Inside CVA - How It Works
The product makes use of real-time METAR observations to establish
C&V (Ceiling & Visibility) conditions at approximately 2000 observing points.
At display points between METAR locations (where there are no direct observations),
CVA ceiling values are derived as follows:
First, nearest-neighbor interpolation is used to find an initial estimate of ceiling height. This initial
estimate is the ceiling height at the nearest METAR location.
Next, the initial height is corrected for any change in surface elevation between the METAR
location and the CVA display point. For example, if
terrain at the display point is 200' higher than at the METAR site, the ceiling value
(given as feet above ground level) is taken as 200' less
than the value at the nearest METAR location. If the resulting ceiling height is negative, a
value of zero is substituted.
Finally, a clear-cloudy test using GOES (Geostationary Operational Environment Satellite) satellite data is applied.
If a conservative interpretation of real-time GOES data indicates that the display point is cloud free,
then the ceiling height at that point is reassigned as ‘unlimited’.
Visibility values at display points are taken as the visibility observed at the nearest METAR site and
are displayed as (i) less than 3 miles (pale yellow), and (ii) 3 miles or greater (pale blue).
Flight category values are derived from ceiling and visibility display values according to the official definition, as given in the table below.
|VFR (Visual Flight Rules)
|| Visual Flight Rules
|| ceiling ≥1000 feet
||visibility ≥ 3 statute miles (sm)
|IFR (Instrument Flight Rules)
||Instrument Flight Rules
||ceiling < 1,000 feet
|| visibility < 3 sm (statute miles)