Chapter 10 - Low level winds Flashcards
What is a katabatic wind and when does it happen?
Wind caused by mountain air cooling which caused the now more dense air to snowball down the surface. Happens at night. “Put the cat out at night and it will get cold.”
What are anabatic winds?
Winds flowing up mountain side terrain due to mountain side heating causing air to rush in to replace the now less dense air.
Which are stronger out of anabatic and katabatic winds and why?
Katabatic because they have a snowballing effect.
How is a valley inversion formed?
Via mountain side cooling cause surface air to get colder and more dense and thus fall below the warmer air above.
What is the Foehn effect?
When the air temperature on the lee side of a mountain is warmer due to condensation occuring on the windward side.
How do you calculate the rise in air temperature, due to the Foehn effect, between the windward and leeward side of a mountain?
The difference between condensation levels X the difference between DALR and SALR (1.2degC/1000ft).
What happens when wind veers?
It turns right.
What happens when a wind backs?
It turns left.
How is low level wind measured?
With anemometers or wind wanes.
How high are anemometers placed to measure surface wind?
10m to avoid mechancal disturbances in wind.
How thick is the friction layer on average?
Around 2000 - 3000 ft thick.
What affects the thickness of the friction layer?
Mechanical and thermal activity.
In the NH in free flow and with straight isobars, how is geostrophic wind achieved between a high and a low?
Initially the PGF pushes the air from high to low.
The coriolis force then acts at 90deg to the direction of flow turning the airmass as it flows. (a veering motion)
It keeps turning until the coriolis force is equal and opposite to the PGF.
This is when geostrophic wind is formed.
This is the same as the SH except it backs instead.
What is geostrophic wind?
Wind parallel to isobars/isohypses.
In the NH with straight isobars i the friction layer, how do surface winds form between a high and a low?
The PGF starts an air mass moving towards the low.
CF acts perpendicular to the flow causing the wing to veer.
As it accelerates, friction builds and acts in an opposite direction to the velocity.
This friction acts as an equalising force before the wind direction becomes parallel to isobars and so the surface wind crosses the isobars.
it also slows the flow.
(veers in SH)
How much does wind back and slack in the NH (or veer and slack in SH) from free flow to surface wind, over land and sea?
Land - backs by 30deg and speed halves.
Sea - backs by 10-15deg and speed is 70% of free flow.
How does wind form on curved isobars, in free flow, in the NH and why is the wind faster around a high than a low?
As the wind is turned a centrifugal force is created.
In an anticyclone this force adds onto the PGF, since PGF point outwards from a high, and so gradient wind is faster.
Around a depression the PGF points inwards so this centrifugal force opposes the PGF causing wind speeds to be slower..
How do sea breezes form?
Land warms during the day (usually in summer).
This causes the air on the land to rise.
air from the cooler sea then rushes in to fill the now low pressure.
The rising hot air eventually reaches a cieling and flows outwards where it then sinks over the cooler sea, creasting a cycle.
This works in the opposite way at night (when the land cools to be colder than the sea) to create land breezes.
What are the Mediteranean winds and how are they made?
Winds that a created by the relatively warmer Med sea forming a low pressure which draws in air from the land.
Forming the Mistral and the Bora from Europe and the Sirocco, Ghibli and Khamsin form Africa.
