Ash is generally anticipated to absorb radiation around , and so is usually associated with a negative signal at [i.e., a lower brightness temperature (BT) for ash than for clear sky]. However, Fig. 4 shows that the presence of ash in some atmospheres can actually increase the radiance received in this channel, giving a positive signal. Detection methods based on exploiting the absorption of ash at this wavelength are unlikely to be successful where Figure 4 shows a mean signal of 0 or greater. This is only seen when the ash is present over land in a nontropical atmosphere at altitudes greater than about 7 and 4 km for day and night, respectively (plots b and d) and, to a lesser extent, for very low concentrations of ash in a nontropical atmosphere over sea during the day (plot f). The positive signals in Fig. 4 are due to temperature inversions, whereby an ash cloud at altitude emits at a higher temperature than the underlying surface, causing the signal to become increasingly positive as ash concentration increases. The tropopause is a temperature inversion that is always present, with an altitude that is generally lower where the surface and overlying air are cooler, which could explain why this effect is seen for ash at lower altitudes over land at night (plot d) than during the day (plot b) (since for warmer surfaces the effect requires the ash to be at a higher altitude). In tropical atmospheres, it is unlikely that the tropopause would be low enough for the modeled ash to be above it, and so it is not surprising that the effect is more pronounced in the nontropical results. Temperature inversions are more prevalent over land than sea, which could explain the relative prevalence of the positive signal in plots b, d, f, and h. The magnitude of the positive change for nontropical profiles over land at night is greater than during the day in plots b and d, suggesting that the altitude difference between the ash and the tropopause is greater for the night-time cases than for the day-time cases (and the air at the ash altitude is, therefore, relatively warmer). For both day and night, the magnitude of the positive change increases with ash amount. The mean signal for ash at higher altitudes in nontropical atmospheres over sea (plots f and h) is more difficult to interpret, particularly at night where the signal becomes less negative and then more negative as altitude increases (plot h). This could indicate the presence of some weaker temperature inversions in the dataset (weaker relative to those in the nontropical land groups). Figure 4 suggests that the single channel ash signal becomes less negative (and possibly less discernible) as ash altitude increases toward the tropopause, above which it may become positive. This altitude-dependency could make discrimination of ash from clear sky problematic when the altitude of either the ash or the tropopause is not known reliably. If our interpretation is correct and the altitude at which such effects come into play is related to the tropopause, then this could be a problem for ash detection at altitudes used by aviation, particularly over cold, dry land at night when the tropopause is likely to be lower. In contrast to nontropical atmospheres, Fig. 4 shows that ash in tropical atmospheres always produces a negative signal. At low concentrations, the signal becomes increasingly negative with increased concentration indicating absorption (plots a, c, e, and g). It should be noted that we are investigating variability in the signal used to discern ash from clear sky and are not considering meteorological clouds, which are likely to be more prevalent in tropical atmospheres where more water vapor is available.