The Soufriere Hills Volcanic Eruption
On Monday, July 28, 2008, at approximately 11:45 pm, the Soufriere Hills
Volcano in the West Indies on the island of Montserrat erupted, resulting
in a large ash plume that could be followed for miles on satellite imagery.
This particular volcano is very active and often emits ash into the atmosphere
on a regular basis. The last major eruption on the island was back
in mid-July 2003 causing portions of its volcanic dome to collapse and creating
huge pyroclastic flows (fast-moving clouds of hot ash and rock) that spread 2
kilometers out over the ocean.
In this latest event, the Soufriere Hills Volcano emitted ash several thousand
feet into the atmosphere overnight. The animated satellite image below clearly
shows the eruption. For your reference, the island of Montserrat is labeled with
the four-letter identifier: TRPG. This is the identifier for Gerald's Airport
located on the island. As you watch the satellite loop, you will see ash spreading
west, southeast, and northwest from the island.
Figure 1. Infrared satellite imagery depicted as an animated GIF showing the
geographic landscape of the northeastern Caribbean, including the island of Puerto
Rico. In this animation, one can see the volcanic ash plume being emitted from
the Soufriere Hills Volcano on the island of Montserrat labeled with the four-letter
identifier TRPG.
As the ash rises from the volcano, it begins to encounter various wind currents at
different levels in the atmosphere. The 1200Z upper air sounding from San Juan, Puerto
Rico is depicted. On the right side of the sounding plot, you can see where the winds
are originating throughout the depth of the atmosphere. In the lowest levels of the
atmosphere, winds are out of the east. In the mid-levels of the atmosphere, winds are
out of the southeast. In the upper levels of the atmosphere, winds are out of the northwest.
These differing wind directions make up what forecasters coin directional wind shear.
So, as the volcanic ash encountered each of these atmospheric levels, it responded by
being blown in the direction the wind was traveling.
Figure 2. The 1200Z upper air sounding taken at the National Weather Service (NWS)
Weather Forecast Office (WFO) in San Juan, Puerto Rico during the morning after the
eruption is shown. Notice how the winds significantly change direction with height,
causing the volcanic ash to respond to multiple directional forces.
Following the volcanic ash from the island of Montserrat northwestward toward the U.S.
Virgin Islands and Puerto Rico, you can see this mid-level ash interacting with mid-level
moisture causing brief shower formation (in the form of colder cloud tops depicted at
1015Z between St. Maarten (TNCM) and St. Thomas (TIST) before succumbing to the
surrounding drier air.
Volcanic ash can be extremely dangerous, not only to your respiratory health, but also
to aircraft. Ash that gets ingested into airplane engines can cause serious damage to
the engine itself. The National Weather Service in San Juan, in conjunction with the
Volcanic Ash Advisory Center in Washington DC, and area airports within Puerto Rico and
the U.S. Virgin Islands, have fostered partnerships to ensure that the people we serve
stay safe. At the moment this volcanic ash was detected, area airports were alerted to
its presence so that customers such as airlines can take the appropriate actions to
protect life and property.
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