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) 
   (4)  map decoding (station plot): please click on 
        Please click on the following URLs for surface station plot:
   (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 
         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 
             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: 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.
          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.
          E. Types of winds: 
             (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 
                   (c). Straight contours or isobars: no centripetal
                   (d). The wind blows parallel to contours or isobars. 
              (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 
                   (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
                    (e). The wind blows parallel (more accurately, 
                        tangent to) contours or isobars. 
              (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
                  (c). The wind blows across isobars from high to 
                        low pressure areas 
   (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
         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.
                 (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. 
                 (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 
             (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 
                  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 
              (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 
              (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 
              (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:
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
   (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
   (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 
   (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 
           (b). Wind blows parallel to isotherms and/or contours.
   (6). Map decoding (500-mb station Plot)  
        Please click on the following links: station plot.jpg
        (Please click on magnifier, +, or just click mouse anywhere 
        on the figure to enlarge the figure). station plot.pdf
        $$$:    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 
         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, particularly from the trough
                 axis to the subsequent ridge axis (front or east
                 portion of a trough, the area of cyclogenesis).
        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 
           (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 
           (B)   Stormy weather (an exchange between cold and warm air
        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:  
            (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 
                     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 
                     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 
         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 
        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 
        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       
  (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 
   (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 
            (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
            (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


         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

             (15o S to 35o 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


         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). 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


          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: