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2.1 WIND INTENSITY Wind is caused by pressure differences to equalize the pressure differential and the rotation of the planet. This differential is created by unequal heating of the planet’s surface. When a difference in pressure exists, the air is accelerated from the higher to lower pressure. Wind pressure can be approximated by: pressure = ½ x (density of air) x (wind speed) 2 x (shape factor). Shape factor is the drag coefficient and it depends on the shape of the wind pressure. Winds are classified by their spatial scale, their speed, how long they last, and their effect. In Christchurch, the main northwesterly wind is a foehn type wind, hot and dry, it comes from the Southern Alps and can raise the temperature by 10 to 15 degrees within an 210
Micro-Urbanism | Analysis
hour. When wind is combined with cold temperatures, they have a negative impact on livestock by affecting their food stores, as well as natural hunting capabilities. Wind contributes to the spreading of wildfires. Wind disperses seeds from plants, which enables survival through propagation, as well as feeding flying insect species. Wind is one of the causes of soil erosion through deflation, which is the lifting and moving of small particles to another region. These suspended particles cause erosion by abrasion when they impact on other solid objects. Wind erosion occurs in regions with little vegetation, often associated with low rainfall.
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