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23 January 2004

Science Notebook: Investigating the stratosphere from the ground

by Forrest M. Mims III
www.forrestmims.org
Geronimo Creek Observatory

Students and citizen scientists can easily observe and study the presence of aerosols in the stratosphere using little more than a watch and a camera. This column explains why these observations are significant and how they are conducted.

Structure of the Atmosphere

If the Earth were the size of an official NBA basketball (228.6 millimeters or 9 inches in diameter), the atmosphere would be only about as thick as a 0.13 mm (0.05 inch) pencil lead. When the atmosphere is stable, smoke, dust and clouds tend to form layers in the sky. These layers can be very flat and extend for thousands of kilometers. This layering tendency characterizes the atmosphere itself, which is organized into several distinct layers. The lowest two layers are especially important to life on Earth.

This chart indicates the approximate height of aerosol layers in the stratosphere (y axis) for a range of twilight durations (x axis) and latitudes. Click image to enlarge.

The troposphere is the layer nearest the surface where weather occurs. Around 99 percent of the atmosphere's water vapor is found in the troposphere, and most clouds are found there. Air in the troposphere is often in motion. Yet even the troposphere itself has layers. That's why the bottom sides of puffy cumulus clouds are so flat. Water vapor condenses into liquid droplets that form clouds when the temperature falls below the dew point. The bottom sides of cumulus clouds denote this point.

The atmosphere's column water vapor or precipitable water (PW) refers to the thickness of the layer of water that would result if all the water vapor in a vertical column through the entire atmosphere could somehow be precipitated as liquid water. Total column water vapor can range from 70 mm or more in humid, tropical regions to a few millimeters at mountain tops and in arctic regions. Most of this water vapor is found in the first few thousand meters of the troposphere.

The stratosphere is the very dry, clean and stable region of the atmosphere above the troposphere. Only about a millimeter or so of column water vapor is found in the stratosphere. While our daily existence might seem to be influenced much more by tropospheric weather than happenings in the stratospheric, the stratosphere plays a key role in the survival of life on Earth. That's because the stratosphere is home to around 90 percent of the ozone layer. The remaining 10 percent is found in the troposphere. The ozone layer is critically important to blocking dangerous wavelengths of ultraviolet sunlight.

The stratosphere is clean and dry because very little air from below manages to reach it. The boundary that separates the troposphere and the stratosphere is called the tropopause. The flat top of large thunderstorms often delineates the tropopause. Ordinarily, air in the troposphere and stratosphere do not mix. Mixing can occur, however, when the tops of giant thunderstorms poke through the tropopause into the stratosphere.

The cleanliness of the stratosphere is important for maintaining the integrity of the ozone layer. Gases such as chlorine, bromine and methane can lead to significant destruction of ozone. Such destruction can be exacerbated in the presence of particulate matter from volcanic eruptions, which is why considerable effort is made to monitor the stratosphere using instruments in satellites or suspended from balloons.

Monitoring the Stratosphere

A Sun photometer (www.concord.org/haze/) can measure the aerosol optical thickness of the entire atmosphere with a high degree of accuracy. But how does one subtract the huge tropospheric amount from the total to arrive at the much smaller stratospheric contribution?

One way is to measure the optical depth of the entire atmosphere for several years when the stratosphere is very clean. Similar measurements made after a major volcanic eruption will show a significant increase in the aerosol optical depth. Careful analysis will allow the stratospheric optical depth to be estimated from a time series of such ground measurements.

But this method requires a long term series of measurements that require considerable time to accumulate. I know this first hand, for I have making such measurements on a regular basis since 1989.

There is a much simpler way to keep track of stratospheric aerosols and derive qualitative information about their abundance. It's a method pioneered after the historic volcanic eruption of Krakatoa (or Krakatau) in Indonesia in 1883. Many observers reported intensely colorful sunsets and twilights following this huge eruption. The duration of twilight glows was much longer than usual, and scientists could estimate the height of the volcanic aerosols by measuring the duration of twilight glows.

Similar twilight phenomena have followed other volcanic eruptions that propelled large volumes of particulate matter and sulfur dioxide into the stratosphere. Long-lasting, colorful twilight glows followed the eruption of Agung (1963), El Chichon (1982) and especially Pinatubo (1991). During the first few years after the Pinatubo eruption, I measured and photographed hundreds of such twilights, many of which lasted a full hour.

Determining the Height of Aerosol Layers

Aden and Marjorie Meinel described a very simple method for estimating the altitude of the aerosol layers that cause extended twilights. The method is described in "Sunsets, Twilights, and Evening Skies" (Cambridge University Press, 1983), a fine book which belongs on every sky watcher's bookshelf.

The Meinel method is based on the duration of the twilight glow. If you watch a sunset on a clear evening, you will notice that the entire western sky is brightly illuminated for at least 10 minutes. Gradually, the twilight forms a bright arc in the sky. The time between the setting of the Sun and the upper edge of the twilight glow is closely related to the altitude of aerosol layers in the stratosphere.

The nearby chart for estimating the height of stratospheric aerosols from the duration of the twilight glow was made using the equations given by the Meinels in "Sunsets, Twilights, and Evening Skies." The spreadsheet from which the chart was made is available at www.forrestmims.org (click on the Scientific Research page).

Currently, the stratosphere is exceptionally clean and at or near its background condition. Thus, twilight glows are much more subdued than when aerosols are present, and the twilight glows may be too faint for the Meinel method to be applied.

An alternative approach is at least as useful, and all that is needed is a watch and a camera. The watch is not necessary if the camera time stamps photographs optically or digitally. The camera is not necessary if all you wish to do is measure the time between sunset and glow set. Having both instruments will allow you to refine your observations and schedule photographs at uniform intervals, say, every 5 minutes. If possible, try to take all the photographs with the same f/stop and exposure time.

Twilight is bright within 20 minutes of sunset when the stratosphere is clean. When stratospheric aerosols are present, the twilight glow may fade 5 to 20 minutes after sunset. Beyond 20 minutes past sunset, Then the glow may become much brighter and intensely orange and even red in color.

The idea is to collect a record of twilight photographs at known times and intervals on very clear days. If you begin such a program now, when the stratosphere is very clean, you will have a record of enormous changes should a major volcanic eruption occur. You may even observe a fairly rare Tropical Stratospheric Reservoir event, like the one I found in 1995. These events occur when sulfur dioxide normally present in a belt around the Equator breaks away and spirals toward the poles. The 1995 event was so significant that it produced twilights that closely resembled those produced by a major volcanic eruption. Because this event was not associated with a volcanic eruption, it was missed by the satellite community. For details, see the report by Mims at al., in the Bulletin of the Global Volcanism Network 21, February 1996 (http://www.volcano.si.edu/gvp/reports/bulletin/contents.cfm?issue=2102&display=complete).

Meanwhile, be sure to keep an eye on the twilight glows.