Synoptic Weather Systems
 
1. Surface Weather Map
 (1) Sea-level air pressure (all station pressures are converted to
  equivalent sea-level pressures)
 (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.
 (4) Map decoding: please see Quiz 2 Study Guide in Content Modules
  and Image Database.
  Please click on the following URL: print this page for quiz 2.
  http://www.hpc.ncep.noaa.gov/html/stationplot.shtml
 (5) High Pressure System (High, Anticyclone):
   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):
 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.
   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.
  (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.
   C. Friction force: 
 (A). Affects the boundary layer wind.
 (B). Slows down wind speed and changes wind direction.
 (C). Makes wind blowing across isobars.
 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.
 (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
   A. Definition
 (A) A boundary between the cold air and warm air masses.
 Air mass: A large body of air, normally designated 
  as a High with the exception of the 
  thermal low over the desert area in 
  summer.
 (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
 (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.
 (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)
 (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 a warm air pool aloft.
  c. Surface air pressure starts to increase due to 
  cold air advection (cold air is heavier than warm 
  air).
 (D) Stationary front
 a slow-moving front.
 (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.
 b. Can be a warm front, a cold front, an occluded 
 front and/or a stationary front.
 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.
 
 
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). 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.
 (6). Map decoding: Please see Image Database.
 
 http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cyc/upa/hght.rxml
 
 (7) Weather Systems
   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
 (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.
 D. Ridge
 (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
 (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.
 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.
 G. Zonal Flow
 (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 to the 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.
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).
 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 
  compress ional heating.
 (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 and 
  Surface convergence (net inflow of air).
 C. Thermal Low (heat Low): Warm air rises and is the cause 
  of a surface Low: a warm Low.
  D. Dynamic Low: Either warm or cold air can rise due to the
  upper-level divergence that results in the surface
  convergence (a Low).
 (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) 
   and an upper-level High. 
 (6) Warm core weather system
 A. The air column within a weather system is warmer than the 
  Air column surrounding the weather system.
   B. A surface High intensifies with increasing altitudes
  (hydrostatic equation: warm air promotes an upper level 
  High).
 C. A surface Low weakens with increasing altitudes.
 (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 increasing altitudes.
 C. A surface Low intensifies with increasing altitudes
  (hydrostatic equation: cold air promotes an upper level 
  low).
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.
 C. Hurricanes may develop from an area in the ITCZ.
 (2) Easterly wave
 A. An elongated low in the tropical easterlies.
 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
 A. An intense low or cyclone in the tropics:
 (A). Central pressure of the low is lower than 990 mb.
 (B). Wind speed at the hurricane eyewall 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.