Synoptic Weather Systems
 
1. Surface Weather Map
 (1) Sea-level air pressure (all station pressures are converted to
  equivalent sea-level pressures by the hypsometric equation)
 (2) Different locations have the same elevation (sea-level or zero
  height) but may have different air pressures.
 (3) Isobar: Line of equal air pressure. Isobar interval 
  on weather map: 4 mb (National Weather Service)
http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/isobars.rxml?hret=/guides/mtr/fw/grad.rxml 
 (4) map decoding (station plot): please click on 
 
 Please click on the following URLs for surface station plot:
 
  http://www.hpc.ncep.noaa.gov/html/stationplot.shtml
 
http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/sfcobs.rxml?hret=/indexlist.rxml
 
 (5) High Pressure System (High, Anticyclone):
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/prs/hghdef.rxml 
   A. Area of relatively high pressure (more molecules).
   B. Clockwise circulation (in the Northern Hemisphere):
 Caused by the Coriolis force.
 Friction wind or boundary -layer wind (wind blows across
  isobars away from the High center)
 C. Surface divergence (net loss or outflow of air 
  horizontally).
 D. Subsidence (sinking): cloud-free (clear sky), 
  Compressional heating.
 E. Warm and sunny weather (the result of the high air 
  Pressure but not the cause of the formation of high 
  Pressure system): cloud-free or partly cloudy, more 
 solar radiation, compressional heating. 
 (6) Low Pressure System (Low, Cyclone, Storm):
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/prs/lwdef.rxml 
 A. Area of relatively low pressure (fewer air molecules).
  B. Counterclockwise circulation (in the Northern Hemisphere): 
 Caused by the Coriolis force.
 Friction or Boundary-layer wind (wind blows across isobars
  toward low pressure center due to the friction force).
  C. Surface convergence (net gain or inflow of air horizontally).
  D. Rising air: expansion cooling, cloudy or rainy.
 Cold or cool weather: Clouds decreases the solar radiation. 
  Lack of compressional heating due to rising air.
 (7) Forces causing winds (circulations):
   A. Pressure Gradient Force: Pressure difference per unit 
  distance. 
  triggers (starts) the wind.
  http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/pgf.rxml 
   B. Coriolis Force:
 (A) Causes the clockwise circulation around a High and the
  counterclockwise circulation around a Low in the
  Northern Hemisphere.
 (B) The apparent force caused by the rotation of the earth.
 (C) Deflects the wind direction (circulation) to the right
  (look downwind)in the Northern Hemisphere: at the 90-
  degree angle to the right (look down wind) of the 
  wind direction.
  (D) Twice the spinning force around a local vertical axis.
   (E) The Coriolis effect is zero at the equator and is 
  maximum at the Poles.
  (F) Affects wind direction but does not affect wind speed.
  (G) The magnitude of Coriolis force is determined by the 
  wind speed, the angular rate of the earth’s rotation 
  (a constant)and the latitude.
  http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml 
    C. Friction force: 
 (A). Affects the boundary layer wind.
 (B). Slows down wind speed and changes wind direction.
 (C). Makes wind blowing across isobars.
 http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/fric.rxml 
 D. Centripetal force
 (A). Inward-directed force to make an air parcel moving in 
  A circular path.
 (B). The balance force among pressure gradient, Coriolis, 
  and friction forces.
  E. Types of winds: 
 http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/home.rxml 
   (A). The Geostrophic wind
  (a). The upper-air wind: no friction force.
  (b). The pressure gradient force is balanced by 
  the Coriolis force: When the 2 forces are 
  balanced (same magnitude but in opposite 
  direction), the pressure gradient force 
  directs the air parcel from high to low 
  pressure areas and is perpendicular (at the 
  90-degree angle) to contours or isobars on 
  the weather map. The Coriolis force directs 
  the air parcel from low toward high pressure 
  areas.
  (c). Straight contours or isobars: no centripetal
  force.
  (d). The wind blows parallel to contours or isobars.
 http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/geos.rxml 
  (B). The Gradient Wind
  (a). The upper-air wind: no friction force.
  (b). (pressure gradient force + Centripetal force)
    is balanced by the Coriolis force.
  (C). Curve contours or isobars: The centripetal
  force plays an important role in creating 
  winds.
 (d). In a very small section of neighboring
  contours or isobars, curves approximate
  straight lines. Hence, on an upper-air map,
  the winds are general described as gradient
  winds.
   (e). The wind blows parallel (more accurately, 
  tangent to) contours or isobars.
  http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/grad.rxml 
  (C). The Boundary-layer wind (friction wind)
  (a). The lower atmosphere or surface (ground 
  surface) wind: significant friction force.
  (b). (pressure gradient force + centripetal force)
  is in balanced with (Coriolis force + friction
  force)
   (c). The wind blows across isobars from high to 
   low pressure areas
 http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/bndy.rxml 
 (8) Causes of the formation of the surface (sea level) High 
  and Low:
 A. Thermal effect: air temperature difference.
 (A) Thermal Low (Heat Low): Warm air rises and induces
  surface convergence.
 (B) Thermal High: Cold air sinks and induces surface
   divergence.
 B. Dynamic effect: motion of air currents.
 (A) Dynamic Low: surface convergence or upper-level
  divergence induces rising motion.
 (B) Dynamic High: surface divergence or upper-level
  convergence induces sinking motion. 
  C. A High may contain either warm or cold air as compared 
  To the surrounding air.
 D. A Low may contain either warm or cold air as compared 
  to the surrounding air.
 (9) Intensification of a High or a Low:
   A. High: The upper-level convergence exceeds the surface
  divergence. Sinking air (subsidence)
   B. Low: The upper-level divergence exceeds the surface
  convergence. Rising air (ascent)
 (10) Front
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/frnts/home.rxml 
   A. Definition
 (A) A boundary between the cold air and warm air masses.
 (a). Air mass: A large body of air, normally 
 designates as a High with the exception 
 of the thermal (heat)low over the 
  desert area in summer which is denoted 
 as Low.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/af/arms/trp.rxml 
   (b). Air mass Symbols: based on the source, moisture,
  temperature, and stability.
 cP: continental polar air mass. The air mass 
  originates from the Arctic lands (about 66oN, 
  Canada and Siberia).
 mP: maritime polar air mass. The air mass 
  originates from the Arctic oceans (the Gulf 
  of Alaska and the North Atlantic ocean 
  between Canada and Europe).
 cA: Arctic air mass. The air mass originates from 
  the north polar area (Please note that polar 
  and Arctic air masses are named in opposite
  source regions. Air mass originating from 
  the polar area should be names as the polar 
  air mass instead of the Arctic air mass)
 mT: maritime tropical air mass
 cT: continental tropical air mass
  (thermal or heat low in the desert)
 mE: equatorial air mass
 (B) A zone of strong horizontal air temperature gradient
  (temperature difference per unit distance).
 (C) A low pressure zone (elongated shape of a low): 
  Surface convergence and rising air.
 (D) A zone of strong horizontal wind shear (shift in wind
  directions and speeds):
 Winds veer (turn clockwise) as the time proceeds.
 (E) A zone of large horizontal moisture gradient.
 B. Types of front
 http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/cond/frnt.rxml 
 (A) Cold front
 a. Cold air replaces warm air at the ground level as 
  the time proceeds.
 b. Ahead of a cold front (east of a cold front): 
 SW or S winds.
 Warm and moist air (warm sector) from the tropical
  maritime air mass (mT).
 c. Behind a cold front (to the west of a cold front):
 NW or W winds.
 Cold and dry air from the polar continental (cP) 
  or polar maritime (mP) air mass.
  d. veering: Winds change from SW to NW , a clockwise 
  change with time, during the passage of the front. 
 (B) Warm front
 a. Warm air replaces cold air at the ground level as 
  the time proceeds.
 b. Ahead of a warm front (to the north or east)
  SE to NE winds (mP or cP air mass).
 c. Behind a warm front (south of a warm front):
 S or SW winds (warm sector)
  e. winds veer during the passage of the front.
 (C) Occluded front
 a. The merge of the cold and warm fronts.
 b. The surface warm air in the warm sector is 
  uplifted to form an warm air pool aloft.
 c. Surface air pressure starts to increase due to 
  cold air advection (cold air is heavier than warm 
  air).
  e. winds veer during the passage of the front.
 (D) Stationary front
 a slow-moving front.
  b. Winds on the either side of the front are 
  parallel.
 (E) Polar front
 a. The boundary between the polar air mass (cP or mP) 
  to the north and the tropical air mass (mT) to the 
 south (cP/mT or mP/mT).
 b. Can be a warm front, a cold front, an occluded 
 front and/or a stationary front.
  c. Common fronts visiting the USA.
 (F) Arctic front
  a. cA/cP 
  b. mostly in Alaska and Canada
  c. rarely visits the USA.  
 C. Weather associated with a front
 (A) Cold front
 a. Cumulus type clouds, Cu, Cs, Ac, Cb (Ns viewed 
  from the ground surface).
 b. Shower precipitation (heavy but short duration) 
   covering a smaller area (compared to warm front).
 c. Stable air: stratus-type clouds, foggy and/or 
 smoggy.
 (B) Warm front
 a. Stratus type clouds, foggy or smoggy.
  b. Drizzle type precipitation (light but long 
  duration)covering a larger area.
 c. Unstable air: cumulus-type clouds, shower 
   precipitation.
 (C) Occluded front
 a. The weather is similar to either warm front or 
 cold front.
 b. Air pressure in the center of cyclone starts to 
   rise due to the intrusion of cold air (denser).
 C. A warm air pool aloft (inversion).
  (D) Stationary front
 The weather is similar to either warm or cold front.
 D. Life cycle of a wave cyclone
 (A) Definition of a wave cyclone:
 a cyclone with fronts.
 (B) stages
 a. Initial stage: stationary front.
 b. Mature stage: cold and warm fronts.
 c. Dissipating stage: occluded front.
 (11). Dry line:
  A. The boundary between the dry and moist air currents
  B. Moist air (lighter) override dry air (heavier); the
  molecular weight of water vapor (18) is lighter than the 
 mean dry-air molecular weight (about 28).
 C. Frequently occurs in the southern Great Plains (Texas,
  Oklahoma, and Kansas: Dry and sinking air from the Rocky 
  mountains (New Mexico and Colorado)is overrun by the 
  light moist air from the Gulf of Mexico.
 (12). Frontal symbols:
  Please click on the following URLs for surface weather maps
  and front symbols:
 http://www.hpc.ncep.noaa.gov/html/fntcodes2.shtml
 http://www.hpc.ncep.noaa.gov/html/sfc2.shtml
2. Upper-level weather systems 
 (1). Definition of an isobaric surface
 A. A surface on which every point has the same air pressure 
   but different elevation.
 B. 850-mb, 700-mb, 500-mb, 300-mb, 200-mb, 100-mb isobaric
  surfaces.
 (2). Contour line (contour)
 A. line of equal elevation on an isobaric surface.
 B. shows the direction of air flow (wind direction): gradient
  wind (curved contours) or geostrophic wind (straight
  contours).
 (3). The elevation of a point on an isobaric surface is determined
  by the average air temperature from surface to that point.
 (4). Hydrostatic Equation
 A. The air pressure decreases more rapidly (at a larger rate) 
  in a cold air column than in a warm air column.
 B. A cold air column is associated with an upper-level Low 
  and a warm air column is associated with an upper-level 
  High.
 (5). Advection
  A. At a given location, a cold-air advection (inflow of cold 
  air) in the low-level decreases the elevation of that 
  location on an isobaric surface aloft. Warm air advection
  in the low-level increases the elevation of that 
  location on an isobaric surface aloft. 
 B. Warm air advection (in the baroclinic atmosphere
  where there is a horizontal temperature gradient).
  (a). Contours crosses isotherms (baroclinic atmosphere) 
  (b). Wind blows from a warmer air region to a colder 
 air region.
 C. Cold air advection (in the baroclinic atmosphere
  where there is a horizontal temperature gradient).
 (a). Contours crosses isotherms (baroclinic atmosphere).
 (b). Wind blows from a colder air region to a warmer 
 air region. 
 D. Absence of advection (in the barotropic atmosphere
  absent of a horizontal temperature gradient).
  (a). Contours run parallel to isotherms (barotropic 
  atmosphere).
  (b). Wind blows parallel to isotherms and/or contours.
 (6). Map decoding (500-mb station Plot) 
 Please click on the following links:
 http://www.csun.edu/~hcgeg004/500-mb station plot.jpg
 (Please click on magnifier, +, or just click mouse anywhere 
 on the figure to enlarge the figure).
 http://www.csun.edu/~hcgeg004/500-mb station plot.pdf
 http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/upaobs.rxml?hret=/indexlist.rxml
 http://www.srh.noaa.gov/ohx/educate/current_environment.html
 $$$:  Please note that air temperature and dew point (or 
  dew point depression) are in units of oC on the
  upper-air weather map while they are in units of oF on 
  the surface weather map.
 (7). Weather Systems 
http://mapmaker.meteor.wisc.edu/~jbrunner/ackerman/upperair/upperairbkgrnd.html 
   A. Low: 
 (A) The circular area of the relatively low elevation on 
  An isobaric surface.
 (B) Counterclockwise circulation.
 (C) Cloudy and/or stormy weather.
 B. High
 (A) The circular area of the relatively high elevation on 
   An isobaric surface.  
  (B) Clockwise circulation.
 (C) Fair weather.
 C. Trough 
http://www.indiana.edu/~geog109/topics/10_Forces&Winds/sfc_trough.html 
 (A) The elongated area of the relatively low elevation.
 (B) The equatorward (southward in the northern hemisphere)
  extension of a Low.
 (C) V- or U- shaped contour lines.
 (D) Counterclockwise circulation.
 (E) Cloudy or stormy weather, particularly from the trough
  axis to the subsequent ridge axis (front or east
  portion of a trough, the area of cyclogenesis).
 D. Ridge http://www.geologywales.co.uk/storms/upthere.htm 
 (A) The elongated area of the relatively high elevation.
 (B) The poleward (northward) extension of a High.
 (C) Inverted V- or U-shaped contour lines.
 (D) Clockwise circulation.
 (E) Fair weather.
 E. Blocking High http://weatherfaqs.org.uk/node/144 
 (A) A migratory High that blocks and splits the 
  westerlies into two branches of flows.
 (B) The west part of the High: warm and moist air
  associated with the SW wind.
 (C) The east part of the High: cold and dry air 
  Associated with the NW wind.
http://climb.mountainzone.com/everest/2003/html/fagin-weather.html 
 F. Cutoff Low 
 (A) An upper level Low that drifts toward south, away 
  From the main westerlies.
  (B) The cutoff Low is south of a jet stream.
 (C) Cloudy or stormy weather.
http://www.weather.com/outlook/weather-news/news/articles/massive-cutoff-low_2009-11-18 
 G. Zonal Flow:
http://www.islandnet.com/~see/weather/elements/jetstream2.htm 
  (A) The west-east flow (the contour lines run more or 
  Less in west-east direction, parallel to the 
  latitudes).
 (B) Fair weather (lack of the exchange of cold and 
 warm air masses).
 H. Meridional Flow
 (A) The north-south flow (the contour lines run more or 
  Less in the north-south direction, parallel to 
  meridians).
 (B) Stormy weather (an exchange between cold and warm air
  masses).
 I. Zonal Index cycle
 (A) The time required for the change of the upper air 
  Flow patterns from the zonal to the meridional then 
  back tothe zonal flows: one to six weeks.
 (B) Zonal Index
 a. The difference in the sea-level air pressure 
  between 35 and 55 degrees latitudes.
  b. High zonal index: strong westerlies.
 c. Low zonal index: weak westerlies.
  J. Jet stream: 
  http://squall.sfsu.edu/gif/jetstream_pac_init_00.gif
   http://squall.sfsu.edu/gif/sathts_pac_500_00.gif 
  http://www.avsim.com/avwx/avwx_files/jetstream.gif 
  (A) Definition
  A band of strong air current in the upper-atmosphere,
  normally identified from the 300-mb isotach (line 
  of equal wind speed) of 75 knots or stronger, 
  separating the cold air to north from the warm air 
  to south.
  (B) Types
   (a). Polar-front jet
 In the extratropic (north of 35 oN to about 60 oN).
  Large meandering and displacement in location.
  Associated with the surface polar front, a zone
  of strong horizontal temperature gradient.
  Exists all year round.
  (b). Subtropical jet
  In the subtropic latitudes (around 25 to 35 oN)
  Small variation in its location.
  Occurs along the north edge of the subtropical 
  High.
  Exists in winter only.
  (C) Causes
  Jet stream is a zone of strong horizontal temperature
  gradient, similar to the surface front.
  (a). Thermal wind theory: 
  Upper-level wind results from the lower-level
  horizontal temperature gradient.
   The upper-air pressure gradient is determined by 
  the lower atmosphere horizontal temperature 
  gradient.
  A strong horizontal temperature gradient in the 
  lower atmosphere created a strong upper-air 
  pressure gradient, and hence a strong wind in 
  the upper atmosphere because the wind strength 
  is determined by the pressure gradient force.
  Surface front is the area with a strong horizontal
  temperature gradient and thus is associated with 
  the jet stream in the upper atmosphere.
  (b). Confluence theory
  A cold air current from north flows side by side 
  with a warm air current from south.
  The confluence occurs in the area south of a Low 
  and north of a High, usually identified from the 
  700-mb weather pattern.
3. Surface High or Low vs. Upper-Level High or Low
 (1) Surface High
   A. The warm air (or warm weather) is the result of the
  compress ional heating of the sinking air and more 
 solar radiation (clear sky). Warm air is not the cause 
  of a surface High but rather the result of a High.
 B. A surface High is characterized by the subsidence 
  (sinking air) and the surface divergence (net outflow of 
  air).
   C. Thermal High: Cold air sinks and is the cause of a 
  Surface High (a cold High). the Siberian High and the 
  Canadian High in winter, for example. 
 D. Dynamic High: Either cold or warm air can sink due to 
  the upper-level convergence that results in the surface
  divergence (a surface High): a warm High due to 
 compressional heating. Subtropical Highs (Anticyclones)
  such as the Hawaiian High and Bermuda High)
 (2) Upper-level High
   A. The warm air in the lower-level (includes the ground 
  surface)is the cause of an upper-level High (the 
  hydrostatic equation).
 B. There can be either a High or a Low on the surface.
 (3) Surface Low
 A. The cold air (or cold weather) is the result of cloudy
 weather that reduces the solar heating on the ground
  surface. The cold air is not the cause of the surface 
  Low.
 B. A surface low is characterized by the rising air,
  surface convergence (net inflow of air into an area), 
  and upper-level divergence (net outflow of air from 
  an area)
 C. Thermal Low (heat Low): Warm air rises and is the cause 
  of a surface Low: a warm-core Low. Examples: California 
  heat low, Thar Desert Low (northwest India/Pakistan), 
  Llanos heat low (Columbia and Venezuela), Sahara desert 
  Low, and Chinese monsoon heat Low in summer, etc.
 D. Dynamic Low: Either warm or cold air can rise due to the
  upper-level divergence that results in the surface
  convergence (a Low). The Arctic lows (Aleutian and 
  Icelandic lows, for example.
 (4) Upper-level Low
   A. The cold air in the lower-level is the cause of an
  upper-level Low (the hydrostatic equation). 
   B. There can be a surface High or Low.
 (5) Summary (The thermal effect and the hydrostatic equation)
 A. A cold air column establishes a surface High (cold High) 
 and an upper-level Low.
 B. A warm air column establishes a surface Low (heat Low 
  or thermal Low)and an upper-level High. 
 (6) Warm-core weather system (thermal Low or heat Low)
 A. The air column within a weather system is warmer than the 
  air column surrounding the weather system.
   B. A surface High intensifies with increased altitudes
  (hydrostatic equation: warm air promotes an upper-level 
  High).
 C. A surface Low weakens with increased altitudes.
  D. California heat low (thermal Low) in summer, for example.
  (A). Rarely produces precipitation because the Low is 
  shallow and air is relatively dry (desert air).
  (B). The warm air rises from the equator moving northward and sinks at the 
  subtropical latisudes. The subtropical sinking air meets the surface rising  
  dry and warm air (Low) and diverges outward creating a High and the trade 
  wind inversion aloft (in average, 1,500 m height). 
  (C). Warm rising air is dry and not deep enough (topped 
  by a trade wind inversion) to produce Cb clouds 
  or other types of rain-making clouds such as Ns,St, 
  As and/or cumulus congestus). Please click on the 
  following link to see the vertical structure of a 
  therma LOw  
 
   http://en.wikipedia.org/wiki/Thermal_low 
 
 (7) Cold-core weather system
 A. The air column within a weather system is colder than the 
 air column surrounding the weather system.
 B. A surface High weakens with increased altitudes.
 C. A surface Low intensifies with increased altitudes
  (hydrostatic equation: cold air promotes an upper level 
  low).
 (8) Hydrostatic Equation:
  A. Air pressure decreases more rapidly (at a larger rate) in
  a cold air column than in an adjacent warm air column 
  because the cold air is heavier than the warm air of the 
  same volume.
  B. The cold air in the lower atmosphere is associated with
  the low aloft (in the upper atmosphere) while the warm air
  in the lower atmosphere is associated with the High aloft.
 (9) Intensification of surface cyclone (low) and anticyclone (high)
  A. Intensification of surface cyclone:
   (a). The Upper-level divergence exceeds the surface 
  convergence (loss of air in the air column).
  (b). The area ahead of the trough axis (front portion of
  the trough or from the trough axis to the next ridge 
   axis)
  (c). Cyclogenesis: cyclone formation or intensification.
  B. Intensification of surface anticyclone
  (a). The upper-level convergence exceeds the surface 
  divergence (gain of air in the air column).
  (b). The area between the ridge axis and the next trough
  axis.
  (c). Cyclolysis: surface cyclone weakening. 

4. Tropical weather systems

(1) Intertropical convergence zone (ITCZ, Doldrum)

A. A convergence zone between trade winds (easterly winds)

from both hemispheres.

B. A major rainy zone in the tropics (accounts for 80% of

rain).

C. Hurricanes may develop from an area in the ITCZ.

D. Doldrum: the average location of the ITCZ.

E. The location of the ITCZ is more variable in Asia

(15^o S to 35^o N)

(2) Easterly wave

A. An elongated low in the tropical easterlies (prevailing

winds are from E, NE, or SE).

B. Travels from east to west (driving by easterlies or east

winds).

C. Behind the wave: surface convergence and stormy weather.

D. Ahead of the wave: surface divergence and fair weather.

(3) Hurricane

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hurr/grow/home.rxml

A. An intense low or cyclone in the tropics:

(A). Central pressure of the low is lower than 990 mb.

(B). The sustained wind speed (one minute average wind

speed)at the hurricane eyewall equals or exceeds

74 mph.

(C). May have a hurricane eye (clear sky, warm, subsiding

air).

B. Originates from an easterly wave or ITCZ.

C. Most frequent in the western North Pacific and in summer

and early fall.

D. Saffir-Simpson hurricane damage potential scale: 1 to 5

with 5 as the most potential damage.

Dr. Lin’s Publications: http://www.csun.edu/~hcgeg004/articles/