TWC Today Forums
Present - The Weather Channel 2000 => Local Forecast => Topic started by: cc17926 on March 10, 2010, 06:06:14 AM
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This one has me baffled, I was wondering if someone can clear this up for me..
What declares "Fair" skies. On my IS, the hourly obs says Clear skies (night), and on my XL, it says Fair skies, both for the same reporting station. Checking the report directly from the metar, the visibility is 10 miles, and ceiling is Unlimited, so it's Clear. Checking from the NWS' Regional Weather Roundup, same location Partly Cloudy, and the Point Forecast shows "Fair". I go outside and look up, and the sky is clear, but off to one side of my house, there are clouds around the horizon. Thanks to anyone who can clear this up for me.
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Many reporting stations switched from human to automated observations with the installation of ASOS (Automated Surface Observing System) in the 1990s. The ceilometers only report cloud ceilings up to 12,000 ft, and so to avoid reporting clear conditions when the possibility exists that higher clouds exist, the system reports fair. To tell if your station is automated, look for AO2 in the remarks. The weather service may use satellite data to augment the METAR, and thus sometimes reports sunny (clear) skies even though the actual observation is only fair. I would assume TWC HiRAD similarly assimilates data from multiple sources.
We can determine more if you show the associated METAR or give the station identifier.
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Twcfan gave a pretty good description. This is why in the early '90s you'll see 4000s, 3000s, and juniors giving "Clear" as an observation. In the late '90s-early '00s, many switched to automated sites. Like he said, the automated sites can only report up to 12,000 ft.. so when they reported clear (CLR), it would show up as "Fair" or "Clear Below 12,000 ft" on the legacy STARs.
Example:
METAR KWVI 101253Z AUTO 00000KT 10SM CLR 04/03 A3011 RMK AO2 SLP196 T00440033
This would show up as "Fair" on the XL and "Clear Below 12,000 ft" on the 4000 and junior.
METAR KRHV 081850Z VRB05KT 10SM SKC 25/06 A2986
This would show up as "Sunny/Clear" on the 4000 and XL (depending on time of day), and simply "Clear" on the junior.
Hirad is totally different. It takes into account so many things...like nearby observations, radar, satellite, etc. This is why your IS observations may be different from what's on the XL.