EAS 380 Day 2 Content - Use of Upper Air Charts

Material Covered:

1. Reading upper-air charts.
   1. A water vapor satellite loop (Satellite sensor detects infra-red radiation from water vapor at 400-700 mb) can show the mid-tropospheric wind flow and the progress of mid-level troughs.
   2. The warm front process (warm air overrunning cooler air underneath) can be seen by warm-air advection at 700 mb and 850 mb.
   3. Dew point depression is plotted instead of dew point on an upper-level chart. (See handout of Upper-Air Plotting Guide)
   4. Height gradient is shown on an isobaric chart and is equivalent to the pressure gradient on a surface chart. (See Upper-Air Plotting Guide)
2. Uses of upper-air charts
   1. Scale height of the Rockies is about 3 km (700 mb)
      1. Neither Pacific air masses nor Pacific troughs and cyclones can climb over the North American Cordillera
      2. Analysis showing this is just wrong!
   2. 850 mb used to represent air masses (free of local topography). Dew points are representative of potential moisture. (NWS Storm Prediction Center analyses start at 8°C). Warm-air advection is associated with sinking; cold-air advection is associated with sinking.
   3. 700 mb analysis functions as the 850 mb chart over the Rockies. Can track upper-level troughs and lows more easily than at lower levels. 10°C is indicative of a lower tropospheric warm air inversion that would inhibit convection. This is not a hard and fast rule (your mileage may vary). Also, the axis of dilatation at 700 mb often marks the northern edge of precipitation during a frontogenesis event.
   1. 500 mb chart used to determine steering flow for storms. Examination of vorticity field determines potential for cyclone intensification.
   2. Use 200, 250, 300 mb chart, depending on the day, to find the jet streaks and jet streak circulations (looking for upper-level divergence).

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Last updated: Friday, September 28, 2007 8:16 AM

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