Tropical cyclones and severe storms

Tropical cyclones are violent storms that form over warm tropical oceans and move to the coasts, bringing violent winds, very heavy rain and storm surges: one of the most devastating natural hazards. Names by region: cyclones (Indian Ocean), hurricanes (Atlantic), typhoons (western Pacific and South China Sea), willy-willies (western Australia).

Conditions for formation:

• a large sea surface warmer than 27°C;
• the Coriolis force (so not right at the equator);
• small changes in wind speed with height;
• a pre-existing weak low or low-level cyclonic circulation;
• upper-level divergence above the system.

Energy and life: the storm draws its energy from condensation in towering cumulonimbus clouds around the centre, fed by moisture from the sea. When it reaches land, the moisture supply is cut and it dissipates. The place where it crosses the coast is its landfall. Cyclones that cross 20° N usually recurve and are more destructive.

Structure of a mature cyclone:

• A calm centre, the eye, with sinking air.
• Around it the eye wall, with strong spiralling ascent up to the tropopause, the highest winds (up to about 250 km per hour) and torrential rain. Rain bands spiral outward.
• The circulating core is about 150-250 km across; over the Bay of Bengal, Arabian Sea and Indian Ocean the whole storm may be 600-1,200 km wide. It moves about 300-500 km a day.
• Storm surges flood low coastal land.

Thunderstorms and tornadoes are short, local, violent storms:

• Thunderstorms come from intense convection on hot, humid days: a towering cumulonimbus cloud with thunder and lightning, strong updrafts and later downdrafts of cool air and rain. If the cloud reaches sub-zero heights, hail forms; with too little moisture, a thunderstorm can raise a dust-storm.
• Tornadoes: from severe thunderstorms, a spiralling wind descends like an elephant's trunk, with very low pressure at the centre and massive destruction. They occur mainly in middle latitudes. Over the sea they are water spouts.
• These storms convert heat and potential energy into kinetic energy, after which the atmosphere settles back to a stable state.

Example (new): before a severe cyclone reaches the Odisha coast, people are moved to raised cyclone shelters because the storm surge, not only the wind, is what floods low villages near the shore.