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Weather Systems

• Stability

• Clouds

• Storms

• Applications

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Stability

• warm areas on the ground develop lower pressure, as air is forced to rise

• expansion upward: thermal buoyancy of warm air "parcels, or thermals" in heavier surrounding (ambient) air

• in rising, parcels change in character through reduction in pressure:• Universal Gas Law: K = PV/T • an increase in elevation accompanies a decrease in pressure• i.e. the temperature in a rising parcel lapses (decreases) with height• in dry air (without condensation) this is 1° per 100 m of rise• the Dry Adiabatic Lapse Rate (DALR) is a constant.• this is hypothetical only (in order to ignore any mixing effects):

• An adiabatic process such as this is one that is closed to exchanges of mass or energy with its surroundings;

• it is independent of the ambient• the rising air is seen as a closed “parcel”, subject only to laws of physics .• the assumptions are unrealistic but will be relaxed later, as necessary to

add realism

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Stability

• graph paper was devised to show temperature (° T) by elevation (pressure: φ phi)

• T-phi diagram or tephigram depicts selected temperature “ascent paths” for air parcels that may change elevation

• to predict the temperature in upwelling air (cooling) or sinking air (warming)

If air at 2000m is at 12°C and sinks 1000m it will have warmed by 10C° to 22°C.

If air at the surface is at 20°C and rises to 1500m, it will have cooled by 15C° to 5°C.

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Stability

• If the parcel is warmer than its surroundings it will float upward (instability)• but if cooler it will sink downward (stability)

Measure the temperature T at any heights: measured ambient T, to compare with expected dalr predictions of T in a displaced parcel:

• if ambient is less than predicted parcel temperature T1a < T1dalr there is increased buoyancy and parcel ascends: unstable air

• if ambient is more than predicted parcel temperature T2a > T2dalr there is decreased buoyancy and parcel descends: stable air

• If ambient equals predicted parcel temperature T3a = T3dalr there is no buoyancy parcel remains stationary: neutral atmosphere

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Stability

• an air parcel warmer than its surroundings will float upward (instability)

• an air parcel cooler than its surroundings will sink downward (stability)

500

1500

2500

0

1000

2000

205-5-10 1510

T1a

T3a

T1dalr

T2dalr T2a

T3dalr

Air temperature

Elev

ation

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Stability

• surface heating raises T of the hypothetical surface "parcel" (dashed lines show effect of surface heating from predawn to afternoon)

• as surface warms, higher adiabats are followed• radio-soundings of ambient temperatures at heights produce an environmental

temperature curve (red) that is ambient to the hypothetical parcel (blue line); • if a rising parcel cools at the DALR(blue line), and this is cooler than the

measured ambient temperature with height (red line), then the parcel temperature remains below that of the ambient environment. Decreased buoyancy in the parcel forces it to descend, inhibits vertical mixing: stable air.

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Instability

• surface heating raises T of the hypothetical surface "parcel" (dashed lines from predawn to afternoon)

• as surface warms, higher adiabats are followed• radio-soundings of ambient temperatures at heights produce an environmental

temperature curve (red) that is ambient to the hypothetical parcel (blue line)• if a rising parcel cools at the DALR (blue line), and this is warmer than the

measured ambient temperature with height (red line), then the parcel temperature remains over that of the ambient environment. Increased buoyancy in the parcel forces it to ascend, promotes vertical mixing: unstable air.

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Clouds

• dalr assumes no moisture in the hypothetical parcel

• air always contains some moisture both as vapour and in liquid form as sub-microscopic droplets

• steady state exists in which atmospheric water is constantly evaporating and condensing. (and if near 0°C, freezing and thawing)

• temperature in the air determines the balance among these processes:• as air cools, the evaporation rate decreases more rapidly than does the

condensation rate (less energy is available)• a temperature (the dew point temperature) may be reached at which

evaporation is less than the condensation and the tiny droplets collide and coalesce into larger (visible) cloud droplets

• further collisions of these are what forms actual raindrops that are heavy enough to fall (Ice Crystal Process (Wegener -Bergeron - Findeison Theory

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Clouds

Maximum Specific Humidity by Temperature: dependency of maximum moisture upon temperature (some texts show this as vapour pressure, or % by mass)

• at any given temperature, there is a steady state of water vapour:

e.g. @20°C: 15 gm-3

@10°C: 9 gm-3

• conversely, for every moisture content, there is a temperature at which the air will begin to display net condensation: Dew Point temperature

e.g. for 15 gm-3: 20°C

Relative Humidity = actual moisture content / maximum moisture content for air at that temperature e.g. air @20°C with 11 gm-3, RH=9/15 = 73%,but if it cools to 10°C, RH=9/9=100% T (°C)

(gm-3)

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Condensation

• a rising parcel of air will cool at the DALR only to the point that it reaches its Dew Point, called the lifting condensation level (LCL)

• if rising continues, the parcel will then cool at the Saturated Adiabatic Lapse Rate (SALR), which is a variable lapse rate

• the cooling rate is governed by the latent heat (QE ) released which depends on absolute humidity

LCL

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Clouds

As a fall day warms up the surface temperature rises the most : • e.g. from 4°C to 20°C or more.

At each point in time the adiabats shift, • e.g. by the time the surface is nearing 12°, 15°, 20°C as shown, adiabats

show more instability, buoyancy increases

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Clouds

• Therefore clouds form at the elevation at which the dew point is reached

• if the saturated adiabat soon crosses the ambient environment temperature sounding, thin layers of clouds form

• if the adiabat does not intersect the ambient temperatures, and free convection extends upward toward the Tropopause, cloud tops will be high (unrestricted – free convection)

The shaded areas are proportional to the degree of instability or the energy of uplift (turbulence)

If the air then descends, it will warm at the SALR until clouds evaporate, then will warm at the DALR

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Cloud classification and inference

Luke Howard (1802) a pharmacist and amateur meteorologist, identified three distinct cloud forms: • Cirrus (curl of hair) “Parallel, fibres” • Stratus (layered) “A widely extended horizontal sheet, increasing from below.” • Cumulus (heaped) “Convex heaps, increasing upward from a horizontal base.”

Nimbus is applied to clouds from which rain is falling: generally nimbostratus and cumulonimbus.

Classification by height, and hybridization of forms: • High clouds(to near tropopause; ice crystals): cirrus, cirrostratus, cirrocumulus • Middle clouds: altostratus, altocumulus • Low clouds: stratus, stratocumulus, cumulus

All clouds form by vertical uplift of air, with greater vertical development (towering cumulus) associated with instability.

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Cloud forms:Cirrus

Cumulus

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Cloud forms: Stratus

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Cloud forms: Cumulonimbus

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Mechanisms causing air to rise:

Orographic uplift: over higher land

• A Chinook (Föhn) may develop if descending air is warmed by a net release of latent heat, to temperatures warmer than ascending moist air.

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Mechanisms causing air to rise:

• Convective Uplift

• especially on surfaces exposed to the sun and are dry, but also oceanic cells are also significant

• if a stable atmosphere, uplift is subdued and these clouds are generally ephemeral, evaporating as rapidly as they form

• if air cools to its dew point, clouds appear and motions are visible

• Virga are whisps or streaks of precipitation falling out of a cloud which evaporate before reaching the ground

• individual convective cyclonic cells are usually short-lived but intense with heavy showers capable of generating flash floods, strong winds and hail posing risks for people and property

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Mechanisms causing air to rise:

Convective Uplift • upward displacement as air is heated by the ground below

• if the air becomes unstable, thermals intensify, downdrafts may develop adjacent to the strengthening cyclone

• some clouds dissipate, but certain ones may continue to grow, producing turbulence and potentially damaging conditions

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• Tornadoes • especially hazardous are the tornadoes which may develop• energy of convection is accentuated when vertical vectors of up- and

down-drafts occur in conjunction with a squall line (a line of thunderstorms, leading a cold front)

• these conditions permit the out-flowing down-drafts (also called downbursts, microbursts or plough winds) which interact with other storm cells, and produce the rotating motion of the tornado vortex

• initially around a horizontal axis, but is drawn to a vertical position in forming the funnel

• especially violent when there is a strong contrast in air masses along the front, with hot humid maritime tropical air being suddenly uplifted in what are termed supercells

• description of their geographic distribution that has provided clues to their origins, and perhaps will lead to better explanations, predictions and options for what to do about tornadoes

• The most common documentation of tornadoes (~800 per year) is in the US (http://www.nws.noaa.gov/om/brochures/tornado.shtml ): east of the Rocky Mountains during the spring and summer months

• not common in Canada • research continues on prediction

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Watching for tornadoes, and chasing them have become fashionable. Reporting is therefore subject to bias, favouring the more densely populated areas, until automated observation is universally available via satellites and/or networks of Doppler radar. This radar identifies rotating motion in a cloud via the juxtaposition of shortening (red) and lengthening (blue) reflected radar signals. (Christian Doppler, 1840s: when objects move away from the observer, the radiation waves they emanate tend to exhibit a lengthening of their frequency).

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(from NOAA, 2011: http://www.prh.noaa.gov/cphc/pages/FAQ/Hurricanes_vs_tornadoes.php)http://www.aoml.noaa.gov/hrd/tcfaq/A1.html

Midlatitude tornadoes Tropical Cyclones (Hurricanes, Typhoons)

 form atmospheric vortices - extreme Lows; from a single convective storm (i.e. a thunderstorm or cumulonimbus)

atmospheric vortices - extreme Lows; several to dozens of convective storms

diameter

100’s of meters 100’s of kilometers

origin large temperature gradient; upward thrust is due to land-based solar heating (water spouts occur over water); change of wind speed and/or direction with height creates a strong vertical shear of the horizontal winds

near zero horizontal temperature gradient; over equatorial oceans only , latent heat release fuels further uplift/downdrafts in spiralling rays around the calm eye (descending air); die out over land as moisture source is lost

duration

minutes days

winds strongest tornadoes (Fujita Tornado Damage Scale 4 and 5) have estimated winds of 333 kph and higher

strongest hurricanes ( Saffir-Simpson Hurricane Scale 4 and 5) have winds of 210 kph and higher; however landfall may spawn tornadoes, since surface winds decelerate sooner than those above Hurricane tracks

Convective Storms

• Hurricanes • tropical cyclones are the largest of convective storms• hurricanes, typhoons as well as by other names• among the most deadly of naturaltropical storms in Canada: wind, rain, storm

surge, and ocean waves hazards• normally hundreds of kilometres (even 1000 km) across and, rotating around a

relatively calm “eye”, are winds that exceed 118 kmh -1. • surrounded by several convective cells including tornadoes• may persist for several days as they track across the Atlantic, Pacific or Indian

Oceans• formation associated with:

• a very warm ocean surface (>26°C)• in the tropics (beyond a few degrees of the equator, where there is no Coriolis

Force)• away from Jet Stream interference• a seasonal pattern of beginning around the time of the fall equinox in the

Atlantic• initially a simple tropical thunderstorm• may intensify in pressure gradient and therefore wind speed and expand in

size eventually reaching hurricane status as it migrates westward, then curls north (and sometimes even eastward)

• patterns became much clearer once orbiting and geostationary satellites provided visible-light images of cloud cover and thermal infrared data.

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• Hurricanes • Many web sites offer video/animations of the development of hurricanes, both

from the past and as they develop: http://www.atl.ec.gc.ca/weather/hurricane/bulletins_all_e.html

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• Hurricanes:• See: http://www.nhc.noaa.gov/ current tropical storms/hurricanes• http://fermi.jhuapl.edu/hurr/10/igor/igor_gs_overview.gif• http://www.cawcr.gov.au/ research programs

for background theory and timely observations and predictions of hurricane activity as well as advice on how to react to the warnings of hurricanes.

The energy of a hurricane is derived from the very high amounts of latent heat released as huge amounts of water vapour condense, so as lower temperature s or limited moisture supply is encountered, it is weakened and eventually dissipates. Monitoring of the winds and updrafts in hurricanes is conducted by government research agencies primarily to track their trajectories, enabling prediction of where they will contact coastlines (landfall). In fact the US National Weather Service categorizes the severity of tropical cyclones and hurricanes using the Saffir–Simpson scale, based in a part on its association with shoreline risks).

Note: new GIS products: http://www.nhc.noaa.gov/gis/ current systemshttp://www.srh.noaa.gov/gis/tropical/Models/kml/NHC_Model_Forecasts_AutoUpdate.kml and http://radar.weather.gov/GIS.html

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Hurricane-proof engineering and architectural solutions are being developed:• sea walls, buildings on stilts, wind-resistant shutters and heavily anchored

roofs• not widely accepted, especially where their costs are prohibitive• Galveston, Texas which lies at or below the elevation of its sea wall after the

experiencing the most devastating natural disaster in US history (a 1900 hurricane that killed over 6000 people), has adopted many of these procedures.

• in general, society is not willing to avoid the risk; in fact shorelines are increasingly inhabited. Instead ,the response is to expect agencies to issue warnings when it is necessary to evacuate, and in many situations to provide safe shelter.

• Canada is not exempt from the risk of hurricanes, but most have weakened by the time they reach here. The most devastating hurricane reach Canada was Hurricane Hazel in 1954 which killed 81 people, 35 of whom were drowned when the Humber River flooded their homes on Raymore Drive near Lawrence and Weston Road.

• http://www.ec.gc.ca/ouragans-hurricanes/default.asp?lang=En&n=E1111740-1

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“On October 15-16th 1954, Hurricane Hazel dumped 210 millimetres of rain in the Toronto region within 12 hours. Flooding was inevitable: steep slopes along rivers and soil saturated by previous rainfall funnelled 90 per cent of the rain directly into rivers and streams. Flows in the Humber River were four times greater than previously recorded. Hurricane Hazel caused the most severe flood in the Toronto area in recorded history.

Eighty-one people died and thousands of people were left homeless. Most of the bridges on the west side of Toronto were destroyed or badly damaged, as were many on the Don River. Many roads, parks, public utilities - even an entire street of houses - were washed out. The damages were astronomical, reaching an estimated $25 million in 1954 ($169.5 million in 2000 dollars).“ Because of its immediacy, many compelling accounts of the event have been archived, for example, the recorded images of CBC television (http://archives.cbc.ca/environment/extreme_weather/topics/77/ among others.

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• The ramifications of that event have been far reaching.

• Not only has its intensities of rainfall become the standard "event" for designing bridges, culverts and dams, but flood plains now have the protection of law in order to limit the public's exposure to flood hazards.

• Because of it, Conservation Authorities were empowered to manage runoff, to identify, acquire and protect flood plains from development, to diminish the effects of flooding, and have become ecosystem-based stewards of watersheds and natural river functions.

• http://www.ec.gc.ca/ouragans-hurricanes/default.asp?lang=En&n=5C4829A9-1 (extratropical transition of Hurricane Hazel to a midlatitude frontal storm)

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• Frontal Uplift

• Air masses of contrasting properties (temperature and moisture)collide as warm and cold waves develop along the Polar Front.

• When the Polar Front migrates northward (spring) and southward (fall), the contrasts between warm and cold air masses and the succession of changes from one air mass to the other creates abrupt changes in conditions.

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Mechanisms causing air to rise

Frontal Uplift

• Along the front, centres of low pressure develop at the apex of wave-like forms (Rossby Waves) along the front.

• The lead edge of the wave becomes a warm front and the trailing edge a front http://www.theweathernetwork.com/index.php?product=weathermaps&pagecontent=weathermaps&maptype=sys

From the Weather Network2011.09.29

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Frontal uplift

• Gradual uplift of the whole air column occurs along warm fronts • rapid uplift of surface air at cold fronts • Expressed as clouds:

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When a warm front passes:• the temperature increases as the warm air mass gradually replaces the

cold• uplift is gradual as the warm air shears up over the cold air mass• precipitation is steady and seldom intense, falling from overcast skies

(stratus clouds)• in winter the precipitation may be in the form of freezing rain/drizzle

When a cold front passes: • the temperature decreases suddenly as the cold air mass replaces the

warm• precipitation is characterized as sudden intense showers and

thunderstorms of relatively short duration, falling from cumulus clouds• if temperatures drop sufficiently in the cold sector, the rain may change

to snow squalls• cold fronts tend to migrate more rapidly than the warm fronts

• the low centre acts as a hinge creating a wedge of warm air• close to the centre, the pressure gradient steepens, winds intensify and

converge on the cyclone, and precipitation is more probable• if the cold front overtakes the warm front, an occlusion forms, in which

the warm front only exists aloft, above colder air masses below

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• uplift initiates condensation

• dew and fog are not adiabatic, condensation alone without uplift; surface cooling through:

• advection• cold air drainage• most commonly, overnight loss of longwave radiation

• continued condensation in rising air produces precipitation

Advection cooling• advective cooling (warm air cooling by passing across a warm surface) can

also cause "lake effect" precipitation, such as the high snowfall (and rain) downwind of water bodies that is common around the Great Lakes. See:

http://www.noaa.gov/features/02_monitoring/lakesnow.htmlhttp://www.islandnet.com/~see/weather/elements/lkefsnw3.htm

• precipitation prediction involves not only expected condensation quantities, but also rates of coalescence of cloud droplets into rain or snow (etc) of sufficient mass that they fall

Applications

To assess risks – e.g: • getting wet - outdoor activities • getting hit by lightning (Each year in Canada, lightning kills an average of 16 people and causes more than 20 per cent of all forest fires http://cwfis.cfs.nrcan.gc.ca/en_CA/fwmaps/fdr http://www.ec.gc.ca/foudre-lightning/default.asp?lang=En&n=48337EAE-1

• getting hit by hail (While there has not been a single recorded death attributed to hail in Canada, hailstorms probably cause the greatest economic losses of any natural hazard in Canada in terms of property and crop damage http://www.canhail.com/

http://www.icomm.ca/hazards/meteorological/hail.html being late – transportation, communication

• getting too cold or too hot

• preventing or monitoring the spread of: • wild-fire Lawson, O.B. Armitage, W.D. Hoskins 1996 Diurnal Variation in the

Fine Fuel Moisture Code http://www.for.gov.bc.ca/hfd/pubs/docs/Frr/Frr245.pdf • pollution (affected by atmospheric stability)

.

From: http://www.theweathernetwork.com/pollenfx/powcu

Note: The red bar indicates historically when pollen is active for that source.Data provided by: Aerobiology Research Laboratories

Severe storms in Canada• Types of storms • Blizzards• Hail• Heavy rain• Ice storms• Lightning• Thunderstorms

From http://www.getprepared.gc.ca/knw/ris/str-eng.aspx#b6 http://ontario.hazards.ca/data/intro3-e.html

Physics, chemistry, history and geography of atmosphere all of considerable significance to our understanding of it:

• processes of change are ongoing• detected by observation (including now routine measurement) of

• temperature• energy• pressure• winds• gases (including water, emissions etc).

1. Descriptions are ongoing

2. Development of further explanations

3. Weather predictions are improving though imperfect

4. Management of individual, communal and societal responses continue.

Increasingly atmospheric concerns are become climatological…

References: Ahrens, D. C., 1994: Meteorology Today. West Publishing, Co., Minneapolis-St. Paul.

Bluestein, H. B., 1993: Synoptic-Dynamic Meteorology in Midlatitudes. Oxford University Press, New York. 2 vols.

Fujita, T. T. 1973. Tornadoes around the world. Weatherwise, 26: 56-62.

Fujita, T. T. and B. E. Smith. 1993. Aerial survey and photography of tornado and microburst damage. In: The Tornado: Its Structure, Dynamics, Prediction, and Hazards. C. Church, D. Burgess, C. Doswell and R. Davies-Jones (Eds), Washington, D. C., Amer. Geoph. Union, Geophysical Mono. 79: 479-493.

Environment Canada, Glossary Weather Related Terms http://www.ns.ec.gc.ca/weather/glossary.html