Upper Winds Flashcards
Thermal wind
Winds aloft are rarely the same as sea level wind because temperature influences a vertical pressure change.
Cold air - pressure reduces quickly
Warm air - pressure reduces slowly
Sea level pressure is 1000hpa, by 40,000ft cold air pressure is reduced to 190hpa where as warm air pressure is 300hpa.
Tries to flow high to low.
Flows perpendicular to Pressure gradient.
Known as thermal wind.
Upper air flows.
Upper westerlies - above 20N and below 20S.
Tropical easterly- between 20N and 20S.
Jet streams
Very strong upper winds.
Wind strength must be greater than 60kts.
Can be up to 350kt.
Jet stream dimensions
Fastest airflow in the core.
2nm deep, 200nm across, 2000nm long
1:100:1000
Global jet stream distribution
Artic front jet (westerly), tropical easterly jet are seasonal.
Polar front jet x2 (1 warm, 1 cold), sub tropical jet, x2. ( all westerly) are permanent.
Permanent jet streams
Polar front jet: 50-60N, 30,000ft, 150kt - 200kt.
Sub tropical jet: 30-40N, 40,000ft, 150kt.
Sub tropical jet: 30-40S, 40,000ft, 80-90kt.
Polar front jet: 50S, 30,000ft, 120-150kt.
Flying across the polar front jet
Tropopause height either side of jet stream will influence temperature change.
Below the jet stream core, temperature colder when flying into the polar air.
Above the jet stream core, temperature warmer when flying into the polar air.
Jet stream surface protection
The jet stream will appear as if it lies in the cold air when viewed from above, due to sloping temperatures devision.
Polar front jet (PFJ)
PFJ lies closer to the cold front (50-200nm) than to the warm front (300-450nm).
At the cold front the PFJ wind directions backs compared to gradient wind (NH).
At the warm front the PFJ wind direction veers compared to the subgeostrophic wind (NH)
Seasonal jet streams
January, winter: 60N, 20,000ft, 80kts.
July, summer: 10-20N, 50,000ft, 90kts
Seasonal artic front jet (winter)
Interaction between artic and polar air in winter.
FL 200.
Occurs over Greenland.
Seasonal tropical easterly jet. (Summer)
Temperature difference between hot land of Asia and Africa verses cooler Indian Ocean.
Up to 100kts - unusual
Known as easterly wave
Jet stream - CAT
The most servere CAT is found near the frontal zone, especially at higher levels on the cold side and just below the core.
Jet streams on SIGWX
Displayed at 80kts and above.
Change bars ll indiacate change of 20kts or 3000ft.
Broken line indicates area affected by CAT
Low level jet streams
Narrow, strong winds (approx 70kts)
Located between 500-5000ft, several hundred miles long.
Four main type: Nocturnal jet, valley inversion, coastal jet, ahead of cold front. Boundary layer jets - form in clear skies at night when a strong nocturnal inversion occurs.
