Strong winds (typhoons, hurricanes, cyclones, tropical storms and tornados)

Mechanism of destruction

Pressure and suction from wind pressure, buffeting for hours at a time. Strong wind loads imposed on a structure may cause it to collapse, particularly after many cycles of load reversals. More common damage is building and non-structural elements (roof sheets, cladding, chimneys) blown loose. Wind-borne debris causes damage and injury. High winds cause stormy seas that can sink ships and pound shorelines. Many storms bring heavy rains. Extreme low air pressure at the center of a tornado is very destructive and houses may explode on contact.

Parameters of severity

Velocity of wind. Wind scales (e.g. Beaufort) gale severity scale. Local hurricane/typhoon scales.

Causes

Winds generated by pressure differences in weather systems. Strongest winds generated in tropics around severe low pressure systems several hundreds of kilometers diameter (cyclones) known as typhoons in the Pacific and as hurricanes in Americas and elsewhere. Extreme low pressure pockets of much narrower diameter generate rapidly twisting winds in tornados.

Hazard assessment and mapping techniques

Meteorological records of wind speeds and direction at weather stations gives probability of high winds in any region. Local factors of topography, vegetation and urbanization may affect microclimate. Past records of cyclone and tornado paths give common patterns of occurrence for damaging wind systems.

Potential for reducing hazard

None. Cloud seeding may dissipate rain content.

Onset and warning

Tornados may strike suddenly but most strong winds build up strength over a number of hours. Low pressure systems and tropical storm development can be detected hours or days before damaging winds affect populations. Satellite tracking can help follow movement of tropical storms and project likely path. The movements of weather systems are however, complex and still difficult to predict with accuracy.

Elements most at risk

Lightweight structures and timber housing. Informal housing sectors and shanty settlements. Roofs and cladding. Loose or poorly attached building elements, sheets and boards. Trees, fences, signs etc. Telegraph poles, pylons and high-level cables. Fishing boats or other maritime industries.

Main mitigation strategies

Engineering of structures to withstand wind forces. Wind load requirements in building codes. Wind safety requirements for non-structural elements. Good construction practices. Micro-climatic siting of key facilities, e.g. in lee of hillsides. Planting of windbreaks, planning of forestry areas upwind of towns. Provision of wind-safety buildings (e.g. strong village halls) for community shelter in vulnerable settlements.

Community participation

Construction of wind-resistant or easily rebuilt houses. Securing fixing of elements that could blow away and cause damage or injury elsewhere, e.g. metal sheeting, fences, signs. Preparedness for storm action. Taking shelter in strong, wind-resistant buildings. Protection measures for boats, building contents or other possessions at risk.