Upper Air Met (ATPL) Flashcards
Define height contour
- Lines drawn on upper air charts
•Pressure is constant everywhere on the chart
•Contour lines join places of equal height
•Height is in meters AMSL
What are the height differences for hPa and FL
700hPa - FL 100
500hPa - FL 180
300hPa - FL300
200hPa - FL390
100hPa - FL530
Define Isotherm
Lines drawn on chart joining equal temperature
Describe the use of height contour charts in the forecasting of upper charts
- In conjunction with the thermal wind they can be used to forecast the upper air wind direction and speed
•A radiosonde reports the pressure levels of significant changes in temperature and humidity
•MSL charts cannot be used due to density error
State the info that can be obtained from spacing and orientation of height contour lines
Act in the same way as isobars
•The closer they are together, the stronger the wind
•Wind blows parallel to contour lines
•Unlike isobars there is no deviations caused by surface friction
•Accurate to 10-20° in direction and 5kt in speed
Outline definition of thermal wind
•Caused by the temperature difference horizontally
•The steeper the temperature gradient, the stronger the wind on upper air charts
•Is the vector difference between the geostrophic wind at upper altitudes minus that at lower altitudes in the atmosphere
With reference to Buy Ballots law in the southern hemisphere the thermal wind blows with the cold centre to one’s right
•In the northern hemisphere the thermal wind blows with the cold centre to ones left
Outline how wind at higher altitudes is a vector sum of the lower level wind and the thermal wind through the layer
Using vector addition you can calculate how the surface wind changes direction and speed with altitude
•Surface wind + thermal wind = accurate upper air wind
Explain why wind at progressively higher altitudes in mid-latitudes tend to become stronger and more westerly
•The equator is hot, the poles are cold
•The average thermal wind blows from hot to cold
•It is then deflected by the Coriolis force to become westerly
•Stronger because it is added on top of existing surface wind
Define jet stream
•Any sustained wind of 60kt or more
•A strong narrow current in the upper troposphere below the tropopause
•Strong vertical and horizontal wind shear with one or more wind maxima
•Caused by large surface temperature differences
Describe the structure of a jet-stream
- The core has the fastest wind speeds
- Above and below, wind speed slows down creating horizontal wind shear, Same occurs either side of the jet-stream
- If jet-stream is unstable, wind shear can cause turbulence
Identify 4 jet streams
- Sub tropical jet-stream norther and southern
•Polar jet-stream norther and southern
With regard to the Southern Hemisphere polar jet-stream, describe its connection to low-level fronts
•The jet exists because of the front
•The polar air and sub-tropical air create a large temperature gradient over a small space
•Increases thermal wind, therefore upper wind speed
•Contour lines become closer together due to large changes in height of tropopause over shirt distance
With regard to the Southern Hemisphere polar jet stream, describe its location relative to the frontal interface
Behind the cold front due to it tilting backwards
•Ahead of the warm front due to it tilting forwards
With regard to Southern Hemisphere describe its typical altitude jetstreams polar
300-500 hPa
30,000ft - 18,000ft
With regard to the Southern Hemisphere polar jet stream, describe its variation in intensity and latitude from winter to summer
- Moves 15° north in winter as it follows the heat equator
- Moves back 15° south in summer
-Strongest during winter due to largest temperature contrast between consistently warm air near the equator and the now much colder Southern Hemisphere
With regard to the Southern Hemisphere polar jet stream describe its probable areas for turbulence
•Jet-stream is located on warm side of front
•Worst turbulence is in the lower quadrant on the cold side of the jet-stream when looking downstream through the core
•Caused by the horizontal and vertical wind shear being greatest in this area
With regard to Southern Hemisphere subtropical jet stream, describe its disconnection from low-level fronts
•Hot air rises from the equator then hits the tropopause and moves south with the thermal wind
•At the equator the Earth spins faster than at 30°S but the air keeps its momentum
•As it moves towards 30°S it is deflected due to Coriolis force and becomes a westerly jet-stream
Regards to SH subtropical jet stream describe its location relative to the fractured tropopause
•Located on the warmer side above the lower tropopause
•This is the area above the semi-permanent subtropical anticyclones/highs at roughly 30°N/S (horse latitudes)
What are typical altitudes for tropical jet streams
200hPa
40,000ft on average but can be higher if it’s closer to equator
Regards to SH subtropical jet stream, describe its variation in intensity and latitude from winter to summer
•Strongest during winter due to largest temperature contrast between consistently warm air near the equator and the now much colder Southern Hemisphere
•Weakest in summer due to the hot southern hemisphere being similar temperature to the equatorial area
•Small movements north in winter and south in summer
Regards to SH, subtropical jet stream describe the probable areas for turbulence
Same as polar jet-stream
Strongest near to, or just below the jet axis on the cold air (low pressure) side with a secondary area above the axis.
Explain where and why cirrus cloud is likely to form in relation to a jet stream
- Extensive cirrostratus/cirrocumulus on the warm side flowing in the same direction as the jet-stream
- Due to the fractured tropopause usually only cirrus on the cold side as it is lower down
- Clouds created through frontal activity
Explain the characteristic ‘tilt’ with height of developing mid-latitude depressions
- Depressions intensify with altitude if they are cold
- This causes them to tilt towards the south as its colder
- This combined with the rotation of the Earth causes a depression to slope SW with height
Explain the characteristics ‘tilt’ with height of developing mid-latitude anticyclones
- Anti-cyclones intensify with altitude if they are hot, causes them to tilt towards the north as its hotter
- This combined with the rotation of the Earth causes a anti-cyclone to slope NW with height
Describe how mountain waves can combines with jet-streams to generate severe clear air turbulence
Mountain waves have been felt as high as 80,000ft’
•Like swells on the ocean, mountain waves can buckle a jet-stream as they flow through it.
•This buckle compresses the isotach’s (equal wind) in the jet-stream increasing vertical wind shear
Describe how pilots can anticipate location and altitude of jet streams
- Upper air SIGWX charts
- WINDTEMP charts by looking at the barbs
- Satellite imagery
- ROFORS (route forecast) by looking at the wind speeds at appropriate levels
Explain the telltale signs that are often present in flight to indicate a jet stream (5)
•Large temperature changes if traveling north or south
•Clear air turbulence (cobblestone turbulence)
•Cirrus clouds
•Aircraft drift or change in groundspeed
•Aircraft wind speed indication increases
