Local Winds Flashcards
What measures surface wind?
Anemometer, 10m above the ground
Conversion for knots to m/s
1kt = 0.514m/s
What speed is calm wind?
1kt or less
What speed is gale force wind?
34-47kt
What speed is storm force wind?
48-63kt
What speed is hurricane force wind?
64kt+
What is a gust?
A sudden increase in wind speed of 10kt or more above the average wind speed , lasting less than 1 minute
What is a squall?
Gust lasting more than 1 minute
What is a lull?
Sudden drop in wind speed
How do we report wind direction?
The direction the wind is coming from
What is for the name wind direction moving in a clockwise direction?
Veering
What is the name for wind direction moving in an anti-clockwise direction?
Backing
What is an isotach line?
A line showing areas of equal wind strength
What are the4 forces contributing to wind direction and strength?
- Pressure gradient force PG
- Coriolis effect CF
- Centrifugal force
- Friction force
What is the pressure gradient force?
The winds desire to flow from high to low pressure
What direction does wind flow in relation to the pressure gradient force?
Perpendicular
What is the Coriolis effect?
Due to different rotational speeds at different latitudes, when a parcel of air changes latitude it retains its original rotational speed
What is the Coriolis force proportional to?
- Wind speed (faster wind, more deflection)
- Latitude (higher latitude, more deflection)
- Density (greater density, more deflection)
What is geostrophic wind?
When PG and CF are equal and opposite and the wind blows parallel to the isobars
What is the equation for geostrophic wind speed?
V = PG/(2 x w x rho x sinLat)
V proportional to PG
V inversely proportional to density and latitude
What is gradient wind?
When air flows in a circular path and is influenced by centrifugal force, when CfG supports PG wind speed will be increasing, this occurs in a high pressure system, the opposite is true in a low pressure area
What direction does wind flow around a high pressure system in the Southern Hemisphere?
Anti-clockwise
What direction does wind flow around a low pressure system in the Southern Hemisphere?
Clockwise
What is buys ballot law?
In the northern hemisphere if you stand with your back to the wind the low pressure will be on your left, on your right in the Southern Hemisphere
What is the vertical extent of the friction layer?
2-3000ft
How does wind vary with surface friction?
Friction reduces wind speed, as wind speed reduces, Coriolis force reduces
How does the friction layer impact wind direction compared to isobars?
Due to the diminished impact of the Coriolis force, surface wind will somewhat flow across isobars towards an area of low pressure
What is the diurnal variation surface wind?
- During the daytime thermal turbulence mixes slow moving surface air with the faster stream above
- No thermal activity at night so therefore no mixi, slower winds
What is the deflection of surface wind from the 3000ft wind during the day over the sea?
10 degrees
What is the deflection of surface wind from the 3000ft wind during the day over the land in the day?
30 degrees
What is the deflection of surface wind from the 1000ft wind during the night over the sea?
10 degrees
What is the deflection of surface wind from the 1000ft wind during the night over the land at night?
45 degrees
What is the surface wind speed as a % of the 3000ft wind by day over the sea?
70% (2/3)
What is the surface wind speed as a % of the 3000ft wind by day over the land during the day?
50% (1/2)
What is the surface wind speed as a % of the 1000ft wind by night over the sea?
70% (2/3)
What is the surface wind speed as a % of the 1000ft wind by night over the land at night?
25% (1/4)
In the northern hemisphere, as you climb or descend how does the wind change direction?
- Veer as you climb
- Back as you descend
Opposite true for Southern Hemisphere
What direction does the Coriolis force act in the northern hemisphere?
Right
Left in the Southern Hemisphere
What time does a sea breeze act?
Begins 10am, peaks 2-4pm
What is the strength of a sea breeze?
10-15kt
What is the horizontal and vertical extent of a sea breeze?
25-40km inland, 2-3000ft high
What clouds are associated with a sea breeze?
Cumulus development over the land
How does a sea breeze effect the temperature?
Air over the land is replaced by cold sea air
When does a sea breeze usually occur?
During anticyclone due to weak PG
What are anabatic and valley winds?
Anabatic - winds flowing up a mountainside
Valley - winds flowing up a valley
Caused by daytime surface heating
What are katabatic and mountain winds?
Katabatic - winds flowing down the mountainside
Mountain - winds flowing down the valley
Caused by night time surface cooling
What is the Bora?
A katabatic wind on the Adriatic coast where winds can exceed 100kt
What is the mistral?
Cold and dry northerly wind which forms between Atlantic anticyclone and low over gulf of Genoa, wind is accelerated passing between the alps and the Massif Central, can reach 70kt
What are Fohn winds?
- Winds flowing perpendicular to a substantial mountain range
- Must have high moisture content
- The leeward side of the mountain will be warmer
What are mountain waves?
- Wind is disturbed by significant mountains
- Forms a wave like pattern on the lee side
- Can reach FL600 and can be found 100-300nm on the lee side
- Distinguished by lenticular and roll clouds
- Extremely dangerous, especially rotor zones near the surface
What are the characteristics of rotor zones?
Severe turbulence, rotor/roll clouds will show as raged and rapidly rotating streaks
How should you fly if you encounter mountain waves?
- Overfly at 90 degrees
- Fly as high as possible to reduce risk
- Avoid flying on the leeward side and in rotor zones
