Low Level Winds Flashcards
Anti-clockwise Movement of winds is referred to as
Backing
Clockwise Movement of winds is referred to as
Veering
Geostrophic winds refers to
PGF + Coriolis
Balanced in speed and direction
Parallel to straight isobars
What increases the Coriolis effect force (Geostrophic force)
Increase in density
Faster winds
Higher latitudes
Buys Ballot law
With back to wind in NH low pressure on your left
With back to wind in SH low pressure on your right
Gradient Wind
PGF + Coriolis + Centrifugal force
Gradient vs Geostrophic wind in a low pressure system
Geostrophic is stronger due to the opposing force on centrifugal which makes the gradient wind slower.
Gradient wind slower
Geostrophic wind faster
Gradient Vs Geostrophic winds in high pressure
Gradient winds are greater than Geostrophic in high as centrifugal force act in same direction.
Gradient is faster
Geostrophic is slower
Friction layer is
From SFC to 2000-3000ft
Affected by terrain etc which impacts wind and stability
At night can reduce slightly by 1000ft
Variation in surface winds
Geostrophic winds above friction layer are faster
Wind speed reduces in friction layer
Coriolis reduces because wind speed is reduced in friction layer
What happens in friction layer in NH with winds
Winds will back and reduce
What will happen to winds in SH in friction layer
They will veer and reduce
Figures for wind/direction change over land in friction layer
50% decrease in speed
25/30 degrees change in direction
Figures for wind speed change over the ocean within friction layer
25/30% reduction in wind speed
Directional changes around 10/15 degrees
Sea Breeze
Air over land warms and rises
Converging air creates onshore breeze
Can lead to convective cloud formation
Land Breeze
Wind from land to sea over night
Weaker PGF than daytime effect
Calm wind strength
Below 1kt
Gale force wind strength
34kt to 47kt
Storm force wind
48kt - 63kt
Hurricane force wind
64kt+
How do we measure winds
Anemometer (33ft AGL)
Mountain Valley Winds on altimeter
Air flows down mountain and along valley
Causes altimeter to over read due to decrease in static pressure
The Mistral Wind is (south of France)
A valley wind
Winds up to 70kt/strong in winter
Cold/dry northerly wind
Blows along Rhône valley
Alps create a Venturi effect
The Boar wind is a
Katabatic wind
Cold/dry north easterly wind affecting the Adriatic Sea
Strongest in winter
Winds can exceed 100kt
Foehn Winds
Stable air that rises up surface of mountain
Cools below dew point and cools at SALR
Rain and cloud on upward side and dry air on lee side
Lower dewpoint on lee due to reduced vapour and warmer
Referred to as a chinook in Rocky Mountains
NE trade Winds
Subtropical high to ITCZ
Blow Easterly
S.E Trade Winds
Subtropical high to ITCZ
Easterly wind
Westerly’s - “roaring 40s”
Faster in south than north
Higher speeds in winter season
West to east
Horse latitudes
Around 30N/S light winds
Doldrums
Light winds at ITCZ
Harmattan - W Africa Wind
NE trade wind over Africa
Dry sand which impacts visibility
Reduced visibility to 1000m
10-15k ft
S America - The Pampers
Burst of cold air from south
Blows across pampas of SA
Squall lines on passage of a cold front
Local wind systems such as katabatic winds/sea breeze/valley wind have what in common
Flat uniform pressure distribution
Mountain Waves develop when
Wind direction is perpendicular 30 degrees
Wind speed strong at least 20kt and increases with altitude
Marked layer of stability/strong inversion around altitude summit