3.3.2. Dust Storm Affecting Sky Temperature
 The statistical parameters of the dusty events are presented in Table 1, and their distributions are illustrated in the last 3 histograms of Figure 3. For a total of 1160 h of BD, the sky temperature ranges between −21°C and 0°C with an average value of −11.5°C. For the dust storm with 109 observed hours, the temperatures lie between −18°C and 4°C, with an average value of about −6°C.
 The wide ranges of sky temperatures in dusty conditions can be due to several factors. For instance, the properties of the air masses that brought the dust storms to the region can lead to such variations [ Alharbi, 2009; Badarinath et al., 2007]. Moreover, the impact of dust aerosols in the atmosphere on the sky temperature depends mainly on the particle characteristics such as size, shape, chemical composition and mineralogy [Dayan et al., 1991]. While these characteristics can change during dust transport [Badarinath et al., 2007; Kambezidis and Kaskaoutis, 2008; Kutiel and Furman, 2003], they are initially determined by their source regions. This finding needs more investigation in the future, but it is beyond the scope of this study to investigate the correlations between dust aerosol sizes and air mass sources and the variations of the sky temperatures.
 It is also interesting to note that the average sky temperature during BD is comparable to that in overcast conditions, and it is higher than the average temperature of partly overcast skies and of skies with scattered clouds by 3°C and 6°C, respectively.
 In the case of SS, the situation is different from that under the other dusty conditions. The sky temperature confined between −3°C to 8.5°C with an average value of approximately 3°C, which is approximately 31°C higher than the mean temperature of clear skies and approximately 13°C higher than the mean sky temperature of totally overcast skies. This result implies that such dusty events tend to warm up the atmosphere and in some situations, such as those during SS or DS, the effect of these events become more than that of the clouds. Consequently, airborne particles from dust storms alter the local climatic conditions by modifying the energy budget through their behavior and causing heating to the atmosphere.
 Figure 12shows the variations of the sky temperature during two dusty periods; the first one is for a DS event occurring on 26 March 2010, and the second is for an SS event on 19 March 2010. In both examples, the sky temperature increased as a consequence of the dust events, and the sky temperature increase was more than 15°C. However, the two events differ from each other by their response to the event. In the latter case, the dust plume was much stronger than that in the former case, and the sky temperature changes dramatically. Additionally, the amount of aerosol particles brought by the storm and the duration of the event may differentiate the two events from each other. Moreover, the recovery time of the event depends on the severity of the storm and its duration. In some cases, short-lasting SS events have more effect than long-lasting DS events [Maghrabi et al., 2011].
Figure 12. Hourly variations of the measured sky temperature and the visibilities for two dusty events: (a) DS, which occurred on 26 March 2010, and (b) SS, which occurred on 17 March 2010.
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 The effect of aerosol and dust particles on the atmospheric thermal radiation spectrum was examined for 14 dusty events that occurred in Riyadh using MODTRAN. The upper air data from the radiosonde for these events were used as a user input into MODTRAN. The visibility values for these events were 9 km, 8 km, 7 km, 6 km, 4 km, 3 km, 1 km, 800 m, 600 m, 500 m, 100 m, 80 m, 50 m, and 10 m.
 Figure 13 shows the atmospheric thermal radiation spectral distribution for some of the selected visibilities. For visibilities greater than 3 km, no considerable changes were found in the atmospheric thermal radiation spectral distribution for most wavelengths, and the spectral distribution resembles that of the clear sky as shown in Figure 1. For visibilities less than 1 km, the atmospheric thermal radiation spectral distribution has two features. First, the atmospheric emission outside the AW does not change greatly and remains fixed. In this case, the effect of the dust storm on the thermal spectral distribution is negligible. Second, the change of the spectral distribution inside the AW region is evident as we move toward lower visibilities (increasing the amount of aerosol). The increase in atmospheric thermal radiation continues up to visibility = 600 m. A dramatic increase in the atmospheric radiation in the AW region occurs for visibility of 10 m. For instance, as visibility reduced from 6 km to 80 m, the increase in the atmospheric thermal radiation in the AW region was approximately 210%. With 10 m visibility, the AW is totally closed, and the thermal emission resembles that of a BB. Therefore, the main effect of a dust storm on the atmospheric thermal radiation was to increase the thermal emission significantly inside the AW, causing the atmosphere to emit as a full BB.
Figure 13. Atmospheric spectra obtained from MODTRAN using the radiosonde profiles for Riyadh as input with different visibility values.
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 Figure 14 shows the variations of the sky temperatures (integrated temperatures in the wavelength range of 5.5–50 μm) with the visibilities for 14 events. In this example, the sky temperature increases by 2°C as the visibility reduces from 9 to 3 km. At these visibilities, the effect of other meteorological parameters such as water vapor content, air temperature, and/or suspended aerosol particles are dominant. However, for visibilities below 1 km, the sky temperature increases dramatically with decreasing visibility. For example, at a visibility of 800 m, the sky temperature was −14°C, while at a visibility of 400 m, the sky temperature was −8°C, which is equivalent to an increase of approximately 6°C. For a visibility of 10 m, the integrated sky temperature was 4°C. This is approximately 37°C warmer than the clear atmosphere with a visibility of 9 km.
Figure 14. Integrated sky temperatures (in the wavelength range of 5.5–50 μm), which are the results of MODTRAN simulations for Riyadh plotted against different visibilities.
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