Upper Air Met 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
Significant height contour pressures at flight levels.
- 700 hPa - FL100
- 500 hPa - FL 180
- 300 hPa - FL 300
- 200 hPa - FL390
- 100 hPa - FL530
Define isotherm.
- Lines drawn on chart joining places of equal temperature
Describe the use of height contour charts in the forecasting of upper winds.
- 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 information 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 degrees in direction and 5kts in speed.
Outline the definition of the ‘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 SH the thermal wind blows with the cold centre to one’s right with their back to the wind.
- In the NH the thermal wind blows with the cold centre to ones left with their back to the wind
Outline
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 the 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 including the occurrence of wind shear and turbulence.
- 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 the jet-stream in unstable, wind shear can cause turbulence
Identify the four principal jet-streams found globally within the troposphere.
- Sub tropical jet-stream northern and southern
- Polar jet-stream northern and southern
With regard to the SH polar jet-stream, describe its connection to low-level fronts and thermal gradients.
- 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 short 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 the Southern Hemisphere polar jet-stream, describe its typical altitude.
- 300-500 hPa
- 30,000ft - 18,000ft
With regard to the Southern Hemisphere polar jet-stream, describe its variations in intensity and latitude from winter to summer.
- Move 15 degrees north in winter as it follows the heat equator
- Moves back 15 degrees south in summer
- Strongest during winter due to largest temperature contrast between consistently warm air near the equator and the now much colder SH
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 the SH sub-tropical 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 degrees S but the air keeps its momentum
- As it moves towards 30 degree S it is deflected due to Coriolis force and becomes a westerly jet-stream
With regard to the Southern Hemisphere sub-tropical 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 degrees N/S
With regard to the Southern Hemisphere sub-tropical jet-stream, describe its typical altitude.
- 200 hPa
- 40,000ft on average but can be higher if its closer to the equator
With regard to the Southern Hemisphere sub-tropical jet-stream, describe its variations 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 SH being similar temperature to the equatorial area
- Small movements north in winter and south in summer
With regard to the SOuthern Hemisphere sub-tropical jet-stream, describe its probable areas turbulence.
- Same as the polar jet-stream
- It is 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 warmer side above side flowing in the same direction as the jet-stream
- Due to the fractured tropopause usually only cirrus on the cold side it is lower down
- Clouds created through frontal activity
Explain with characteristic ‘tilt’ with height of developing mid-latitude depressions and anti-cyclones.
- 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 depressions to slope SW with height
Explain the characteristic ‘tilt’ with height of developing mid-latitude depressions and anticyclones.
- Anti-cyclones intensify with altitude if they are hot
- This 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 combine 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 in the jet-stream increasing vertical wind shear
Describe how a pilot can anticipate the location and altitude of jet-streams.
- Upper air SIGWX charts
- WINDTEMP charts by looking at the barbs
- Satellite imagery
- ROFORS by looking at the wind speeds at appropriate
Explain the tell-tale signs that are often present in flight to indicate a jet-stream.
- Large temperature changes if traveling north or south
- Clear air turbulence (cobblestone turbulence)
- Cirrus clouds
- Aircraft drift or change in ground speed
- Aircraft wind peed indication increases
