"Eruptions on Monserrat island during 5-8 August 1998 observed by the GOES weather satellite"

NOAA operates the Geosynchronous Operational Environmental Satellite (GOES) to take pictures of unpredictable weather as it develops. GOES takes pictures of North America and its surroundings four times per hour in the visible and infrared parts of the spectrum. The thermal infrared is very useful because storm clouds can be tracked day and night.

Volcanic clouds can also be seen by GOES within minutes of their ejection into the atmosphere, so the GOES pictures can be used immediately to warn aircraft in the region. However, in a single channel, volcanic clouds look a lot like ordinary watery clouds.

But, there is a way to tell the difference using two GOES infrared channels originally designed for another purpose.

There normally is a small positive temperature difference between channels at 11 and 12 microns due to absorption by water vapor. In fact, the GOES satellite routinely observes the atmosphere in these two adjacent infrared spectral bands to measure the amount of water vapor in clear air.

Fortunately, with volcanic clouds, there is an abnormal negative temperature difference between the 11 and 12 micron channels. The abnormal difference between these two channels is a unique spectral signature due to the silicate ash clouds ejected by volcanoes. Therefore, we can use infrared temperature differences to help identify ash clouds, and to track their movement among the normally "grey" watery clouds.

In the accompanying movie, we at NASA's Laboratory for Atmospheres applied this technique to a time series of GOES pictures of the eastern Caribbean during a series of eruptions between that occurred there from the 5th to the 8th of August 1997 on the tiny island of Montserrat. While resolution in the thermal infrared is not as high as in the visible, 4 km vs. 1 km (2 miles vs. 0.5 mile), this small island can seen as a few pixels embedded in the arc of the Lesser Antilles in the right side of the picture.

The normally "grey" 11 micron infrared channel is colorized ONLY when and where there is an abnormal temperature difference, and we chose to apply a color that corresponds to the cloud height - red for low, warm clouds; green for middle clouds; blue for high clouds. Remember, color is applied ONLY where there is an abnormal signature.

We see many normal watery clouds streaming from the islands in the trade winds that blow across the eastern Caribbean, since the islands serve as catalysts for clouds and rain in the humid sub-tropical air. There are many such ordinary clouds originating from Montserrat, but every so often a volcanic cloud appears with the abnormal spectral signature.

Most commonly, the volcanic cloud stays low (stays warm and is colored red) and blows to the west with the prevailing trade winds from the east. Sometimes an eruption is strong enough to penetrate higher into the atmosphere where there is an upper level wind from the west or northwest. In that case, the GOES satellite sees the cloud torn in two directions -- the low warm part of the ash cloud is carried to the west, while a high cool part of the ash cloud is carried to the south or southeast. It looks like two clouds pop out of the island in almost opposite directions.

Sometimes, ice-topped storm clouds have a spectral signature similar to volcanic ash clouds. In the GOES movie, these appear as very cold mid-ocean clouds, always colored blue-green. These icy clouds are false alarms which are relatively easy to distinguish from true volcanic clouds.

So, the GOES two-channel test for volcanic ash can be compared to a home pregnancy test -- it is easy to do and fairly reliable, but there are some false alarms that need to be double-checked.

In conclusion, the GOES infrared channels can be routinely used day and night to make cloud images that are colorized where there is an abnormal spectral signature that signifies volcanic ash. The experimental technique was successfully tested on a series of volcanic eruptions from the island of Monsterrat in August 1997.

last updated 15 January 1998
Dennis Chesters
GOES project scientist
Goddard Space Flight Center
Laboratory for Atmospheres
Code 913
Bldg. 22, room 124
Greenbelt, MD 20771
GOES Volcano Watch