What contours are drawn on Upper air charts and how do they work?
- Isoheights, which join areas of equal height
- Charts will be assigned a given pressure for the entire chart. and the Isoheights drawn on the map indicate ALTs that correspond to this pressure.
- Treated exactly the same as Isobar charts (ie; orientation and spacing of contours give wind direction and speed).
What is a thermal wind?
What is another name for a thermal wind?
The differences between winds at differing levels (shears) due to temperature differences, also known as the ‘vertical geostrophic wind shear’.
How are we able to calculate a thermal wind/shear?
Though vector addition
Define the general rule for the affect of differing temperature in the air
Temp differences between two airmasses within a layer of air creates pressure differences between the two columns at the SAME height. Resulting in a slight pressure gradient.
Warmer the air, the larger the distance between the two isobaric surfaces and vice versa.
What wind results from slight pressure gradients?
A light wind from High to Low pressure (Thermal wind)
‘Buys Ballots law’ (not really) for Thermal winds
If the Thermal wind is hitting your back, the cold air lies to your right (in the Southern Hemisphere)
What components are required to calculate thermal winds (and what is the trick?)
- SFC wind direction and speed
- Chosen Upper level wind direction and Speed.
Both these can be used to calculate the thermal through the layers.
(use compass for direction, and 1cm per kt)
- SFC wind head to thermal tail
- SFC wind tail to Upper wind tail.
What Thermal wind does the southern hemisphere experience?
Generally, the temperature gradient that drives the SH thermal wind slopes from the equator (warm) to the Poles (cold). However, due to Coriolis Force, the wind bends to form a westerly thermal wind.
What does a westerly thermal wind tend to do with height?
Where?
- Increases in strength with height.
- Becomes more Westerly with height (ie a TRUE westerly)
- IN MID LATITUDES
Define a Jet Stream
A region of strong winds, sustaining 60kts or more.
Where to Jet Streams tend to occur?
Give an example of where?
- At any altitude, however they are usually associated with strong winds and higher ALTs
- Generally found where there is a strong temp gradient (e.g. fronts and inversions)
- The strong westerly wind belt in upper troposphere.
What are the two Westerly Jet streams of interest to us?
- The polar front jet
- The subtropical jet
What are the characteristics of the Polar front jet (PFJ) (6)
- Driven by large temp differences found above frontal zones (thermal wind)
- Located near 50 - 60 deg Latitude zone, North of cold polar air masses
- Not continuous around the globe (due frontal)
- Often have large Northerly/Southerly component, always a westerly component.
- found at 30,000ft in warm air, a few deg poleward of SFC front.
- Wind Max in NZ area 80-100kts, can reach 200kts
What are the characteristics of the Sub-tropical jet (STJ) (5)
- Driven by conservation of angular momentum as fast moving air from equator descends on mid-latitudes
- Located above sub-tropical belt of high pressure (25 - 35deg)
- Very zonal, only a few deg N or S of Westerly
- Found at 40,000ft
- Wind Max in NZ 80kts in summer, 120kts in winter, can reach 200kts.
Cloud characteristics (associated with jetstreams)
- Little cloud above the Jet axis
- PFJ: Super high cirrus on equatorial side, dense cirrus on poleward side.
- STJ: clear skies on poleward side, cirrus on equatorial side (due ascending air)
Altitude favoured for CAT
30,000 - 45,000ft in unstable conditions
Risks of jetstreams to aviation
- severe turb and vertical shears (extreme difference in wind speeds b/t the core and outer areas of jetstream)
- Clear air turb (slight to severe - severe usually on polar side of jet)
- Cloud?
Likely regions for CAT in NZ (4)
- Near jet streams
- Above areas of considerable tropospheric convection
- Near sharp upper level troughs
- In association with mountain waves
Upper air charts issue times and validity
- Issued four times daily (000/0600/1200/1800 UTC)
- Valid +/- 3hrs from stated issue times
