Meteorology and Atmospheric Electricity at Reading
Dr Giles Harrison
What's Atmospheric Electricity?
This picture was taken from the cliffs above Dover, looking towards France during a summer thunderstorm. It was totally dark at the time, and all the cloud illumination in the photograph came from intra-cloud lightning during the time exposure. Inter-cloud lighning can be seen top left. (picture by RGH).
Yes. It's the study of electric charges and currents in the atmosphere. Fossils suggest that the terrestrial atmosphere has had lightning within it for at least the last 250million years. Lightning has also been observed on Jupiter and Saturn, and it is probably a characteristic property of atmospheres containing both polar molecules (such as water or ammonia) and strong convection.
Everywhere. The atmosphere contains many electric fields of different shapes and sizes. Electric fields are at their most intense in thunderstorms, but even in fine conditions there is a small vertical electric field. This means that in fine weather, there is over a hundred volts of potential difference between an upright person's head and toes, vertically.
Although fine weather electric fields can generate large potential differences, the currents involved are very small. Air fortunately has a pretty large resistance, so even though there may be hundreds of volts around your head, only a tiny current will flow, perhaps a few million-millionths of an amp.
Not exactly. In fact it was one the first areas of experimental research in meteorology. Benjamin Franklin is well-known for his Philadelphia kite-flying experiments in the 1750s, but much more sensitive experiments were being performed in England at a similar time by John Canton. In 1754 Canton observed that clouds that didn't produce lightning could also be electrified, and could carry positive or negative charge. He also identified hail as being important.
Just because fields, currents and charges are small on fine days doesn't mean they're innocuous. Plenty of things in nature seem affected by electric fields, and small particles (aerosols) in the atmosphere can have their behaviour greatly influenced by electric charges. Electrostatic discharges can easily cause catastrophic damage to computers and electronic components and the recent discovery of
lightning above clouds suggests that there are still significant atmospheric electrical processes to be understood.Well lightning's pretty big. It causes the production of Nitrogen Oxides (NOx) from air, at a rate of about 12million tonnes per year. In turn NOx regulates Ozone concentrations, an atmospheric gas of great importance which contributes to the Greenhouse Effect.
Lightning flash rates seem very sensitive to surface temperature, and it's thought that a general atmospheric warming could lead to vastly more lightning. Radio ("
sferics") has been used to monitor lightning for a long time, but now there are satellites too.We're studying the interactions between meteorology and atmospheric electricity, especially on days without nearby thunderstorms. Amongst other things, this means measuring and studying the charges on aerosol particles (like the charged dust which sticks to computer monitor screens), and how the charged particles behave. To do this, we have had to develop ultra-sensitive electrical
instruments, one application of which is to use the electrical effects on particles to measure pollution. Jump on to our research.
Introductory essay on the Atmospheric Electrical system
List of FurtherReading.html
The research at Reading is primarily experimental, requiring the development of new techniques and instruments. We are also interested in the electrification of particles, and the links between aerosol and cloud processes. The research group currently consists of Giles Harrison (Lecturer), and three research students: Janet Barlow, Sachchida Tripathi and Karen Aplin. Our publications are listed here.
Giles Harrison (r.g.harrison@reading.ac.uk) for more specific information.
RESEARCH WORK IN PROGRESS
Karen Aplin is a research student whose work is concerned with using atmospheric electrical methods (such as measuring the conductivity of air) as a means of monitoring pollution and also investigating the fundamental processes of particle formation. In a recent Aerosol Society newsletter article, she discussed the significance of her ion measurements.
Ion concentration measurements and their use in urban pollution
We have also developed a variety of bespoke instruments for our research, such as a novel picoammeter. Some of our instruments use highly sophisticated materials, and we jealously guard them from prying eyes. A hint at the increasing level of our sophistication is given by this letter published in The Times.
Sachchida Tripathi's work concerns the atmospheric significance of charged particles. Charging can change the rate at which particles deposit on surfaces, the rate at which they coagulate and the rate at which they are removed by cloud droplets. These effects have often been neglected hithero, and in the case of radioactive aerosols this may explain large discrepancies between experimental and theorectical results. A preprint of a recent paper on charged particle deposition is available here.
One area of recent research here concerned the turbulent transport of charged aerosol particles, which can be sensed using atmospheric electrical methods. Janet Barlow pursued this work, and was awarded the C.N.Davies award of The Aerosol Society. In articles for their newsletter, she described her progress.
Turbulent Transport of Charged Aerosol
AITKEN CHARGED....Some small atmospheric particles are known as Aitken Nuclei, and whether they are electrically charged or not is of interest to us. The train of thought triggered by an article in The Guardian on legal action against a certain politician is described by Janet, which also emphasises some of the true joys of experimental work.
Cosmic rays and climate: some recent controversy about possible effects of the atmospheric electrical system on climate change is briefly discussed. You can also jump directly to Fred Pearce's summary article in The Guardian or read the press release from the International Space Science Institute workshop held in June 1999 in Bern.
A short essay on ionisers and the supposed significance of atmospheric air ions for human health
Research presented at 1996 Aerosol Society Meeting on ionisers
Sonning Field Experiment
In the summer of 1998, a field experiment was undertaken at Sonning, not far from the University, where a unique combination of micrometeorological and electrical measurements was obtained. More details are available here.
Other Atmospheric Electricity Home Pages:
http://ae.atmos.uah.edu/
AE
Current European lightning maps
Although they have recently received considerable attention as a new discovery and called Sprites (or Jets), reports of lightning travelling upwards from clouds are not new, and were probably first made up to a century ago. Current images from recent work are available at:
