Space Weather, Mountain Waves & Föhn Wind

What is space weather?

How the sun's energy interacts and influences the earth's space environment and magnetic field

What two types of radiation does the sun emit?

- Electromagnetic radiation

- Corpuscular radiation

What are coronal holes?

- Massive sunspot

- Large dark areas

- Rooted in large cells of unipolar magnetic fields

- Allows for continuous outflow of high-speed solar wind

What are coronal mass ejections?

- Large scale magnetic structures

- Contains coronal material ejected from the sun

- Travels faster than solar wind

- Creates shockwaves

What are solar flares?

- Due to constant fluctuations in sun's magnetic field

- Plasma energy shoots out at high speed (localised CME)

- Increases at solar maximum

What is corpuscular radiation?

- Small particle radiation emitted by sun

- Has charged particles forming magnetised plasma

What is solar wind?

- Created by corpuscular radiation emission

- Flows out counter-clockwise from sun

What is the heliosphere?

Region of space affected by solar wind

How does the solar wind interact with earth's magnetic field?

- When it hits the magnetic field, plasma is diverted

- Creates a bubble around earth (Magnetosphere)

- Charged particles steered away

How does the Aurora Borealis and Aurora Australis form?

- Charged particles in solar wind enter atmosphere

- Caught by magnetic field

- Steered towards poles

- Collision and then lose energy which turns into light

What is a mountain wave?

Laminar wave airflow, downwind of mountainous terrain

What are the requirements for a mountain wave (6)?

- Wind within 30° perpendicular to ridge line

- 20kt wind speed at ridge top

- Wind increasing with height

- Little variation in wind direction

- Stable layer above ridge top

- Big mountain

What factors affect amplitude of mountain waves?

- Wind speed

- Topography (of upslope)

- Changes in stable/unstable layers around mountain

- Second mountain range

What factors affect the wavelength of the mountain waves?

- Wind speed

- Stability

Increasing wind speed or reducing stability increases wavelength

How do jet streams interact with mountain wave formations?

- As air rises, waves in higher altitudes push up into jet stream

- Jet stream will buckle

- Generates severe vertical wind shear

What are weak dissipating mountain waves?

- Relatively light winds

- Any wave forming is small and weak

- Doesn't extend far downwind

- Not a large affect on flying

What is a normal pattern of mountain wave?

- Stronger winds

- Increasing wind speed

- Sustained linear wave formations far downstream

Trapped - Won't extend above a layer centred above the mountain crest

Untrapped - Extends further up into troposphere

What is a vertically propagating mountain wave?

- Hydraulic jump

- Fast moving air hits zone of slower/stationary air

- Instead of mixing, it leaps vertically over it

What is a Föhn wind?

- Warm, dry and gusty wind

- Blows on lee side of large mountain

- Air cools at slower rate on windward side than warming on lee side

What are the requirements for a Föhn wind?

- High moisture content on windward side

- Forced rising of saturated air (windward)

- Cloud formation and precipitation (windward)

- Large differences in adiabatic cooling of rising air and warming on leeward side

What is isentopical formation of Föhn wind?

Mass of air can't sink on one side but travels over a mountain and then it can descend

What is rotor streaming?

- Migrating rotor zones forming immediately in the lee slope or below ridge level

- Not anchored to mountain

- Broken up by internal motion

- Strongest at ridge height

- Falling/breaking away from mountain waves

- Keeps tumbling

What are the meteorological conditions associated with mountain waves?

- Rotor zones

- Cloud development

- Heights of friction layer

- Turbulence zones

- Airframe icing

Where are rotor zones found within mountain waves?

- Beneath downstream waves

- Anchored in position below each crest

- Spins towards mountain

What is the associated cloud development with mountain waves?

- Orographic (windward)

- Lenticular (along wave crests)

- Rotor clouds (in rotor zones)

How does the height of the friction layer change with mountain waves?

- It extends to the top of the mountain

- Adopts shape of mountain wave

- Increase in wind increases height of friction layer

What are the meteorological conditions associated with Föhn wind?

- Lower cloud base on windward side

- Saturated air, close to dew point on windward side