Raw water line data from a single track for one WLM box. The figure on the left shows the observed WLM temperature as a function of time. The two main features in this plot are the overall gradual, U-shaped structure to the WLM temperatures over time, and the periodic, abrupt changes in the WLM measurements. The gradual changes are due to following a source in elevation as it rises (high WLM temperatures), transits (low WLM temperatures), and sets (high WLM temperatures) in the sky. As the elevation of the source changes, the total path length through the atmosphere will vary, and hence the total amount of water vapor along the line of sight will change. The abrupt changes in the WLM signal occur when slewing between sources during standard calibration observations. The calibrator sources are generally at a different elevation than the astronomical source, which leads to a sudden change in the WLM temperature.

The information on the water vapor fluctuations occur on a smaller scale than can be been in the figure on the left, but are highlighted in the inset on the right. This inset shows the observed WLM temperatures over a 5 minute scan during this track. WLM temperature fluctuations are easily seen on the 2 second time samples in which the data are recorded. Some of these fluctuations are due to noise in the WLM boxes themselves (about 8 milliKelvin), but also represent real fluctuations in the amount of water vapor in the atmosphere.