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Extreme Weather

Otago's weather spans the spectrum from hot dry summers to long, cold winters. These extremes can present challenges to those of us who live here.

Knowing what the risks are can help us be prepared for them. Visit the Otago Civil Defence and Emergency Management website for tips on how you can get ready for extreme weather events. 

Droughts

A drought can best be defined as a period of unexpected rainfall deficit resulting in a shortage of water. Drought hazards develop slowly, with generally no clear beginning or end. Drought conditions can extend over a wide area, and this extent also can be difficult to define.

Drought is not necessarily caused by low rainfall amounts, but rather by the lack of 'expected' rainfall. Otago has a wide range of climatic types, and different communities have adapted to the different rainfall levels that normally occur in each area. When these are reduced, this is when a drought hazard may be created.

The effects of drought are wide-ranging, but can generally be grouped into social, environmental, or economic categories. The social impacts of drought in Otago include challenging and stressful conditions for people, changes in lifestyle, population decline in drought affected areas, and conflict over available water resources.

The environmental impacts of drought may include loss of vegetation or reduced species biodiversity, migration changes, reduced air quality, and increased soil erosion. The economic impacts of drought in Otago may include financial losses for those involved in, or dependent on, pastoral industries. These losses may also be passed on to consumers in the form of higher prices.

The Otago Natural Hazards Database does not map drought hazard as such, but rather provides a series of five maps which show unusually low annual and seasonal precipitation totals. These colour-coded maps show the rainfall total (in mm) for a dry year or season, based on data from the period 1970-2001.

Snow

Snowstorms can represent a severe hazard for people in Otago, particularly when deep snow accumulates in densely-populated coastal areas which are not regularly affected by snow. Heavy snow loads can collapse roofs and gutters, and bring down power and telephone lines.

Snow can also close roads and affect transport links such as airports, rail lines and bus services. Road access across the region can be affected, particularly the alpine passes such as the Lindis and Haast, and SH1 north and south of Dunedin.

Snow may fall to sea level for one to three days and form into drifts. When such an event is followed by an advancing anticyclone, clear skies and cold temperatures may mean that the snow persists for several days, resulting in frosts and widespread ice.

ORC does not currently hold a snow hazard map covering the entire Otago region.

Wind

Extreme winds can represent a major hazard for people in Otago. They can damage roofing, fences, and signs, and can blow down trees. Previous events have resulted in closure of airports, power lines being brought down, and electrical sparks igniting fires.

Strong winds can occur under a number of different atmospheric conditions, including:

North-west winds: These can occur when a deep trough advancing across the Tasman Sea is squeezed against an intense blocking anticyclone to the east of the South Island. A very strong pressure gradient develops between these two features, resulting in strong winds from the northwest. Such windstorms can last for 6-12 hours.

Southerly gusts: Extreme wind gusts, but of a shorter duration can occur with a sudden change to a southerly air flow. These events are associated with a rapid progression of cold air up the eastern coast of the South Island. They tend to be of short duration, lasting less than one hour.

Wind information in the Otago Natural Hazards Database is mapped at a regional level, and as such detailed variations in wind speed cannot be shown. The wind hazard map shows average annual (January to December) maximum wind speed (km/hr) at 10 m above the ground over tussock or rough grass for the period 1970-2001.

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