Yesterday afternoon turned cloudy and very dreary, as thick cirrus streamed in from the west-southwest. There was a bit of dust in the air also, as the 5 pm MST view of the Catalinas (above) indicates. However, the surface observations from TUS (below) indicate that the day was characterized by clear (CLR) skies. The reason is that the automated "observing" system (ASOS) used by the NWS only detects clouds below 12,000 ft AGL. Thus, the discrepancy between actual sky conditions versus the ASOS "observed" sky conditions. At larger airports, as per Phoenix's Sky Harbor, the ASOS observations are augmented through the addition of human-observed cloud-cover information.
The visible satellite image (above, from 3:45 pm MST) shows the thick cirrus streaming east-northeastward from northern Baja. Strong flow over the mountain ridge-line of Baja induced an orographic enhancement of the cirrus and gave us our dreary afternoon. Direct solar radiation, as measured at Atmo (green curve below) indicates the impact of the cloudiness beginning after 10 am MST, with very low solar radiation values after 2 pm.
At times limited cloud information from the NWS can have impacts beyond the aviation weather system. For example, when air-quality modeling was done to estimate the impacts that the proposed Rosemont copper mine might have, the ASOS surface observations were used from TUS as a surrogate for solar radiation at the project site to the east. The observed sky cover at TUS was used internally within the model to deduce what the solar radiation might have been. Situations like yesterday were probably not handled well by the modeling approach used by consultants to the project. For those interested, more complete technical details are available at: http://www.squidinkbooks.com/madweather/pdfs/Rosemont-DEIS-Comments-Air-Quality-RAM.pdf
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