a weather hazard

A tropical cyclone is a powerful rotating tropical storm. It becomes a natural weather hazard because its winds, rainfall and effects on the sea can threaten people, buildings and the environment. A cyclone’s danger is not limited to its strongest winds: flooding can be deadly even in a lower-category hurricane.

High winds can damage roofs, snap branches, uproot trees and bring down electricity poles. These effects connect together: fallen trees can block roads, while damaged power lines interrupt electricity supplies. The strongest hurricane winds can cause roofs and walls to fail and leave residential areas isolated.

How intense rainfall causes flooding and landslides

Intense rainfall can deliver water faster than the soil can absorb it or rivers can carry it away. Water that does not soak into the ground flows over the surface as runoff, collecting in low-lying areas and entering streams and rivers. River levels can rise until water overflows the banks. Rapidly accumulating water can also cause flash flooding, inundating roads and homes with little warning.

Concrete and asphalt surfaces allow little water to soak into the ground, increasing runoff in urban areas. Steep slopes move runoff downhill quickly. A slow-moving cyclone can bring rain to the same area for longer, increasing the amount of water that must drain away. Rain-induced flooding can therefore threaten places well inland, not just the coast.

Heavy rainfall can also trigger landslides: the movement of rock, soil or debris down a slope under gravity. As water saturates slope material, it adds weight, weakens the material and reduces friction. If the forces resisting movement become too weak to hold the material in place, the slope fails. Sliding material or flowing mud and debris can bury buildings and block roads. This explains why intense cyclone rainfall creates an additional hazard in steep terrain.

How storm surges cause coastal flooding

A storm surge is an abnormal rise in sea level caused by a storm. Its main cause is strong winds pushing seawater towards the coast, where it piles up. The cyclone’s low atmospheric pressure makes a smaller contribution to the rise in water level.

The raised sea level can flood low-lying coastal land. Large waves on top of the surge can damage sea defences or spill over them, adding to the flooding. Water and waves can erode beaches, undermine building foundations and damage coastal roads and bridges. A surge arriving at high tide can be especially dangerous because the sea level is already high.

Storm surge and coastal flooding are closely connected, but they are not identical: the surge is the rise in sea level, while coastal flooding is the inundation of normally dry land. Rainfall can add further water to an already flooded coastal area.

These hazards need to be considered together. Rain-induced flooding comes from rainfall over land, whereas surge-related flooding comes from the sea. Knowing a cyclone’s wind category alone does not tell us how dangerous its rainfall, coastal flooding or landslides will be.

Measuring wind intensity with the Saffir–Simpson scale

The Saffir–Simpson Hurricane Wind Scale assigns hurricanes to five categories using their maximum sustained wind speed. It estimates potential wind damage; it is not a combined score for every cyclone hazard.

Categories 3, 4 and 5 are called major hurricanes. However, every hurricane category can produce life-threatening winds and deadly flooding.

To determine a category from weather data, identify the maximum sustained wind speed, check its units and find the interval containing that value. This is a classification task, rather than a calculation using a formula.

For example, a weather report giving a maximum sustained wind speed of 190 km/h would indicate Category 3, because 190 lies within 178–208 km/h. This tells us about wind intensity and potential wind damage. It does not provide a rainfall total or predict the height of a storm surge.

Reading graphs of climate change

Weather describes conditions at a particular time; climate concerns average patterns over a long period. Graphs make it easier to identify long-term changes while recognising variation between periods.

Start with the title and axes. Establish what is measured, its units, the geographical area covered and the time span. Then describe the overall direction of change, support it with values, and identify any exceptions to the pattern.

Data for Interpreting an overall temperature trend

This constructed graph represents a fictional location, not observed climate data. Its mean temperature rises overall from 12.0°C in the 1980s to 13.0°C in the 2020s, an increase of 1.0°C. Yet the 2000s are cooler than the 1990s. An overall upward trend does not mean that every successive period must be warmer.

When comparing two trend lines, distinguish their height from their gradient. A higher temperature line indicates warmer conditions; a steeper upward gradient indicates faster warming, provided the axes and scales are comparable. A graph for one location cannot, on its own, establish the pattern for the whole world or explain the cause of the change.

Following cyclone movement using GIS

A geographical information system, or GIS, links data to locations. To track a cyclone, each observation needs a position, a date and time, and an identifier showing which cyclone it belongs to. The identifier prevents observations from different storms being joined into one misleading track.

A GIS can display these observations over a base map, connect positions belonging to the same cyclone and replay them in time order. Selecting a track allows the user to inspect its observations. A time window can show only positions recorded during a chosen period, making a crowded map easier to read.

In this fictional local grid, one cyclone moves from A to B to C to D at six-hour intervals. Its path initially heads east-north-east, then turns more towards the north. The changing direction is established by comparing successive positions, not simply by looking at the shape of the whole line.

The grid demonstrates the method only: it is not a real cyclone track, location or GIS output. With authentic GIS data, use the map’s coordinate system, scale and time information, then examine where the track passes in relation to actual coastlines and settlements. Lines between observations join recorded positions; they do not supply additional measurements of the route between them.

The ArcGIS hurricane-tracking example uses Atlantic observations from 2000, grouped by hurricane name and linked to date and time. Its records contain four observations per day. This illustrates why observation frequency matters: many dots may represent repeated measurements of one storm rather than many different storms.

Tracking past positions helps describe movement. Those recorded positions must be distinguished from a forecast of where the cyclone may move next.

Main hazards

• High winds: roof damage, fallen trees, blocked roads and damaged power lines.
• Intense rainfall: water exceeds absorption and drainage capacity → runoff and rising rivers → flooding.
• Storm surge: winds push seawater towards the coast → raised sea level → coastal flooding; high tide and waves can worsen it.
• Landslides: rain saturates slope material, adding weight and reducing strength and friction → downhill movement.

Wind category

Saffir–Simpson uses maximum sustained wind speed only:

Categories 3–5 are major hurricanes. Category does not measure rainfall, storm surge or total damage.

Climate graphs

Check axes, units, location and dates. Describe the overall trend with figures and note exceptions.

• Line height: the value of the variable.
• Gradient: its rate of change.
• Short-term variation does not necessarily reverse a long-term trend.

GIS tracking

Use position + date/time + cyclone identifier. Group observations by cyclone, read them chronologically and compare successive positions to establish direction. Distinguish recorded tracks from forecasts.