Pana Hema-Taylor, of Ardgowan School, asks :-

Where does wind come from?

Marlyn Jakub, a physicist at Otago University, responded.

Wind is motion of the air over the Earth's surface. It is driven by the effects of different air densities at different places. These different densities are caused by an uneven distribution of heating and cooling over the surface of the Earth which includes lowlands, mountains, oceans and polar ice caps. The sun can heat a patch of air, causing it to expand and become less dense. In Earth's gravitational field, this lighter air tends to rise. Any nearby colder, more dense air moves horizontally beneath the rising warm air, creating a cool surface wind.

However, the whole process is further complicated by the rotation of the Earth. The air motion is on such a large scale that circular patterns of air movements are formed, resulting in rather stable high and low pressure circular air masses. These rotating air masses tend to drift eastwards at New Zealand's latitudes, and one often sees these on the TV-weather reporter's moving, elapsed-time images of daily cloud patterns, as viewed by weather satellites.

Warm and cold fronts arise from the interactions of these air masses. As a front passes overhead, there will usually be changing winds at ground level.

On a smaller scale, localized breezes often occur along the New Zealand coast. In the daytime, the coastal land is heated by the sun and the air above it rises. The nearby air over the cool sea remains fairly cold and dense so it flows in to replace the rising warm air. The wind coming in from the sea is called the `sea breeze'. Sometimes at night the air over the land becomes cooler than the air over the sea and this generates a coastal breeze flowing in the opposite direction, from the land toward the sea.