NSA/IRT/C1 - Measurement below minimum threshold

When the actual sky temperature is less than the minimum measureable of 213.2K (-60C), the 
IRT (serial) value is mostly -9999 (missing) with an occasionally valid but usually not 
accurate reading, resulting in the "noisy" data. This situation will occur only during 
periods of very clear skies with low precipitable water vapor (cold and dry). 

The problem was be solved by upgrading the IRT to a model with a lower measuring threshold 
of 173.2K (-100C).

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• Sky Infra-Red Temperature(sky_ir_temp)

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