WIND Flashcards
When are winds given in magnetic?
In ATIS and by ATC.
Otherwise it’s in True
Where are winds measured?
10m above ground level as friction has less of an impact there
What forces acts on air?
- PGF, moves air from H to L
2. Coriolis force
When is the wind called Geostrphic?
When PGF and Coriolis forces have been balanced.
It only blows above the friction layer as PGF and coriolis is not balance below it
In which direction does the Coriolis force act in the northern hemisphere?
It acts at 90 deg to the right of the wind
It’s max at the poles and zero within 15 deg of the equator.
How do you know where a Low pressure system is located?
Stand with the wind in your back and the Low is on your left side.
What happens to wind below the friction layer?
It slows down. As speed slows down the GF is reduced and that deflects the wind towards the Low pressure cause PGF now becomes more dominant
Wind backs in the northern hemisphere
What happens with wind speed/direction over land and sea?
Over land: 30 deg deflection and 50% of speed
Over sea: 15 deg deflection and 75% of speed
Describe wind changes diurnaly between the ground and 1’500ft
Night - Day: Surface wind veer and increase
At 1’500ft wind backs and decrease
Day - Night: Surface winds back and decrease
At 1’500ft wind veer and increase
Describe katabatic/anabatic winds?
Katabatic flows down a hill at night
Anabatic flows uphill by day against gravity so they are weaker
What’s a seabreeze?
Heating of air over the shore creates a Low. Relatively High pressure air over the sea blows in to replace it
Describe Geostrophic wind around a Low and High with curveHd isobars
Low: PGF works towards the Low. GF works in opposite direction. Centrifugal force works opposite to PGF and resultant wind is lower than GW. Winds move anti-clockwise around a Low
High: PGF works from the High. GF works towards the High. Centrifugal force acts with PGF and the resultant wind is faster than the GW.
Wind moves clockwise around a High in the northern hemisphere
How does the wind blow within 15 deg of the equator with straight isobars?
Directly from High to Low due to lack of GF
What can happen with curved isobars within 15 deg of the equator?
Centrifugal force can be so large that it balances PGF and TRS can form
When are winds usually fastest and why?
Usually around 15:00 due to thermal mixing
What causes wind in the upper part of the atmosphere?
Temperature variations control the upper air pressure variations. Cold air causes lower pressure in the upper atmosphere compared to warm air. Reason is because the pressure lines get compacted more in cold air. Lapse rate is higher in colder air.
Wind will blow from the warm air which has a higher relative pressure than the cold one at the same altitude.
Definition of a Jetstream?
Wind speed greater than 60 kts
1’500nm length
200nm width
12’000ft deep
What causes a Jetstream?
Large temp difference in the horizontal
Describe the Subtropical Jet?
Is created by the temp difference between the Hadley and the Farrel cells.
Air rises from the equator and moves north in the Hadley cell. This air meets air from higher latitudes in the Farrel cell. Due to the large amounts of air not all of it descends. Some is forced to flow horizontally. GF turns it to the right giving us a westerly jet.
It moves with the heat equator and is at 20 deg in winter and 40 deg in summer
Are stronger in winter cause the temp difference is greater
Found at about 40’000ft
Is lower in winter and higher in summer.
Describe the Polar Jet?
Between the Farrel and Polar cells
Are westerly in direction
45 deg in winter and 65 deg in summer
Speeds are less than for the Subtropical one due to smaller temp differances.
Fastest in winter time due to higher relative temp diff.
Found at about 30’000ft
Is lower in winter and higher in summer
What’s CAT?
Clear Air Turbulance
Caused by windsheer within and around jetstreams
Most severe is found level with the core in the cold air side.
Secondary area is found above the core above the warm air tropopause
What can cause windsheer?
- Inversions
- CB clouds
- Frontal passage
- Mountain waves
Which direction does jetstreams travel around the World?
The major jetsstreams are westerly except for the equal
torial easterly which as its name suggest is a easterly jetstream.
