Winds occur when air masses of different pressure meet and air flows from the higher to the lower pressure. The greater the pressure difference, the faster the wind. The air flow continues until a balance is reached. Regional wind, one of the most common local winds, occurs in coastal regions. In these regions, the surface temperature difference between land and sea, which causes in turn a pressure gradient, often generates day-time onshore sea breezes and night-time offshore land breezes. Winds may bring life-giving rain like the monsoon, but also can be destructive, like Hong Kong's typhoons. Migrating birds choose their route according to the prevailing winds. A 100-kilometre per hour tailwind means even small birds can travel the long distances between their breeding and wintering grounds easily. Some plants depend on the wind to carry pollen from the male flowers to female flowers so that they can set seeds. Certain seeds have 'wings' of silken 'parachutes' so they can be carried by wind to distant plots of ground, where they germinate and grow into mature plants. Toxic particles from car exhausts and factory chimneys hang around in still air, damaging our lungs, but winds disperse the pollution. Wind also makes the arms of a windmill turn. This produces energy for electricity and irrigation. On a hot summer's day, a breeze helps us to cool down. So where would we be without wind? WWF HK is a local charity environmental organisation whose mission is to build a future in which people can live in harmony with nature. For more information, call 2526 1011.