The Line Of Sight Sky Absorbtion Monitor (LOSSAM)

Variability Local Humidity Thin Cirrus

The Line Of Sight Sky Absorbtion Monitor (LOSSAM)

Variability of the Extinction during photometric nights

Example of two consecutive nights of similar average extinction (0.12 magnitude per airmass) but of different variability (6Feb00:rms=0.008m, 7Feb00:rms=0.013m): both were declared photometric by the observers. In the night of 7 February (below), the sharp decrease of the extinction coefficient at about 1h45 is coincident with the abrupt change in air temperature, indicating a change of wind flow pattern in the first km above the site.

Example of a clear night which shows a change of extinction correlated with the local humidity: this night is not photometric according to LOSSAM criteria.