Today: Wed 18 Oct 2017

Vertical Velocity or Precipitation

Wind flows in the horizontal at a much higher average wind speed than in the vertical. Vertical motion is roughly two orders of magnitude smaller than horizontal motion. The average vertical wind speed is only a few centimeters per second. An uplift of 6 centimeters per second leads to a pretty significant distance given enough time. For example moving 6 centimeters per second in one hour produces 216 meters of vertical distance. Give it a few hours, and that parcel of air can rise in the vertical over a kilometer. An upward vertical velocity of just 6 centimeters per second can produce a large volume of precipitation if the moisture is present to be condensed.

  • The vertical velocity at 700mb (in mb/hr) is shown instead of precipitation for the 00hr forecast.
    • Negative values indicate ascending air, and positive values denote sinking air. Ascending motion is associated with cloudiness and rain. Large negative values of vertical velocity correspond to areas of heavy rainfall if moisture is available (see description of panel 4). These areas tend to correspond with the storms in the first two panels.
  • The remaining forecasts panels indicate 12 or 24 hour accumulated precipitation, measured in millimeters.
    • The total is the amount of rainfall forecast during the 12 or 24 hours immediately preceding the verification time in the lower lefthand corner of the map.
    • Comparison with the 540 thickness line in panel 2, and the 0�C isotherm in panel 4 can give a good indication of the dividing line between snow and rain.

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