WIND: WIND Chapters 9 &10


This week: Different to chapter structure in Ahrens: This week: Different to chapter structure in Ahrens Pressure differences Lead to wind How do they arise? Local winds Sea breezes Urban heat islands Mountain and valley winds Synoptic scale winds


Pressure: Pressure Gravity But, the air doesn’t fall down!!! Balanced by air pressure. Pressure: Force due to gravity that the mass of the atmosphere exerts on a unit area of surface.


Pressure: Pressure Atmospheric pressure proportional to temperature and density. Two identical columns. Cooled Warmed


Forces of Nature -- Air in Motion: Forces of Nature -- Air in Motion Horizontal pressure differences are caused by Radiation! The daily variation in the air temperature near the surface of the earth is controlled by: Input -- Energy from the sun Output -- Energy from the earth


Slide6: Forces & Motion Pressure forces are only one influence on the movement of atmospheric air. Air responds similarly as water to this force, moving from higher pressure to lower pressure. Centripetal, friction, and apparent Coriolis are other forces, however, determining winds. Figure 9.16


Circulation: Circulation Air cannot just vanish, hence… We talk about circulations rather than winds


Scales of Motion: Scales of Motion Global (planetary) Size: Global! Time: Days to weeks Synoptic Size: 100’s to 1000’s kilometers Time: days


Scales of Motion: Scales of Motion Mesoscale Size: kilometers Time: minutes to hours Microscale Size: meters Time: seconds


Local Thermal Circulations: Local Thermal Circulations Differences in small scale heating can produce changes in the horizontal pressure gradient. Initially After Heating Warm Cool H L


Local Thermal Circulations: Local Thermal Circulations The horizontal pressure gradient force acts over a small distance (so the Coriolis does not balance – see later). A thermal circulation occurs. Warm Cool H L H L


Land and Sea Breezes: Land and Sea Breezes We can apply this to see where it might work in the real world. Sea Breeze Land Breeze ©1998 Wadsworth Publishing Co.


Slide13: Cold & Warm Cores Figure 10.21A Land heats more quickly than water, creating land-water temperature differences along a coastline. During the day the land's warm-core thermal low draws a sea breeze, while at night, the warmer sea draws a land breeze.


Slide14: Sea & Land Breezes Figure 10.22 Opposing breezes may converge on an isthmus of land, and this rising moist unstable air will trigger thunderstorms. The leading edge of land-water breezes can bring rapid changes in humidity or pollutants.


Heat Island Circulation: Heat Island Circulation The average temperature of a city is typically slightly higher than that of the surrounding countryside. Some days the city temperature may be as much as 10oC higher. This “island” of warmer temperatures is called a heat island.


Heat Island Circulation: Heat Island Circulation Why is this so? More heat sources in the city People, Cars Industry, Furnaces Better absorption of solar radiation Less foliage and evaporative cooling Heat island effect causes a slightly elevated layer of pollution above the city. Stronger winds may form a “plume” of pollution from the city.


Microscale eddies: Microscale eddies Wind will be forced around obstacles It may go over them Or around them Obstacles include: Mountains Buildings Trees


More obstacles: More obstacles When wind is deflected around an obstacle it will produce ‘eddies’ This will result in slower winds very close to the obstacle And stronger winds away from the obstacle If there are a series of obstacles then the wind may be funnelled eg. city streets see OHPs


Slide19: Turbulence & Eddies Figure 10.2A Surface obstructions slow surface winds, which due to viscosity creates eddies of whirling winds in a much thicker layer. The vertical depth of these eddies and gusting winds thickens with surface wind velocity.


Slide20: Scales of Eddies Figure 10.4 Whirling eddies of wind are generated at various scales and orientations. Here are horizontal whirls leeward of an obstructing mountain and vertical rotors, or roll eddies, that can extend for several kilometers. Figure 10.5


Slide21: Wind Forces & Concern Figure 10.6 Reverse flows of wind, triggered by strong eddies, can upset ground traffic as well as aircraft. Clear air turbulence (CAT) refers to invisible wind shear hazards aloft.


Winds in cities: Winds in cities Oke - Boundary layer meteorology


Wind and pollution: Wind and pollution Winds distribute airborne particles Downwind of cities and industrial complexes - more pollution But serve to remove pollution from cities Just think about pollution in the city on still days


Momentum and energy: Momentum and energy Motion is a form of energy (kinetic) This energy is provided by the solar radiation Therefore if we move air which has momentum from one place to another we are transporting energy Therefore this is a mechanism for moving energy from areas of high input


Moisture transport: Moisture transport Moisture will be advected with the wind. Can have dry winds and moist winds examples of dry winds: Foehn, sirocco Recall that moisture transport is equivalent to energy transport


Summary: Summary We have seen How winds are produced How winds are directed What controls the strength of winds How wind distributes energy and moisture


Homework: Homework Be careful: from 2 different chapters: Chapter 9, questions for thought, page 245: Numbers 9 & 12 Chapter 10, questions for thought, page 280: Numbers 5 & 12