Upper Air Met (ATPL)

Question 1

Define height contour

Answer 1

• 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

Question 2

What are the height differences for hPa and FL

Answer 2

700hPa - FL 100
500hPa - FL 180
300hPa - FL300
200hPa - FL390
100hPa - FL530

Question 3

Define Isotherm

Answer 3

Lines drawn on chart joining equal temperature

Question 4

Describe the use of height contour charts in the forecasting of upper charts

Answer 4

• 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

Question 5

State the info that can be obtained from spacing and orientation of height contour lines

Answer 5

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

Question 6

Outline definition of thermal wind

Answer 6

•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

Question 7

Outline how wind at higher altitudes is a vector sum of the lower level wind and the thermal wind through the layer

Answer 7

Using vector addition you can calculate how the surface wind changes direction and speed with altitude

•Surface wind + thermal wind = accurate upper air wind

Question 8

Explain why wind at progressively higher altitudes in mid-latitudes tend to become stronger and more westerly

Answer 8

•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

Question 9

Define jet stream

Answer 9

•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

Question 10

Describe the structure of a jet-stream

Answer 10

• 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

Question 11

Identify 4 jet streams

Answer 11

• Sub tropical jet-stream norther and southern
  •Polar jet-stream norther and southern

Question 12

With regard to the Southern Hemisphere polar jet-stream, describe its connection to low-level fronts

Answer 12

•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

Question 13

With regard to the Southern Hemisphere polar jet stream, describe its location relative to the frontal interface

Answer 13

Behind the cold front due to it tilting backwards
•Ahead of the warm front due to it tilting forwards

Question 14

With regard to Southern Hemisphere describe its typical altitude jetstreams polar

Answer 14

300-500 hPa

30,000ft - 18,000ft

Question 15

With regard to the Southern Hemisphere polar jet stream, describe its variation in intensity and latitude from winter to summer

Answer 15

• 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

Question 16

With regard to the Southern Hemisphere polar jet stream describe its probable areas for turbulence

Answer 16

•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

Question 17

With regard to Southern Hemisphere subtropical jet stream, describe its disconnection from low-level fronts

Answer 17

•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

Question 18

Regards to SH subtropical jet stream describe its location relative to the fractured tropopause

Answer 18

•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)

Question 19

What are typical altitudes for tropical jet streams

Answer 19

200hPa

40,000ft on average but can be higher if it’s closer to equator

Question 20

Regards to SH subtropical jet stream, describe its variation in intensity and latitude from winter to summer

Answer 20

•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

Question 21

Regards to SH, subtropical jet stream describe the probable areas for turbulence

Answer 21

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.

Question 22

Explain where and why cirrus cloud is likely to form in relation to a jet stream

Answer 22

• 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

Question 23

Explain the characteristic ‘tilt’ with height of developing mid-latitude depressions

Answer 23

• 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

Question 24

Explain the characteristics ‘tilt’ with height of developing mid-latitude anticyclones

Answer 24

• 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

Question 25

Describe how mountain waves can combines with jet-streams to generate severe clear air turbulence

Answer 25

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

Question 26

Describe how pilots can anticipate location and altitude of jet streams

Answer 26

• 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

Question 27

Explain the telltale signs that are often present in flight to indicate a jet stream (5)

Answer 27

•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