Pressure and Winds

• • • • • Forces Global and Local Circulation Models Air Masses Data presentation Applications 30

Pressure and Winds

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Pressure and Winds

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Pressure and Winds

http://www.ec.gc.ca/ouragans hurricanes/default.asp?lang=en&n=502E94BA-1 33

Pressure and Winds: Forces

• subject to impelling and impeding forces • response to these is that of gases (liquids and gases are capable of “flow” motion) • their constituent molecules and atoms are loosely enough held together that they are displaced continuously • gases are also compressible, including under the weight of the air being supported • pressure in a gas is also dependent on temperature: where a gas is heated, it expands upward (“warm air rises”, as a “thermal”), reducing its density and therefore the weight of gas pressing downward • pressure is measured as kPa (= 100mb), 1013.25kPa is sea level standard 34

•

Pressure and Winds: Forces

•the lower atmosphere is heated differentially from below due to the differences in the energy balance of the earth’s surface • largely arising from differences in the abundance of moisture • other influences are soils and vegetation, but also human disturbance of these • parts of the atmosphere heat more rapidly and to higher temperatures than other parts • temperature gradients produce lateral gradients in atmospheric pressure • rising air has lowered pressure • draws adjacent air towards it. Air therefore moves from high pressure to low pressure along the gradient (a change of a measured value over distance). For air pressure, the steepness of its gradient dictates how strongly air is drawn towards the centre of lower pressure (cyclone).

Air is drawn “down” a

Pressure Gradient

(from high to low)

H Pressure Gradient L Warmer surface Pressure Gradient H

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Pressure and Winds: Forces

Impelling Force acting upon the air: ΔF = ΔF

ρ

(P 2 d - P 1 ) ρ (P 2 - P 1 ) -------------- d impelling pressure gradient (air acceleration) density of air difference in pressure between any two points distance between the two points

ρ

P 1 d P 2 36

Pressure and Winds: Forces

• in the air, there are really very low gradients and densities, and long distances which produce very low accelerations, however, the winds do blow strongly, so impelling forces are very successful at overcoming resistance • opposing all impelling forces are impeding forces defining the “strength” of the stressed substance: S = C + (P 2 - P 1 ) σ ------------ d S impeding strength of the air (inertia) C cohesiveness between particles (negligible between gas molecules, strong within)

σ

friction (drag) of contacted surfaces P 2 -P 1 difference in pressure between any two points d distance between the two points C

σ

P 1 d P 2 37

Pressure and Winds: Forces

5.0ms

-1 6.5ms

-1 | 1.0

| | 2.0 3.0

Wind speed (ms -1 ) | 4.0

after Fons and Kittredge, 1948 | 5.0

| 6.0

• Impeding forces within the air are very low, both due to the lack of internal cohesiveness and the generally low friction with adjacent surfaces • What friction results in is a resistance that prevents the air from continuing to accelerate from the impelling force 38

Pressure and Winds: Forces

Coriolis Force (in Physics) diverts motions to the right in the northern hemisphere and to the left if south of the equator. • a rotating frame of reference and apparent displacement of straight-line trajectories: http://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page 01.cfm

) • The three influences on winds are therefore: • the pressure gradient which impels the wind in a particular direction and determines wind speed, • Coriolis force which imposes a right angled force upon wind direction over very long distances (in the northern hemisphere) • friction which diminishes wind speed and at least in part restores the direction of the wind to that of the pressure gradient 39

Pressure and Winds: Forces

H L

Wherever the ground surface heats up differentially, a pressure gradient develops, whether at a local, continental or global scale Air flow becomes: • where there is high pressure (anticyclones) gradual and widespread descending and spreading outward; clockwise, (blue arrows) • rapidly rising, counterclockwise vortex spiralling inward (cyclones) ; into concentrated low pressure centres (orange arrows) 40

Pressure and Winds: The General Circulation Model (GCM)

• • • • • • Global patterns are well known: as the equatorial surface heats up, the atmosphere becomes warmer and uplift is initiated lateral movement replaces the uplifted air Hadley cells form as air diverges at the tropopause belts of winds develop: NE and SE Trade Winds Westerlies develop poleward of subtropical highs Arctic Front develops between subtropical and polar air masses • These wind and pressure zones shift annually corresponding to the relative position of the sun: • northward in the northern hemisphere’s summer, pressing the Arctic Front closer to the pole • southward in the winter, pressing the westerlies towards the subtopics

L

Arctic Front Westerlies

L H H

NE Trades

L

SE Trades

H

Westerlies 30 N Equator 30 S 41

Pressure and Winds: The Local Circulation Models

• • Similarly diurnal heating can act at a very local scale air flows from cooler surface to warmer surface breezes named for their source area (lake/sea or land) • • Land Breeze (water warmer): summer nights winter

H

cooler surface Lake Breeze (water cooler): • summer days

L

warmer surface

L

warmer surface

H

cooler surface Seasonal changes in pressure centres at a continental scale explain the “reversing” winds of monsoon climates (wet summer, dry winter) 42

Pressure and Winds: Air Masses

• wind sources define properties of descending air by: • humidity: continental or maritime • temperature: polar or tropical Four broad classes: dry humid cool cP continental Polar mP maritime Polar warm cT continental Tropical mT maritime Tropical • boundaries between air masses are called fronts • if a front passes a location then the point is now under the influence of air with properties contrasting to those experienced prior to the front passing • e.g. a temperature drop would be experienced if a cold front passed by: Polar air has • • • replaced Tropical air or, as a warm front passes, Tropical air is replacing Polar air air masses are not static spatially nor temporally; maritime-Polar air over the north Pacific Ocean becomes modified significantly as it passes over the western cordillera, again over the prairies and then again over the Great Lakes 43

Pressure and Winds: Data Presentation

National Climate Data Archive of Canada http://www.msc-smc.ec.gc.ca/climate/data_archives/climate/index_e.cfm

• • • • from point-based anemometers aggregated (averaged) over time periods corrected for elevation before spatially interpolating mapping enables patterns to be recognized http://weather.unisys.com/surface/sfc_con.php?image=ws&inv=0&t=cur • • in the absence of anemometer data, winds are inferred from pressure data graphic displaying directional data often uses wind-rose diagrams: wind frequencies and magnitudes by source directions.

Directions of Winds by Frequencies 6 4 Direction of Extreme Gusts 6 4 2 2 Months Months Winds are reported and predicted to enable decisions regarding individual behaviour, local community activities, and broader societal interests 44

Pressure and Winds: Applications of Wind Meteorology

individual/domestic • • modifying local energy budgets (air drainage) to eliminate frost pockets landscaping for boundary layer enhancement to reduce thermal gradients and • mixing for efficient heating and cooling snow drift management for livestock and doorways community/planning • • • snow fencing for highways, route planning to avoid dangerous winds • wind breaks /shelter belts: to reduce soils erosion, to trap drifting sand • diffusion of contaminants! tall stacks, avoid lee of the escarpment for EFW, lakeshore sites for stacks; dispersal of fumes, smoke from spills/fires/accidents lakeshore sights for aerogenerators noise barriers to trap sound, but effectiveness? and collateral impacts on pollutants and snow • standards for construction, to withstand wind stresses, to provide sufficient heating/cooling global/societal • • • • forecasting weather: heat ⇒ pressure ⇒ wind, and wind chill effect of (differential) global warming on wind patterns Jet Streams for eastbound efficiency, avoid for westbound efficiency monitoring, diagnosing trans-boundary contaminant plumes (acid rain, smog, • • • Chernobyl) wave prediction for shipping wind chill prediction warnings wind shear prediction, warnings 45

References

http://www.canwea.ca/farms/wind-farms_e.php

Benoit, R., Wei Yu and A. Glazer, n.d., A Wind Energy Atlas For Canada : Solving The Challenge Of Large-Area Wind Resource Mapping, Environment Canada http://www.2004ewec.info/files/23_1400_robertbenoit_01.pdf

Fleagle, R. G., and J. A. Businger, 1963, An Introduction to Atmospheric Physics, International Geophysical Series, Vol. 5, New York, Academic Press, 346 pp.

Herschel, W., 1800, Experiments on the Solar, and on the Terrestrial Rays that Occasion Heat; With a Comparative View of the Laws to Which Light and Heat, or Rather the Rays Which Occasion Them, are Subject, in Order to Determine Whether They are the Same, or Different, Philosophical Transactions of the Royal Society of London, Volume 90, pp. 255-283.

King, P., Sills, D., Hudak, D., Joe, P., Donaldson, N., Taylor, P., Qiu, X., Rodriguez, P., Leduc, M., Synergy, R. and Stalker, P., 1999: ELBOW: An Experiment to Study the Effects of Lake Breezes on Weather in Southern Ontario, CMOS Bulletin SCMO, 27, 35-41.

http://www.yorku.ca/pat/research/ELBOW/cmosbull.htm

Newton, I., 1672, New Theory about Light and Colours, Philosophical Transactions of the Royal Society of London.

Planck, M.,1900, Zur Theorie der Gesetzes der Energieverteilung im Normal-Spectrum (“On the Theory of the Law of Energy Distribution in the Continuous Spectrum”) Annalen der Physik (printed 1901).

http://www.canadiangeographic.ca/magazine/mj01/alacarte.asp

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