Local Winds Flashcards
Describe the formation process of katabatic wind.
As the hills or mountains cool at night, the lower layer of air in contact with the sloping ground also cools through the process of conduction and thus becomes denser than the air at the same height above the centre of the valley. This cold, heavy air slides downhill due to the force of gravity.
When do catabolic winds occur?
Most common during winter months but can occur in summer on very clear nights where there is a large river catchment system. Onset can vary from just after sunset to just before dawn. They generally die out by mid to late morning or are replaced by geostrophic winds.
Describe the conditions associated with catabolic winds.
Katabatic wind speed varies from a barely detectable drift to 15 or 20 kt in some gorges. Taken to the extreme, some regions of Antartica are barraged by gale force katabatics for most of winter. Stronger katabatics (like at Ohakea) prevent the formation of radiation fog, where weaker ones often encourage development (Whenuapai).
List three typical examples of katabatics in NZ.
The Greymouth Barber on the west coast of the south island.
Ohakea - from 030 between 5 and 10 kt.
Whenuapai - NW about 2 kt.
What are the three required conditions for a catabolic wind to develop.
Clear nights.
Sloping ground.
A pressure gradient that is not too strong.
Describe the development of anabatic winds.
During a sunny day, warm air in contact with the sloping surface rises and has an upslope component. If conditions are unstable, the ascent of air is realised through convection rather than by anabatic. Flow will be strongest in the early afternoon on sun-facing slopes. Always light, usually 2 to 3 kt.
Define sea breeze.
A wind that blows from the water onto the land which develops in coastal areas, whether at the seashore or at the foreshore of a large lake.
Describe the formation process of a sea breeze.
Begins with a slack pressure gradient in a coastal location, so there is little or no wind. As the day progresses, the sun heats the land at a faster rate than the sea (due to its lower specific heat). Conduction and low-level mixing results in a high pressure aloft over the land. The resultant pressure gradient at about 1000 ft creates an offshore flow. With further heating and the shift of air from land to sea, the pressure over land falls. Creates lower pressure immediately above the land compared to same height at sea. The remaining three legs of the circulation kick in as a result and the sea breeze is fully develops.
At what time in NZ is a sea breeze likely to form?
Occur commonly in summer, occasionally in spring and autumn and rarely in winter. It will develop between mid-morning and late afternoon or not at all.
