Hpa standing waves jetstreams

1. HPA Theoretical Online
Knowledge Course
Meteorology
Revision 1, 30/04/2020 Title: Meteorology 1

2. HPA Theoretical Online
Knowledge Course
Jet Streams
• A strong narrow current of air on a nearly horizontal axis in the upper
troposphere or low stratosphere
• Jetstream's have strong lateral and vertical shears
• The wind speed must be more than 60 knots to be titled a ‘Jet stream’
• Formed at frontal air mass boundaries – the less dense warmer air
overlies the colder air, where the pressure gradient is the greatest
Revision 1, 30/04/2020 Title: Meteorology 2

3. HPA Theoretical Online
Knowledge Course
Polar & Subtropical Jet
Streams
• Jet streams are faster in winter
and closer to the equator
• Polar Jet stream is found at
around 30,000 ft with a core
wind speed of 200 kts
• The sub tropical jet stream is
found higher (due to warmer air)
around 40,000 ft with a core
wind speed of 100 kts
• Key components of a Jet
Stream:
• 1000 nm long
• 150 nm wide
• 10,000 ft in depth
• Strongest turbulence on the
cold air mass, slightly below
the core
Revision 1, 30/04/2020 Title: Meteorology 3

4. HPA Theoretical Online
Knowledge Course
Standing Waves
• Standing waves are also known
as lee waves or mountain waves
• A standing wave is virtually
stationary with respect to the
Earth’s surface
• Lee waves are a classic example
and often occur in stable air when
a temperature inversion sits just
above a long hill or mountain
barrier
• Turbulence and downdrafts are
found on the lee side near the
surface
• Conditions that are required:
• > 15 kts wind
• Wind speed increasing with
height
• Stable air mass
• Wind direction constant
• Wind direction crossing
ridgeline within 30 degrees
Revision 1, 30/04/2020 Title: Meteorology 4
The Föhn wind when blowing over the Alps causes turbulence
on the lee side (e.g. northern Italy) – lenticular clouds are an
indication that standing waves exist

5. HPA Theoretical Online
Knowledge Course
Revision 1, 30/04/2020 Title: Meteorology 5
Severe
turbulence exists
in the rotor zone
Lenticular clouds are
a visual clue that
standing waves exist
(caused by moist air
rising in the up-cycle
phase of the wave)