Cloud Top Height Determination

Many types of weather analyses using satellite images focus on the qualitative nature of the images (e.g., signatures of short waves in the water vapor imagery and cloud patterns). There are times, however, when the quantitative information of the images, i.e. the brightness temperatures of the pixels, becomes useful. Examples of this use are: locating forest fires and volcanic eruptions in the 3.9 um channel, and identifying the cooling rate and height of overshooting cloud tops for severe storms in the 10.7um channel. As another example of using quantitative information from satellite images, this page shows how to estimate cloud height using the 10.7 um channel imagery and a sounding. The underlying assumption behind estimating cloud height is that the temperature of the cloud top, as sensed by the satellite at 10.7 um, is the same as the environmental temperature at cloud top. A sounding is then used to match the brightness temperature of the cloud to the pressure with that same temperature.
10.7 um Image

The focus of this example is a thunderstorm in north-central KS (yellow box). The minimum temperature at 10.7 um within the yellow box is -60 C (see color bar at the base of the image). The minimum temperature is the best estimate for the temperature of the highest cloud because the cloud element emitting the minimum temperature is most likely optically thick, minimizing the risk of contaminating the cloud top emission with radiation from below the cloud. Referring to the Topeka sounding, valid 2 hours and 15 min. before the satellite image, -60 C occurs near 125 mb. The conclusion is that the pressure at the storm top is approximately 125 mb, clearly overshooting the tropopause.