NZ Climatology Flashcards
(39 cards)
What effect does the Waitakeres and Coromandel ranges have on AKL?
W: protects AKL from SW & W winds
C: protects AKL from E winds and therefore prevents low cloud bases and rain
What are the most common winds in Auckland? When are they most likely and how strong are they?
What effect will a tropical cyclone have on wind?
SW (most common) spring/summer.
NNE even throughout the year.
When +20kt often due SW. If +30kt due passing cold front. Rarely ever +G50kt.
Bottom of TC will move NW-SE hence NE—E—SE winds
When is the sea breeze most common in AKL?
How strong is it and what direction does it blow?
What can help/destroy the sea breeze?
SSW 5-10kt Oct-Mar.
SW wind will help enhance breeze to 20-25kt and extend upto 3000ft, whilst a NE breeze may eliminate it entirely or significantly weaken it so it only reaches 5-600ft (strong shear).
When does the land breeze occur in AKL?
From what direction and speed?
ENE 5-10kt.
Starts around midnight.
Common in summer and winter but will start later/weaker in winter.
Is turbulence common in AKL?
No surface friction is uncommon.
Thermally induced friction around hills is more common.
When is radiation fog common in AKL?
What winds can enhance/destroy it?
April to August under high pressure conditions.
Will generally form 0300 and dissipate 0900.
Less issue with fog when NNE winds
More fog when W/NW winds and will form 2200 and persist longer (due Waitakeres blocking wind) especially over Papakura channel.
A sea breeze as the day begins to warm the land may cause fog to blow over airport.
What is the most common cause for low cloud in AKL?
What are some other causes?
How high can cloud be expected and how long will it last?
Moist NE is the most common cause with cloud bases 5-600ft, as low as 300ft. Can persist 24h and will be lower of Manawatu Heads.
Slow moving SF will have cloud bases 300ft and rain if wind is moist NNE. Can last 12h.
Wind from W/SW with low SE NZ can bring instability and rain, Cb and cloud to 100ft
When is poor vis most common?
What are common reasons?
Poor vis due to low cloud is often absent in summer and worst may-aug and is worst with N/NE winds less 20kt
Due to rain in most common due to frontal passages and will last 1-2h.
Due to pollution is worst in light wind and stable air conditions.
What effect does terrain have on Wellington airport winds?
NI/SI produce a channeling effect through the Cook St, prevailing winds are NNW/SSE. Hills either side of the airport provide additional channeling which causes a N/S wind
Does the surface wind follow more or less the gradient wing at Wellington?
Describe common reasons for northerly and southerly winds. What percentage of winds are N/S?
Due to complex terrain surface winds do not always follow the gradient wind.
98% of the time the wind will be N or S.
Northerly are common with trough/depression moving E in spring/early summer. The accompanied CF will bring NW winds 50-60kt.
Southerly winds are common in winter with highs over SI or low over/east of NI. Sudden S winds can be experienced with a squall line through the region with winds of 50kt lasting 30-40min.
When does the sea breeze in Wellington occur? (Year/day)
What requirements are there for this formation?
What direction is the breeze?
What can enhance the sea breeze?
Occurs late Oct-Apr. Requires sufficient surface heating and anti cyclonic conditions (high/col) when the pressure differences is low between sea/land.
Occur 0900-1800, reduced hours in spring/autumn and rare in winter.
Is normally SSE 6-12kt. Although with weak pressure gradients, can have weak S ar one end of RWY and N at the other end.
A 12-14kt SB May occur if wind above friction layer is E/NE as Rimutaka/Tararua ranges shelter winds, allowing SB to prevail…will die down in evening.
When does a land breeze at Wellington occur? From what direction?
What can influence this?
Occurs all year round from NNE, with weak pressure gradients.
In winter LB can occur mid afternoon.
Katabatic effects from N/NE can influence the strength and time of day.
What degree of turbulence can be expected when there is a northerly/southerly wind on landing and T/O phase?
LANDING: N: Bumpy on approach and landing S: Approach bumpy but smoother landing TAKE OFF: N: Turbulent until 3000ft S: Little to none climbing out of Lyall Bay. Low level turbulence when Cook St wind 240-290*, and due to hills to W of airport low level shear 500-1000ft.
Degree of turbulence is proportional to wind direction fluctuations
How often will fog occur at WLG? During what season?
What are the common causes?
Occurs Jan-Mar 5 times a year.
A southerly wind can blow fog from EC SI upwards. Or very rarely radiation fog in Hutt Valley blown S by NE winds.
Describe how a moist northerly or southerly airstream will produce low cloud
N: Cook St is generally covered with low cloud but WLG protected by high ground to the north. Lee waves can form so cloud base not uniform, but is lower over Cook St. If due to trough or front, OVC over airport and well to south (approach side) with strong wind, rain and poor vis.
S: ONLY with low/mod strength winds. Low pressure system to N or NE WLG with high over Canterbury but relatively weak PG. Or a weakening tropical depression travelling south along EC NI can produce long periods of low cloud.
What wind direction produces more low cloud?
When is low cloud most common?
Southerlies less than 20kt.
Jan-Mar is worst, May-Jul optimal.
Reduced vis to rain is typically 1-3000m, but is not as common as reduced vis due to low cloud.
What are the main orographic influences of CHC airport winds?
Southern alps 90km to west.
Port hills 15km SE at 2000ft.
Canterbury plains.
Mainly the alps and Cook St will affect wind.
What is the predominant surface wind at CHC? When will this wind typically blow?
NE
Is most frequent in summer as the sea breeze is a major contributing factor. Often very cool temperature.
When 3000ft wind is NNW through to NNE, the surface wind is NNE. Is most common with high to E or SE of Canterbury and is due to effect of Alps and Port Hills.
Explain the NW wind on CHC.
When is it likely to occur?
Is a hot and dry Föhn wind that brings turbulent flying conditions. Is most likely to occur ahead of cold fronts moving from the SW.
When 3000ft wind between SSE/SW, the wind at the airport is SW.
How often is wind above 20kt at CHC?
What direction are these winds typically?
3% are +20kt.
Slightly less than 50% S Or SW, with the remainder from NW or NE.
When (year/day) is the sea breeze common at CHC? How strong and from what direction?
Is most common Oct-Apr starting 1-2h after sun rise, peaking 3h later and ceasing 3h after sunset. Requires a weak PG.
Is approximately 15kt NNE
When do land breezes occur at CHC?
What assists them?
On clear sky nights with weak pressure gradients. Katabatic winds can help assist land breezes. Will start 1-2 hours after the sea breeze has ceased.
What wind directions bring turbulence to CHC.
Any other ways create turbulence?
The airport is relatively free of turbulence with a sea breeze but will have large amount of it when the NW sets in.
In NW, there will be low level wind shear with moderate to severe turbulence, and as NW sets in turbulence will reach ground level.
Other ways include convective heating over crop plains which can result in Cb/hail and turbulence
Is fog common at CHC? What type is most common and how does it form?
Can any other types of fog form? How?
Yes, approximate 45 days per year.
Radiation fog most common, will occur on clear nights with weak pressure gradient from a NE wind, before fog forms will become light NW.
Evening fog may clear around 0000 but form again just before sunrise.
Advection fog may occur from Pegasus Bay (NE) or Waimakariri River (N) and blow over, slight lifting of fog from surface will occur as it approaches the airport. Not as common but may also occur Lake Ellesmere from south which is less likely to lift.
