Atmospheric Opacity at the VLA

It is important to quantify the atmospheric opacity at the VLA in order to aid in interpretation of data taken at high frequencies. In addition, in the near future it is envisioned that dynamic scheduling will be used in order to select observing programs. Atmospheric opacity will be an important input parameter into dynamic scheduling. In the absence of a dedicated instrument to measure the atmospheric opacity (e.g., a tipping radiometer), the VLA itself can be used to measure this quantity, via the TIP procedure (Butler 1996). Given that time is relatively expensive on the VLA (and, again, given the lack of existence of a dedicated opacity measuring device), one would also like to know if there are other ways of predicting the atmospheric opacity, from independent information. I investigate in this memo the measurements of atmospheric opacity at the VLA from Cthrough Q-bands, and methods of predicting that opacity given seasonal and surface weather information. Note that I use opacity in this memo to mean zenith opacity, and the symbol τ always represents this quantity.