The Infrared Cloud Camera (IRCCAM) measures the thermal emission of the atmosphere in the 8 – 14 µm wavelength range in order to determine the fractional cloud-cover during day and night, as opposed to visible cloud cameras which only operate during the day.
Data from visible all-sky cameras at Davos, Jungfraujoch and Payerne allow the fractional cloud-cover and cloud types to be automatically determined during the day (Wacker et al., 2015; Aebi et al., 2017). However, in order to complement these methods so that measurements during the night can be conducted, IRCCAM was developed.
IRCCAM consists of a commercial infrared camera (GOBI 640 GigE model) with a microbolometer detector array of 640 x 480 pixels which is sensitive in the 8 – 14 µm wavelength range. The camera points down onto a convex gold-plated mirror which images the sky hemisphere onto the camera detector. Calibration was conducted in the PMOD/WRC reference blackbody over a -20 to +20°C temperature range, with an uncertainty of ± 1°C.
A computer-controlled acquisition system collects sky images with a 1-minute resolution throughout the whole day. After correction for distortion resulting from the convex mirror, images are then calibrated to give the fractional cloud cover as well as a brightness temperature map of the sky and clouds.
Figure 1. IRCCAM measuring at PMOD/WRC.
Figure 2 below shows a sky image from a visible camera (left panel) and IRCCAM (middle panel) in which the fractional cloud cover can be compared. For the period January to October 2016, >123,000 coincident camera images were collected, and ~65% and 85% were found to agree to within a cloud cover of 1 and 2 octas, respectively. The sky brightness temperature [°C] map (right panel) illustrates a wide-range of temperatures.
Figure 2. Sky image from 3 July 2016 showing: left panel) an image taken with a visible sky camera, middle panel) clouds detected by IRCCAM, and right panel) the sky brightness temperature map. The colours in both pictures on the right, represent the sky temperatures [°C] as shown in the colour-bar.
| Wavelength range | 8 – 14 µm |
| Camera properties | GOBI 640 GigE model, focal length: 25mm, field-of-view: 18° x 24° |
| Mirror properties | Gold-coated aluminium mirror, radius sphere: 286 mm, diameter half-sphere: 390 mm |
| Detector array | 640 x 480 px |
| Measurement frequency | 1-minute (standard) |
| Outer dimensions | Height = 186 cm |