Basic considerations

From the bushwalker or skitourer’s point of view, the basic elements of weather are:

  • temperature
  • cloud
  • precipitation (rain, snow, hail etc)
  • wind
  • humidity
  • lightning.

An understanding of some of the basics and the broad reasons for the vagaries of Australian weather will assist your weather prediction skills in the field.

Weather occurs as a result of movement of the atmosphere. The majority of weather phenomena are caused by the vertical movements of parcels of air:

  • rising air expands, cools, and sheds moisture as cloud droplets, rain, hail or snow
  • sinking air is compressed, warms and can carry more moisture – clouds dissipate as water droplets evaporate into invisible water vapour in the warming air.

Winds occur at ground level because of differences in the weight of air above different locations. The weight of air at any location on the ground’s surface is called the air pressure. If the pressure at one location is greater than the pressure at an adjoining location, the surface air will move from the high pressure to the low pressure. This is surface wind. The various atmospheric movements attempt to equalise the weight of atmosphere above points at sea level. Consequently maps of these weights or pressures, at sea level, can show much of the broad-scale horizontal and vertical motion of the atmosphere. The lines drawn on weather maps connect points of equal pressure and are called isobars. Winds do not flow directly from high to low pressures. The rotation of the earth causes the winds to flow almost parallel to the isobars but:

  • outwards from high pressures
  • inwards towards low pressures.

Regions on a weather map where isobars are closer together mean that pressure differences between adjoining locations are higher, and so the winds will be stronger.