NSA/SKYRAD/C2 - IRT measurements during precipitation events

The IRT measurement of sky/cloud temperature was in question during
precipitation events because rain or snow could accumulate in the 
upward-pointed lens barrel. This was corrected by reorienting the
IRT to measure the sky temperature as reflected by a gold mirror.

Use present weather sensor (nsapwsC2.b1) data to determine timing of precip events.

• Sky Infra-Red Temperature Minima(sky_ir_temp_min)
• Sky Infra-Red Temperature(sky_ir_temp)
• Sky infrared temperature, standard deviation(sky_ir_temp_std)
• Sky Infra Red Temperature Maxima(sky_ir_temp_max)