Weather: is the short term, day-to-day condition of the atmosphere

Transcription

1 Weather

2 Weather: is the short term, day-to-day condition of the atmosphere Meteorology the scientific study of the atmosphere They focus on physical characteristics and motion and how it relates to chemical, biological, physical and geological processes Study complex linkages of atmospheric systems Use this to forecast short-term future conditions of the atmosphere

3 Mid-Latitude Weather Systems Cyclone Low-pressure system Rising air, precipitation Relatively small, compact weather system Anticyclone High-pressure system Descending air, clear skies Relatively large, expansive weather system weather caused by the interaction of cyclone and anticyclone

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5 Air Masses: each area of the earth s surface imparts temperature and moisture characteristics to the air above This creates large, regional, homogenous masses of air with specific temperature, moisture and stability characteristics Air mass distinctive body of air with specific temperature and moisture characteristics air masses interact to produce weather systems

6 Air masses are labeled by their source region and also based on thermal and moisture characteristics: 1. Thermal Tropical (T) Air Mass warm Polar (P) Air Mass cold Arctic (A) Air Mass cold Antarctic (AA) Air Mass - cold 2. Moisture Continental (c) Air Mass dry Maritime (m) Air Mass - moist

7 Continental Polar (cp) cool dry air masses associated with anticyclonic conditions Form only in the N. Hem b/c S. Hem doesn t have polar land masses Most developed in winter Major players in high and middle latitude weather systems and climate Colder, more dense air pushes warmer air up over it

8 Maritime Polar (mp) cool, wet air masses associated with cyclonic conditions Occur over northern oceans Subpolar low pressure cells are located within this type of air mass

9 Maritime Tropical (mt) warm, humid masses generally associated with cyclonic activity, however this varies with air mass origin There are two that affect N. America: Gulf/Atlantic very unstable atmospheric conditions Active late spring to early fall (hurricane season) Responsible for the humidity experienced by the SE U.S. Tends to be warmer and wetter b/c of the pressure of the warm Gulf Stream current Pacific stable to conditionally unstable Cooler and less humid than Gulf/Atlantic Reason W U.S. is less humid and receives less rainfall than E Cooler and drier due to cooler Pacific currents along the west coast

10 Continental Tropical (ct) warm, dry masses associated with anticyclonic conditions Typical of tropical/subtropical continental locations Hot, dry air (subtropical high pressure cells

11 Air Mass modification: Air masses that move from their source region are modified by the temperature and mositure characteristics of the locations they pass EX: cp air masses that travel south from their Canadian source will bring frigid temperatures to southern latitudes - They begin with temps as low as -50 C but are generally warmer by the time they reach their southernmost extent warm as they move over warmer surfaces - they also can change their moisture characteristics lake effect snow

12 Atmospheric Lifting Precipitation

13 In order for precipitation to occur, an air mass must: Cool adiabatically (by expansion) Reach the dew point temperature = achieving saturation After saturation, water vapor will condense and form moisture droplets Moisture droplets form clouds and possibly precipitation for this to happen and air mass must be lifted!

14 There are four principle lifting mechanisms: Convergent lifting air along the surface flows toward a low pressure center and rises Convectional lifting local heating differences warm air above causing it to rise Orographic lifting air is forced over a topographical barrier (mountain) Frontal lifting air rises along the boundary between contrasting air masses

15 Convergent Lifting: Air flows from different locations (converges) toward the center of a low pressure system Displacing air upward Common in tropical regions where convergent lifting of warm, moist air leads to the formation of tropical storms Within the equatorial region the trade winds converge along the ITCZ, forcing air up creating large thunderstorms and high precipitaiton

16 Convectional Lifting: When cooler air masses move over warmer surfaces the warmer surface heats the air mass from the bottom up (can be large or local scale) Warmer air from the bottom of the air mass rises causing convection within the air mass Convection will continue within the air mass as long as conditions are unstable This lifting will create clouds and convectional precipitation (Atlanta)

17 Orographic Lifting: An air mass is physically forced upward as it pushes past a mountain As the air rises over the mountain it is cooled adiabatically When it reaches the lifting condensation level, clouds form As it continues up the mountain convectional precipitation will develop Precipitation will continue until the moisture content goes below saturation or the mass moves over the barrier

18 Windward slope is the side of the mountain over which precipitation occurs As the air mass moves up the windward slope it will cool adiabatically and eventually precipitate Much of the air mass s moisture is lost along the windward slope Leeward slope is the opposite side of the mountain Descending air is heated by compression as it moves to lower elevations Any remaining moisture evaporates Air is typically hot and dry

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29 Chinook winds warm downslope airflow characteristic of leeward side Rainshadow term applied to leeward side that receive very little precip due to orographic lifting

30 Frontal Lifting Front: the leading edge of an advancing air mass The leading edge of a cold air mass is a cold front The leading edge of a warm air mass is a warm front

31 Cold Front: Indicates the advance of a cold air mass; cold air is dense and a cold air mass will move as a dense mass of cold air, hugging the ground As a cold front moves into a region occupied by a warmer air mass, the warm air mass will rise sharply up and over the cold air mass The warm air mass will cool adiabatically as it rises over a cold air mass This creates instability and cloud/precipitation (cumulonimbus)

32 Cold Fronts: ctd Cold fronts are recognised by: Marked wind shift (usually northerly winds in the N. Hem Temperature drop Lower barometric pressure due to rising air along the front Thick cloud formation and possibly heavy precipitation Once the front passes, have high pressure and lower temperatures In the U.S. and Canada, the clash of cold cp and mp air masses from the north and warmer mt air masses from the south dominate our weather patterns

33 Warm Front: Indicates the advance of a warm air mass The warm air mass is unable to displace the more dense cold air mass Warm air moves gently over the cold air mass The warm air will cool adiabatically as it rises over the cooler air mass Water vapor will condense and clouds and possibly precipitation will develop (nimbostratus) Precipitation is usually the light gentle drizzle rain

34 How air masses interact in the mid-latitudes

35 Midlatitude cyclone - a mid latitude cyclone develops along the polar front when warm and cold air masses collide resulting in rising warm air and low pressure development

36 Development of a midlatitude cyclone includes four stages that take 3-10 days to complete: } } } } Cyclongensis - development of a low pressure center along the polar front Open stage - northward advancement of warm air in front of the front with cold air advancing south, curculating around a low pressure center Occluded stage - cold front overrides the warm front Dissolving stage - warmer air is pushed aloft and rising stops

37 Cyclogensis - low pressure systems develop and strengthen The polar front presents a discontinuity in temperature, moisture and wind direction leading to unstable atmospheric conditions Air convergence occurs along the cold front but it must be accompanied by diverging air aloft Diverging air aloft allows the surface low to strengthen as more surface air is forced to rise to fill the void at upper levels

38 Open stage - surface convergence strengthens the developing low Warm air moves north east of the cold front while cold air continues to push south Ccw flow around the low draws more cold air from the north and west and more warm air from the south

39 Occluded stage - the cold front overtakes the warm front wedging underneath it The cold air mass is more homogenous in cold temperature and high pressure than the warmer air mass It moves as a unified mass of colder, more dense air It moves faster than the warm front (40 km/hr vs km/hr) Eventually it plows into the warm front forcing the warm front to move over the cold front Occluded front - when a cold front overtakes a warm front Stationary front - stalemate between warm and cold air masses Generally you ll get moderate to heavy precipitation which will eventually taper off as the cold front continues to force the warm air aloft

40 Dissolving stage - the low pressure center dissolves As the cold front continues to move under the warm front the warm air behind the warm front continues to rise As the cold front completely overtakes the warm front the warm air is pushed to upper levels above the cold air mass With no more warm air to move over the cold front the source of energy and moisture that feed the cyclone is gone As the warm air ceases to rise over the cold front the low will dissolve

41 Cold Front

42 Warm Front

43 Stationary Front

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48 A sharp cold front boundary can be seen on both satellite pictures and radar composites

49 Cyclones

50 Characteristics of a wave cyclone Cyclones normally move eastward, propelled by prevailing westerlies aloft Cold front pushes south and east, supported by a flow of cold, dry polar air The wind direction and temperature change abruptly as the cold front passes

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52 Example of an Occlusion Cold Front Approaches a Warm Front

53 Example of an Occlusion Cold front meets the warm front ahead of it Warm air is lifted upward, away from the surface Occluded front becomes longer more of the cold front converges with the warm front

54 Example of an Occlusion Eventually, the cold front completely overtakes the warm front Entire system is occluded

55 Cold front cold air moves toward the warm air mass ahead Warm front warm air mass moving toward a cold air mass Stationary front two unlike air masses remain side by side, but neither air mass has recently undergone substantial movement Occluded front cold front meets warm front, warm air gets lifted upward, away from the surface

56 When Air Masses Collide Dry Line a front between warm, moist air and warm, dry air Lifting is extremely strong Heavy thundershowers, hail, and possible tornadoes are characteristic of a dry line Seen almost exclusively in the southern states west of the Mississippi ct air versus mt air masses

57 Dryline Over Texas Notice the differences in the dew points on both sides of the dryline.

58 Daily weather forecasting: Synoptic Analysis: evaluation of weather data Numerical weather prediction - data base of weather conditions used to forecast future weather events Weather forecasting is difficult b/c the atmosphere is a nonlinear (irrational) system and tends toward chaos Weather forecasts require information on: Barometric pressure and pressure changes Surface air and dew point temperature Wind speed, direction, and character Cloud/sky conditions and visibility Current weather and precipitation record

59 Today s weather