Climate and weather: Tropical cyclones

Tropical cyclones are classified into categories (e.g., Category 1 to 5) based on their intensity and wind speeds. As a cyclone intensifies, its cloud cover becomes more compact. On synoptic weather maps, the movement of air (anti-clockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere around a low pressure) and pressure gradients help identify and assess the storm's stage. A dissipating cyclone over land will show a gentler pressure gradient and a larger, less defined eye. Madagascar often acts as a natural barrier, reducing the intensity of cyclones before they reach South Africa.

Tropical cyclones are intense low-pressure storms that cause significant wind and flood damage. They are known by different names depending on their location, such as 'hurricane' in the North Atlantic and 'typhoon' in the North Pacific. They develop at the ITCZ (Intertropical Convergence Zone) but outside 5 degrees north and south of the equator, where the Coriolis force is required for their cyclonic movement. They typically occur in tropical regions over warm oceans, moving from east to west, mostly in late summer and early autumn. The main energy source is latent heat from cooling and condensing air, and ocean temperatures must be at least 26.5°C.

Key conditions for formation include a large sea surface area with temperatures above 26.5°C, sufficient Coriolis force for a cyclonic vortex, small variations in vertical wind speed to prevent disruption, a pre-existing weak low-pressure area, upper-air divergence above the sea, and a good source of latent heat from high humidity and condensation to sustain the storm. Calm atmospheric conditions are crucial for intensification.

The formative stage involves strong updrafts of air due to high temperatures, intensifying the low pressure. The cyclone is not yet large and lacks a clear 'eye'. The pressure is generally above 1000 millibars. The immature stage sees a further increase in updrafts due to latent heat release, leading to even lower pressure (below 1000 millibars) and the formation of a distinct 'eye'. Wind speeds intensify. The mature stage is characterized by a very strong pressure gradient, extremely low central pressure (e.g., 956 millibars), hurricane-strength winds (up to 250-300 km/h), and a well-developed, calm, clear eye. The 'forward left-hand quadrant' is the most destructive due to combined wind forces. Cyclones can cover vast distances and cause storm surges, massive bulges of seawater that lead to coastal flooding.

The dissipating stage occurs when the cyclone moves over land (due to friction and reduced humidity/latent heat) or into cooler subtropical waters. The pressure in the center increases, the storm weakens, and eventually dissipates. A cross-section of a mature cyclone reveals the eye wall (vortex), which surrounds the calm eye. The eye wall is characterized by intense rising and cooling air, forming massive cumulonimbus clouds up to 15 km high, bringing torrential rainfall and the highest wind speeds. The eye itself is calm and clear with subsiding air, resulting in no cloud formation and high humidity. Rain bands extend outwards from the eye wall, causing additional rainfall.

Tropical cyclones cause significant damage through storm surges, which are large bulges of water pushed onto the shore by strong winds, potentially rising over 4.5 meters above normal tides. Inland flooding occurs due to torrential rainfall, continuing for days even after winds subside. Flash floods are rapid-onset events that can reach heights of 10 meters, moving heavy debris and destroying infrastructure. Urban areas are particularly vulnerable to flooding due to reduced water infiltration from concrete surfaces.

Primary impacts are direct consequences such as deaths, destruction of buildings and infrastructure (roads, transport), and environmental damage like coastal erosion and wildlife mortality. Secondary impacts are long-term effects that follow the primary event, including homelessness, water pollution, waterborne diseases, loss of jobs, high insurance costs, mass migration, and food and water shortages. Poorer countries often face more severe secondary impacts due to limited resources for recovery.

Precautionary measures include evacuating people to higher ground, establishing early warning systems, utilizing GIS monitoring to track cyclone paths, ensuring accessible roads for evacuation, preparing food supplies, and having emergency services (ambulances, hospitals) ready. Some people even elevate their houses in prone areas. Tropical cyclones are increasing in frequency and intensity, with many scientists attributing this to global warming, as higher ocean temperatures provide more energy for stronger storms. This trend highlights the urgent need for global action on climate change.