Categories
METEOROLOGY

WINDS

SEA BREEZE

 

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  1. Blows from sea to land
  2. Occurs especially during warm afternoonswith cloudless skies
  • When insolation heats the land surface more rapidly than sea surface
  1. Layer of air in contact with land = warms up and expands
  2. This creates a Lower pressure areaat the Land surface and Higher Pressure area at surface of sea
  3. Surface wind starts to BLOW from SEA( Higher pressure ) to LAND ( Lower pressure)
  • Typical windspeed : 10kts( Tropis : 15-20kts)
  • Vertical Extend of affected area: up to 3000ft ( 1000ft-3000ft)

 

LAND BREEZE

 

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  1. Surface wind blowing from LAND TO SEA
  2. Occurs at NIGHT
  • Land surface cools down more rapidly than the sea surface
  1. SEA ( Hotter ) = Lower Pressure

LAND ( Colder ) = Higher Pressure

  1. Pressure Gradient will cause the wind to blow from LAND TO SEA
  2. Typical Windspeed : 5kts
  • Vertical extent of affected area : Up to 1000ft

 

 

ANABATIC WIND

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  1. UP-SLOPE wind from colder valley(lower areas) to Mountain top(warmer)
  2. K.A  ‘ VALLEY WIND’
  • Encountered in mountain areas ( Light= 5kts )
  1. Caused : Sun heats areas along sides of the mountain more than the lower areas in the valleys
  2. Increased temperature of the surface at the higher areas causes adjacent air to ‘ warm up ‘
  3. This air expands = lower density ( becomes lighter)
  • This creates a flow of air : UP-SLOPE blowing wind

 

 

 

KATABATIC WIND

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  1. Blows down from HIGHER ( Colder) areas along the mountain sides towards the LOWER(Warmer) areas.
  2. K.A ‘ MOUNTAIN WINDS’ ( light winds = 10kts)
  • Blows Downslope
  1. Occurs at Night
  2. Caused : Rapid cooling of ground at HIGHER Levels( Terrestrial radiation)
  3. Adjacent air cools down as well = becomes Higher density (Heavier )
  • Tends to DESCEND
  • Most pronounced : When mountain sides are snow covered / skies are clear/ Greater radiation cooling
  1. Results in : COLD air DESCENDING into valley

 

GEOSTROPHIC WIND

  • Wind that blows in a straight line Parallel to isobars
  • Occurs only when there are straight(Parallel)and equidistant(constant Gradient) isobar, that are Not Changing with time.
  • Geostrophic wind are affected by 2 forces only : Pressure gradient force and Coriolis Force
  • For Geostrophic wind to exist , Following conditions must be met:

 

  • Latitude is > 15 degrees North/South
  • Straight and Parallel isobar pattern exists
  • Pressure system is not changing rapidly
  • No surface Friction ( Wind blowing above friction layer )

 

CORIOLIS FORCE

  • Varies with latitude and windspeed
  • Is zero at equator
  • Increases with latitude
  • Increases with an increase in windspeed
  • Starts acting on airflow (90 degrees to the airflow direction) as soon as air starts moving and deflects it to the right( northern hemisphere) / Left in (southern hemisphere)

 

SUMMARY OF GEOSTROPHIC WIND

 

Geostrophic wind is :

  • Directly proportional to Pressure Gradient Force ( P.G.F)
  • Inversely proportional to Coriolis force
  • Inversely proportional to Latitude
  • Inversey Proportional to Density of air

 

 

 

 

GRADIENT WIND

  1. Non-Geostrophic wind
  2. Blows Parallel to Curved isobars
  3. More common than Geostrophic wind
  4. Affected by 3 forces : Pressure gradient force, Coriolis forceand Centrifugal Force.

 

Gradient wind circulation around a low:

  • Gradient windspeed is LOWER than a Full geostrophic windspeed
  • K.A ‘Subgeostrophic ‘

 

Gradient wind circulation around a high

  • Gradient windspeed is HIGHER than full geostrophic wind
  • K.A ‘ Supergeostrophic’

 

Highest windspeed

  • Will be experienced as a result of a Large Horizontal PGF
  • PGF is greatest when the Pressure gradient is ‘STEEPEST ‘
  • g. : When pressure changes significantly over a relatively small horizontal distanceor Transition between two air masses

Surface winds 

  1. Surface winds do not blow Parallel to the isobars as in case of the geostrophic and Gradient winds .
  2. Instead , they surface winds cross the isobars at an angle.
  3. It is because The Surface of Earth Exerts a FRICTIONAL DRAG on the air blowing closely above it.
  4. This friction is a resistive force and decreases the ‘wind speed’.
  • Frictional Force is GREATEST at the SURFACE  and gradually DECREASES with ALTITUDE until becoming zero at the top of the friction(Boundary ) layer.
  • At this point the winds blow PARALLEL to isobars at normal geostrophic or gradient speeds.
  • On average this Friction ( Boundary) layer is considered to extend up to about 2000ft.
  • Over Land : Surface wind speed slows down 50% of the geostrophic winds and direction changes by about 30 degrees from the 2000ft winds
  • Over Sea : Surface wind speed slows down to about 70% of the geostrophic winds and direction changes by about  10 degrees from the 2000ft winds.
  • The surface winds are ‘ Backing ” in the northern hemisphere and ” Veering ‘ in the Southern hemisphere from the 2000ft wind directions.
  • ‘Veering’ = a clockwise change in wind direction
  • ‘Backing’ = a anti-clockwise change in wind direction

-When solving questions ,when you are given 2000ft winds( and above) ,use the ‘logic’ of verring/backing application.

  • Given 2000ft winds and you’re asked to determine the Surface wind, the direction change to apply is ‘ backing ‘
  • However, when given surface winds and you’re asked to determine the 2000ft winds , the wind direction change to apply is ‘ Veering ‘.

Summary

  • Geostrophic wind( 2000ft wind) will not be geostrophic wind anymore below the frictional layer ,because it is affected by frictional drag . It changes to become SURFACE WIND
  • How does it change ? : The wind direction ‘BACKS ‘ as it  goes below the Frictional layer to become SURFACE WIND. 
  • How much does it Backs ? : It depends on the surface ( land/sea ) , wind direction backs 30 degrees and speed slows down to 50% of Geostrophic wind ( Land)  and 10 degrees back and slows down to 70%  geostrophic wind( Sea )
  • On the other hand , Surface wind changes to become  2000ft winds with increase with altitude up to 2000ft .
  • How? : The wind direction ‘VEERS’ with altitude up to 2000ft.
  • How much does it back? : Same value as stated above .
  • The above stated are for occurrence in the NORTHERN HEMISPHERE
  • For occurrence in SOUTHERN HEMISPHERE,IT IS THE OPPOSITE .( e,g  wind direction ‘VEERS’ going below the friction layer and ‘BACKS’ with increase in  altitude up to the friction layer)
  • Wind speed  Changes in both hemispheres are the same. ( Decreases going below the friction layer and increases with increase with altitude up to the friction layer )

EXAMPLE QUESTIONS:

  • If the wind at 2000ft is reported as 240/40kts , what is the most likely surface wind at an inland airport.(Northern hemispere)

Answer: 210 degrees/20kts

  • Comparing the surface wind to the 3000ft wind .

Answer : Surface wind blows ACROSS the isobars and is Less than the 3000ft wind.

  • What is the relationship between the 5000ft wind and the surface wind in the southern hemisphere?

Answer : Surface winds are veered from the 5000ft and have a slower speed.