Opaque Clouds

Positive Volcanic Clouds

There are several reasons why a volcanic cloud may be characterized by a positive, rather than negative, band 4-5 brightness temperature difference. The most common reason is that the cloud is still opaque. The temperature difference method relies upon radiation from the surface beneath the cloud passing through the cloud, and being scattered and absorbed by particles in the cloud. If it is opaque to thermal radiation, the signal seen by the satellite comes from emission from the cloud top. At the temperatures of typical clouds, this results in a positive temperature difference. If the cloud is positive due to its opacity, one would expect that it would become negative over time, and thus be able to be discriminated using the band 4-5 temperature difference.

Examples

Mt. Spurr, Alaska: August, 1992
The best example the transition from an opaque volcanic to a transparent one.
The evolution of the volcanic cloud signal can be seen in a series of images of the August 19, 1992 volcanic cloud. Volcanic clouds imaged during eruption and shortly thereafter are optically thick and probably contain abundant water droplets and/or ice, which makes their spectral signal very much like a meteorological cloud. As the volcanic cloud disperses and becomes transparent, the spectral properties of the cloud change, first at the edge and then throughout. This produces a volcanic cloud signal that can be distinguished using a brightness temperature difference determined from thermal bands 4 and 5 of the AVHRR.

Sequence of Band 4 images

Sequence of Corresponding Band 4 minus 5 images
Redoubt Volcano, 1989-90
Typical examples of opaque volcanic clouds with a positive band 4-5 temperature difference. Note that the negative values over land may be due to a temperature inversion.

February 15, 1990 1316Z
Images: left band 4, right band 4-5

March 23, 1990 1331Z
Images: left band 4, right band 4-5
Augustine Volcano, 1986
Typical examples of opaque volcanic clouds with a positive band 4-5 temperature difference. Note that the negative values over land may be due to a temperature inversion.

March 27, 1986 1422Z
Images: left band 4, right band 4-5