Introduction

The amount of water vapor in the atmosphere above the VLA is an interesting quantity for two reasons: water vapor contributes significantly to the opacity of the atmosphere, and the fluctuations in water vapor are the dominant source of interferometric phase fluctuations. We currently have the capability to constantly monitor the phase stability of the atmosphere, with the site-testing interferometer (Carilli & Roy 1998), and we also have the capability to directly measure the opacity in any of the observing frequency bands of the VLA, with the array antennas themselves (through TIP scans - Butler 1996). However, TIP scans are done at essentially random frequency bands and times. Also, the site-testing interferometer has only been operating for a short time, so information over long time periods is not available from that instrument. If information on atmospheric water were available, then it could be used with atmospheric models to produce estimates of opacity quite reliably. The estimation of phase stability from total water is considerably less certain. Although it is generally true that more water vapor means more unstable phase conditions, this is not always strictly true. In the ideal case, it is desirable to know the full vertical distribution of water vapor in the atmosphere, but this is a quantity which is not easy to measure (it can be done with a variety of techniques, but we have no such capability at the VLA). However, even some crude indicator of the total amount of water vapor in the atmosphere is better than nothing. The total column of water is just such a beast. This is the equivalent depth of water which would result if all of the water vapor in the atmosphere were concentrated into a layer of liquid. It is also commonly referred to as the amount of precipitable water. While knowing the precipitable water yields little information regarding the phase fluctuations, it can be used to make a first order prediction of the opacity of the atmosphere.

Given measurements of surface temperature and dew point, a rough estimate of the precipitable water can be made. Uncertainties in the actual profile of water vapor make this a truly rough estimate, but it is probably good to a few 10's of %. Such measurements are available from the electronic versions of the observing logs which have been made for some time now. The logs from mid-1990 up to the present are currently available, so this is the time period that will be presented in this memo.