The Secrets of Cirrus

Cirrus clouds significantly influence our climate but we don't know to what extent because of uncertainty about exactly how the tiny ice crystals they contain scatter light. Researchers at the University of Hertfordshire are developing aircraft instruments and laboratory experiments to help uncover these secrets of cirrus.

Thin and wispy they might be, but cirrus clouds cover the majority of the Earth’s surface and exert a profound influence on our climate.

You may be surprised to know that cirrus can have an overall warming effect – despite being composed mainly of very cold ice crystals.

This is because cirrus tends to reflect heat back towards the Earth’s surface, not unlike greenhouse gases such as carbon dioxide or methane.

And since higher temperatures can lead to more cirrus, potentially causing even higher temperatures, these clouds may have a positive feedback on our climate – in contrast to clouds composed of water
droplets which tend to dampen temperature variations.

While modern climate change models are improving all the time in their accuracy and applicability, cirrus clouds are still not well represented and can result in major uncertainties in predictions.

This is mainly because too little is known about the nature of the tiny ice crystals that make up the clouds, their abundance, shapes (ie: crystal ‘habits’) and sizes, and how they scatter both incoming sunlight and outgoing heat radiation from the earth.

So we don’t yet know how much we have to worry about ‘greenhouse ice’.

The reason why not enough is known about cirrus is because the study of atmospheric ice is very challenging.

The ice crystals can be as small as a bacterium and exist in a delicate balance with their surroundings: slight changes can result in rapid growth or disappearance.

Typically, researchers study cirrus by taking to the skies in specially equipped aircraft that carry wing-mounted instruments or ‘probes’.

These probes attempt to gather information about the cloud particles as the aircraft flies through the clouds at several hundred miles per hour.

Some probes are able to count and size the particles, data from which particle concentrations in the clouds can be determined. However, these probes provide no information on the shapes or orientation of the particles and it is these factors that govern how the particles scatter

Other probes carry high-precision cameras that are able to capture images of individual particles, but the image resolution is such that the shapes of the smaller particles, less than about 20 micrometres in size and potentially of great importance, cannot be deduced.

Hertfordshire’s research

For many years, researchers at the University of Hertfordshire had been engaged in the development of ground-based instruments for the classification and identification of airborne particles in various environments.

This work was primarily targeted at the detection and monitoring of particles that were of concern because of their potential health risk to people who may inadvertently inhale them.

Examples included the detection of airborne asbestos fibres around demolition or refurbishment sites, and the detection of airborne microbes that could represent a military or terrorist threat.

These instruments employed a technique referred to as spatial light scattering analysis.

When a particle is illuminated by light, in our case usually from a laser, it scatters some of the light in a pattern which is determined by the particle’s shape, size, and structure (as well as by the
wavelength and polarisation of the incident light).

The scattering pattern that results is a unique ‘finger print’ by which the particle may be characterised or classified.

Furthermore, such scattering patterns do not suffer the depth-of-field or focus problems associated with images of particles.

In the late 1990’s, Hertfordshire researchers began working with scientists from the Met Office to see whether these spatial scattering techniques could be employed in aircraft-mounted probes.

This resulted in instruments that are now used on the UK’s FAAM (Facility for Airborne Atmospheric Research) aircraft, serving both the Met Office and the academic community.

By 2005, the success of these early instruments attracted the attention of American researchers. The Colorado-based US National Centre for Atmospheric Research (NCAR), was in the process of acquiring
a new research aircraft, HIAPER (High performance instrumented airborne platform for environmental research).

NCAR commissioned the University of Hertfordshire researchers to build an even more advanced spatial scattering probe for HIAPER that
could measure in situ cloud ice particles down to ~1 micrometre in size, at rates of several thousand particles per second, whilst the aircraft was travelling at speeds potentially in excess of 400 mph. A daunting challenge.

The new instrument, referred to rather unflatteringly as ‘SID2H’ (SID from Small Ice Detector), is now in the construction phase and due
for delivery to UCAR in early 2007.

Its designers, Drs Edwin Hirst and Richard Greenaway, are working with the leader of the Hertfordshire team, Professor Paul Kaye, to
provide HIAPER with the most sensitive particle shape instrument built to date.

But this isn’t the whole story. Two other team members, Drs Zbigniew Ulanowski and Evelyn Hesse, are working on an equally critical aspect – how to automatically interpret the scattering patterns that
SID2H will record by the million.

This work, funded by the UK’s Natural Environment Research Council, involves not only the generation of new theoretical models to predict
how ice crystals of particular shape will scatter light, but also the development of unique laboratory-based experimental methods for measuring scattering patterns from individual exemplar crystals, each representing one of the many habits that atmospheric ice crystals can assume.

Since real ice crystals of such small size are extremely difficult to maintain in the laboratory, the researchers have developed ice analogues,
crystals of a material that is stable at room temperature but exhibits the same crystal structures and optical properties as real ice.

Each crystal can be levitated and rotated without contact in a so-called electrodynamic balance, where forces produced by high voltages connected to electrodes counteract gravity acting on the crystal, and illuminated with a laser beam to allow the required scattering patterns to be recorded.

Such patterns can then form the basis of an automated computer-based pattern matching process for classifying the multitude of real patterns that will be recorded by SID2H.

With climate change now rising to the top of both scientific and political agendas, scientists around the world are striving to understand more fully the factors that govern and affect our climate.
The data that will be produced by the new SID2H probe will be a small but important contribution to this process.

Paul Kaye, Edwin Hirst, Richard Greenaway, Zbigniew Ulanowski and Evelyn Hesse are physicists working in the Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, tel: 01707 284173, contact email:[email protected]. Visit their websites at and

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