Cloud base temperature measurements using a simple longwave infrared cloud detection system

Abstract

[1] We have studied the 10 mm temperature of overcast skies, as measured from the ground, in a coastal region of South Australia. The long-term aim of this work is to be able to use measurements of the sky temperature, plus other readily measured ground-based parameters, to recognize the presence, or otherwise, of clouds. Comparisons have been made between the 10 mm sky temperature and cloud height in overcast conditions, measured both with a ceilometer and radiosondes. A low-altitude cloud exhibits a wide range of emissivities, such that clouds with base heights up to 1200 m have emissivities ranging from below 0.2 to close those expected for black bodies at those wavelengths. However, higher clouds exhibit different properties, as their temperatures approach those to be expected for a clear sky with water vapor.

Preliminarily test of physically based models to estimate clear sky emissivity in Tabouk, Saudi Arabia

“…The IR sky temperature (T sky ) was measured using a singlepixel, broadband IR detector designed by our group where the construction of this instrument, its calibration, and all related technical issues have been described elsewhere (Clay et al, 1998;Riordan et al, 2005;Maghrabi, 2007;Maghrabi et al, 2009). The basic detector is a Heimann TPS 534 thermopile.…”

Atmospheric monitoring with an infrared radiometer

“…As cloud shows up as a warming of the sky temperature so, too, will the presence of aerosols, albeit to a lesser extent as they are less efficient blackbody emitters 3 . Figure 4 shows the sky brightness temperature as a function of zenith angle for three different 24 hour periods with very different sky conditions at the H.E.S.S.…”

“…As the sky is not completely covered the telescope rates are unstable during this time and there is a large increase and variance in the sky brightness temperature also. Figure 4c represents data taken after it has rained, the clouds are 3 Which can mean the difference between a brightness temperature of −56…”

“…Since the temperature of the atmosphere drops in a linear fashion as a function of altitude (from above the boundary layer at ∼1 km up to the tropopause at ∼15 km) then with knowledge of the ground temperature and the temperature lapse rate the cloud temperature can be used as a crude estimator of its altitude (see e.g. [2,3], though as cautioned in [3] these estimates can become unreliable due to reflection of IR light from the ground by low altitude cloud, intervening water vapour, or by changes in the emissivity dependent on the cloud composition (e.g. water droplets versus ice crystals).…”

The Central Laser Facility at the Pierre Auger Observatory

“…The CLF can direct light simultaneously into a SD tank via optical fiber to test the relative timing between the FD and SD. The facility also houses a weather station and a radiometric cloud detector [1].…”