Take me to the Steps

Hunga Tonga-Hunga Ha‘apai is a giant volcano under the South Pacific Ocean, 3,000km (1,860mi) east of Australia. The January 15, 2022, eruption of Hunga Tonga propelled a huge dome of water and volcanic ash up through the entire stratosphere and into the mesosphere where meteors burn up (Figure A). It’s the highest eruption ever observed.

A

While Krakatoa and Pinatubo may have launched far more solids than Hunga Tonga, the altitude of Hunga Tonga’s eruption plume and its massive amount of water were the largest of any known eruption. If Krakatoa and Pinatubo were shotgun blasts, Hunga Tonga was a bullet.

B. The Pinatubo eruption in 1991 (left) included far more sulfur dioxide and appeared much larger and denser than Hunga Tonga (right), but Hunga Tonga’s eruption plume was much higher

I made hundreds of photos of twilights caused by the 1991 Pinatubo eruption, the second greatest of the 20th century. Figure B compares Pinatubo and Hunga Tonga sunsets and clearly reveals that the Pinatubo cloud was denser. Nevertheless, Hunga Tonga produced spectacular twilight glows around the world (Figure C). On some days the Hunga Tonga aerosols surrounded the sun with a deep red glow (Figure D).

C. The Hunga Tonga eruption caused spectacular twilights around Earth, like this one over my Geronimo Creek Atmospheric Monitoring Station in Texas.
D. The Hunga Tonga aerosol cloud was dense enough to redden the sky at sunrise and sunset, seen here across the Gulf of Mexico from Port Aransas, Texas.
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The Hunga Tonga eruption increased the water vapor in the stratosphere by 5 to 10 percent. A prominent atmospheric scientist told me the excess would remain in the stratosphere for up to a decade. This is important, for water vapor is the chief greenhouse gas. Thus, adding water vapor to the stratosphere might slightly increase the Earth’s average temperature. It might also slightly reduce the stratospheric ozone layer.

Major volcano eruptions like Hunga Tonga also inject sulfur into the stratosphere, where it is transformed into microscopic droplets (aerosols) of sulfuric acid. The Hunga Tonga cloud contains far less sulfuric acid aerosols than Pinatubo’s. But there’s enough to cause spectacular orange and red twilights around the world.

LIDAR VS. PHOTOMETER

The Meinel method and the Krakatoa studies of 1888 provide credibility for the measurement of the height of the top of aerosol clouds. But how can we know the altitude of the densest bulge in Figure G is correct?

Lidars measure the altitude of aerosol layers by means of a powerful laser. In 2016, I compared one of my twilight photometers with the lidar at Hawaii’s Mauna Loa Observatory, which I was trained to operate by its developer, Dr. John Barnes. After data from six twilight sessions, the stratospheric aerosol layer in the data measured by the lidar and the photometer were compared. The average altitude differed by only 0.9km (2,953ft). This provides credibility for the altitude range in Figure G.

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Project Steps

TWILIGHT GLOW AND VOLCANIC AEROSOL CLOUDS

If your location is experiencing bright, colorful twilight glows from Hunga Tonga, you can measure the maximum altitude of the volcanic cloud using only a watch, or the clock on your phone. This method is described in “The Eruption of Krakatoa and Subsequent Phenomena” (Krakatoa Committee, The Royal Society, 1888). This 495-page report includes vivid descriptions of the famous eruption, the damage it caused, and the brilliant twilight glows that followed. You can read it online or download it for free at Google Books, Open Library, and other sites.

Aden and Marjorie Meinel explain volcano twilights in Sunsets, Twilights, and Evening Skies (Cambridge University Press, 1983). They also describe how to use the twilight method for estimating the peak altitude of volcanic clouds. This remarkable book is available from Amazon, and it’s must reading if you’re interested in studying twilights and the impact giant volcanoes have on the atmosphere.

The Meinel method is easily implemented:

1. Find a location with a clear view of the horizon where the sun will set, free from city lights.

2. Wear sunglasses; don’t look directly at the sun.

3. When the sun touches the horizon, it will disappear in 2 minutes. Make a note of the time the sun disappears, for that’s sunset.

4. After 15–20 minutes, the sky over the sunset will brighten. If volcano aerosols are present, the glow will become bright orange or even brilliant red.

5. Continue watching the sky until glowset approaches. Make a note of the time the upper edge of the glow slips below the horizon.

Haze and the fuzzy, upper edge of the glow can make it difficult to determine the exact time of glowset. That’s why I make a series of photos of the horizon beginning a few minutes before glowset. For best results, mount your camera on a fixed tripod or selfie stick and use a remote control to snap a series beginning just before glowset.

E

F

Depending on your geographic latitude and the height of the aerosol layer, glowset can occur 60 or more minutes after sunset. Figure E is a chart based on Meinel’s formulas that shows the altitude of the top of the volcanic aerosol layer for various latitudes and times from sunset to glowset.

Figure F shows typical Hunga Tonga twilight results at my site, which is just below a latitude of 30 degrees. The box in this chart shows the peak altitude range of the volcanic aerosol cloud over my site during November 2022. The Meinel method is influenced by distant clouds, mountains, and the altitude of your observing site, which should be added to the altitudes in the chart. If you’d like to make your own chart, you can get the complete Excel spreadsheet used for Figure E from here.

The altitudes in Figures E and F are given in kilometers (km). You can multiply kilometers by 3,281 to convert them into feet, but kilometers are much easier to use in atmospheric studies than feet and miles. That’s why altitudes of volcanic clouds and layers of the atmosphere are usually given in kilometers.

The troposphere is the layer beginning at the surface and extending to 10–15km. The stratosphere extends from the top of the troposphere to 50km. Above 50km begins the mesosphere — the region that was penetrated by the unprecedented Hunga Tonga eruption plume.

USE A TWILIGHT PHOTOMETER TO MEASURE THE HUNGA TONGA CLOUD

The twilight method provides the altitude of the top of a volcanic cloud. The profile of the cloud can be determined by lidar, satellite instruments, and the LED twilight photometer I described in these pages in 2013 (Build a Twilight Photometer to Detect Stratospheric Particles).

An LED twilight photometer is an ultra-sensitive light meter that detects the intensity of scattered sunlight directly overhead (the zenith) for around 90 minutes after sunset or before sunrise. My design employs LEDs as photodiodes that detect a narrow band of wavelengths, a principle I discovered in 1971 (“Light Emitting Diodes,” Howard W. Sams & Co.).

In 1990, I began using two LEDs in a sun photometer that provided key data for my paper on 30 years of measurements (doi.org/10.1175/BAMS-D-21-0010.1) of atmospheric haze and the water vapor. That research established the long-term stability of LEDs used as photodiodes and led to the ultra-sensitive LED twilight photometer I described in Make: Volume 44 in 2015.

Hunga Tonga aerosols began arriving over my Texas site in May 2022, and I soon began measuring aerosols higher than 30km (98,425 feet). A NASA scientist read about my findings in emails I sent to scientists studying the eruption. This led to an assignment from NASA through Science Systems and Applications, Inc. (SSAI) to build five LED twilight photometers. My friend Scott Hagerup improved the circuitry and built the five new instruments while I continued measuring the Hunga Tonga plume. These instruments and their findings will be described in a scientific paper that Scott and I are writing with our NASA sponsor.

G

Figure G shows an LED twilight photometer sky profile on November 6, 2022, from the surface to 50km (164,042ft). The profile shows that the densest region of the Hunga Tonga plume was at 25.8km. Figure H is a sequence of some of the twilight photos that evening. The sunset to glowset time (63 minutes) and the graph in Figure F show that the top of the plume still reached 46km (150,918ft) — over 28½ miles high — 10 months after the eruption.

H. Hunga Tonga-Hunga Ha‘apai twilight, 6 Nov. 2022, Geronimo Creek Atmospheric Monitoring Station, Texas (29.6 N, 97.9 W)