Define Anticyclone (high)
A region of relatively high pressure, with winds circulating around the point with the highest pressure anticlockwise in the Southern Hemisphere.
Features of an Anticyclone (5)
- Divergence at the SFC
- Air subsides from above (creating high pressure at SFC)
- Anticlockwise winds in SH
- Surrounded by at least one more or less circular isobar.
- associated with light winds and warm temps
What are the two types of anticyclone?
- Warm
2. Cold
Give an example of a warm anticyclone
Discuss how this works
- Sub tropical highs
1. General circulation causes air to rise at equator
2. This moves South in the high troposphere, cooling and converging - this causes the air to sink to the SFC (convergence occurs faster than outfow @ SFC)
3. As dry air from aloft sinks, it warms at DALR = warm
4. This results in high pressure system created at SFC (generally around 30deg South.
General path for warm anticyclones
Migrate N in winter, and S in summer
How are warm anticyclones intensified
- Increased absolute vorticity above developing high (creates more convergence within jetstream over the high, downward motion is enhanced = intensified high).
WX conditions associated with warm anticyclones (4)
- East of AC: Southerly wind creates slightly unstable conditions = partial cloud/Tcu, light showers (heavier the further east, weaker the further west in zone)
- Clear skies in centre/North of AC
- West of AC: ‘dirty’ side, Northerly being cooled from the SFC upward, and aersols create ‘gloom’, lots of Stratocumulus.
- NW of AC: poor WX.
Type/height of inversion found in warm anticyclones
Usually a Subsidence (temp) inversion b/t 3000 - 8000ft
Discuss the formation of a cold anticyclone
Give an example overseas and in NZ
- Air in contact with SFC cools via conduction and becomes more dense.
- SFC pressure increases as a result, the anticyclone is formed.
E.g. (usually develop in winter)
- Siberian High
- Central Otago in NZ
What is the max depth of a cold anticyclone
Where is the temp inversion located?
- 3000 to 5000ft max
- 3000ft
What are the (7) hazards associated with Anticyclones
- Convective Showers (on exposed coast to E of high)
- Extensive low cloud (100 - 500ft)
- Poor Vis due to Drizzle on W of High
- High winds and turb on fringes of High
- Haze due to trapped aerosols beneath inversions
- Fog due to radiation cooling under clear skies in centre of High.
- Summertime TS due increasing divergence aloft and not enough SFC heating to overcome subsidence inversion. Sea breezes can assist.
