Chapter12

What is Small Scale? : 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


Friction : 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


Turbulence : 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


Using Turbulence: Snow Fences : 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)


Coastal Fronts & Cold-Air Damming : 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


Cold-Air Damming : 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)


Other Examples : 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


Microburst : 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)


Gravity Waves : 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


Lake Breezes : 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


Derecho : 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


Dust Storms : 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


Dust Devils : 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)


Chinook : 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)


Mountain/Valley Breezes : 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


Katabatic Winds : 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)


Gap Winds : 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)


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


Lenticular Clouds : 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


Santa Ana Winds : 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)


Von Kármán Vortex Street : 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)