What is Small Scale

1
What is Small Scale?

• Winds that occur in a time span you can perceive
  (minutes to hours) and across distances you can
  see (a few tens of miles)
• Little is understood about the weather on these
  dimensions (very difficult)
• The Coriolis force is negligible at these scales
• Strong pressure gradients interact with
  topography to produce these motions

2
Friction

• The friction in a fluid is called viscosity
• Molecular viscosity molecules in the fluid
  bumping into each other (a property of that
  fluid)
• Eddy viscosity friction caused by larger swirls
  of air that impede the flow (slower air mixes
  with faster moving air)
• Eddy a swirl in the air

3
Turbulence

• Irregular, almost random pattern of wind
• Caused by eddies turbulent eddies
• Bumpiness experienced in an airplane
• Fluctuations in the wind are called gusts
• CAT Clear Air Turbulence, usually caused by
  wind shear

4
Using Turbulence Snow Fences

• The stronger the wind, the easier it is for the
  wind to pick up snow on the ground and carry it
  horizontally
• This can reduce visibility, cover roads, and
  interfere with life
• Snow fences or windbreaks (lines of trees) create
  turbulent eddies in the wind
• This increases the friction force and causes the
  wind speed to slow (dropping the snow)

5
Coastal Fronts Cold-Air Damming

• Coastal front Boundary between warmer air over
  the ocean and colder air over land in the winter
• Coastal fronts are a small version of a
  stationary front
• Cold air and snow showers may only be a few miles
  away from warm air and rain
• Leads to complicated forecasts

6
Cold-Air Damming

• Cold air that is forced to remain due to high
  mountains blocking its movement
• Shallow layer of below-freezing air lies
  underneath warmer air aloft
• Rain falling into cold air may result in freezing
  rain (lots of ice at the surface)

7
Other Examples

• Harmattan Cool air from the Sahara Desert in
  the winter moves south and displaces warmer air
  (like a mini cold front)
• Southerly Buster Cold air from the ocean is
  funneled and accelerated through mountains in
  eastern Australia

8
Microburst

• Particularly dangerous small-scale wind generated
  from thunderstorm downdrafts
• Downdraft air is cooled by precipitation
  evaporating, making it more dense and causing it
  to fall faster
• Air rushes sideways and swirls upward upon
  hitting the ground
• Can result in wind speeds similar to a weak
  tornado
• Particularly dangerous to aircraft taking off or
  landing at an airport
• Called a downburst or macroburst if the area
  affected exceeds 2.5 miles
• Microburst almost killed President Reagan (winds
  above 150 mph happened 7 minutes after he landed)

9
Gravity Waves

• Straight lines of clouds often appear when stable
  air is disturbed by an obstacle
• Similar in nature to the waves produced when a
  rock is thrown in water
• Gravity is the restoring force to dampen the
  oscillations
• Clouds form where the air is rising
• Can also form due to intense wind shear
• Morning Glory Linear cloud that forms along
  the leading edge of a gravity wave in Australia

10
Lake Breezes

• Formed in the same manner as the sea breeze,
  except the body of water is smaller (lake)
• Similar to a mini cold front
• May initiate thunderstorms in the summertime if
  the convergence is sufficient

11
Derecho

• Strong winds created by a line of severe
  thunderstorms
• Straight-line winds, not rotary winds like in a
  tornado
• Winds can be as strong as 150 mph
• Most often associated with stationary fronts in
  mid summer
• Strong winds aloft mix down in the downdrafts
• Bow Echo line segment that gets pushed
  outward due to the strong winds as seen on radar

12
Dust Storms

• A strong pressure gradient dry ground dust
  storm
• Strong winds pick up dust from the ground
• Can be triggered by thunderstorm outflow, Blue
  Northers, etc.
• Drought combined with poor land use can
  contribute to dry soil conditions

13
Dust Devils

• Thin, rotating columns of air
• Intense heating of the surface causes air to rise
• Stretching of vorticity results in faster
  rotation (conservation of angular momentum)
• Form under clear skies
• Winds are much weaker than tornadoes or water
  spouts
• Willy-Willy (Australia), Simoom (African/Arabian
  deserts)

14
Chinook

• Indian word for snow eater
• Occurs on the leeward side of the Rocky Mountains
  (the Great Plains)
• Air is brought down dry adiabatically from the
  tops of mountains, warming on the way down
• Temperature in Rapid City, SD rose from -4 F to
  45 F in two minutes (record)
• Same phenomenon occurs elsewhere in the world
  Foehn (Alps), Puelche (west slope of Andes),
  Zonda (east slope of Andes)

15
Mountain/Valley Breezes

• In the daytime, the mountains heat up faster than
  the valleys
• As a result, air rises over the mountains and air
  is pulled up from the valleys to replace it
• Valley wind winds from the valley
• At night, the mountains cool off faster than the
  valley
• Air in contact with the mountains gets cold and
  falls down out of the mountains into the valley
• Mountain wind winds from the mountains
• Usually gentle breezes
• Hiraoroshi breeze west of Tokyo

16
Katabatic Winds

• Form in mountainous regions with steep-sided,
  snow-covered large plateaus
• Chilling of the air in contact with the snowy
  plateau creates a strong pressure gradient
• Steep slopes allow air to fall downhill rapidly
• Winds can exceed 100 mph
• Bora (Adriatic coast), Mistral (French Riviera)
  and Fall Wind (Greenland, Antarctica)

17
Gap Winds

• Gaps or passes in the mountains can act like wind
  tunnels and allow the winds to accelerate
• Squamish (British Columbia), Levanter
  (Gibraltar), Tehuantepecer (Central America)

18
Boulder Windstorm

• Downslope winds associated with mountain gravity
  waves can cause very strong winds
• Common near Boulder Colorado
• Known to cause property damage

19
Lenticular Clouds

• Lens shaped cloud
• Hang over mountainous regions for hours
• Resemble hovering UFO motherships
• Winds blowing across mountain ranges can create
  vigorous gravity waves
• Conditions beneath lenticular clouds are
  extremely turbulent

20
Santa Ana Winds

• Downslope wind (similar to Chinook) that is
  stronger (similar to Boulder windstorm)
• High pressure near Rocky Mountains causes dry
  mountain air to move towards California
• Air moves downhill, warming and resulting in a
  lower relative humidity
• Very dry conditions and gusty winds create a
  serious fire hazard
• Berg Wind (South Africa), Leveche (Spain),
  Khamsin (Egypt), Leste (Canary Islands), and
  Sirocco (Mediterranean)

21
Von Kármán Vortex Street

• Appears like a long interlocking chain of eddies
  in the clouds downwind of an island
• Mountain disrupts the smooth flow of the wind
  past it
• Airflow is deflected mainly laterally, not
  vertically
• Wind next to mountain feels the friction, but air
  further away is not slowed much
• Horizontal wind shear is created
• Wind shear causes the deflected winds to rotate
  in an alternating series of vortices (one
  cyclonic, the other anticyclonic)