Image conscious atmospheric science

By Giles Harrison

A frequently-heard mantra in physics is “Like charges repel and unlike charges attract”. At face value this paraphrase of Coulomb’s Law seems useful for clouds too, as, quite apart from the obvious example of thunderclouds, water drops in clouds are almost always charged to some extent. However, as it turns out, there are further subtleties to explore in the case of cloud droplets. The simple summary of the 1785 experimental findings of the French engineer and physicist, Jean Auguste Coulomb, made using a sensitive torsion balance, only applies to point charges, which, small though they are, cloud droplets are not. In fact they are sufficiently large for the charge within them to move around, i.e. to use a technical description, water droplets are polarisable. This means that, should there be another charge nearby, the charges within a water drop will re-arrange themselves in response. If this second charge is carried by another droplet, the charge in one will be re-arranged in response to the charge in the other. This is electrostatic induction: overall, the total charge on each droplet does not change, but its distribution within the droplet alters.

This concept is visualised below in figure 1. In the left-hand picture, there are two drops, both carrying negative charges. If they were solely point charges, they would repel each other in accordance with Coulomb’s Law. In the right-hand picture, in which a smaller droplet has been moved closer to the larger drop, a positive charge – known as an image charge – is induced on the droplet’s side of the drop by the negative charge. If the drops were brought closer still, the induced image charge in one would induce a stronger opposite charge in the other, which, perhaps counter-intuitively for two negatively charged objects, leads to attraction. Consequently when charged drops are driven together by turbulent motions and collide, the strong electrostatic attraction which always occurs between the image charges is likely to make them coalesce, and discourage them bouncing off each other. Collision and coalescence occurs continuously in clouds, and allows drops to grow sufficiently that they can ultimately fall as rain. Our initial work indicates that this process is accelerated by droplet charging.

Figure 1. Electrical forces between a small water droplet and a larger water drop, each carrying an overall negative charge (left). As the droplet approaches the drop, a positive image charge is induced in the drop (right), leading to an attraction.

These and related matters were discussed at a recent workshop at Reading on Microphysics of electrified clouds. In work funded at Reading by the United Arab Emirates Rain Enhancement Programme, a team of scientists and engineers is investigating how droplet charging affects droplet collisions and the formation of rain, and whether this can be used practically to influence clouds. Our project is pursuing these questions using a combination of numerical modelling and experimental work. A novel aspect of the numerical work is inclusion of a full description of the turbulent flow usually present in clouds (figure 2).


Figure 2. A system of droplets subjected to a turbulent flow field. An animation of the simulation is available here.

A second strand of work concerns the electrical environment of clouds in the UAE. This has been little explored, so, to obtain new information, we have established an automatic measurement site that provides a combination of cloud and atmospheric electricity data (figure 3).  

Figure 3. Measuring equipment being installed in the UAE to provide continuous data on atmospheric properties. Data is obtained by remote interrogation from Reading.

Finally, we need an inexpensive and flexible means to actually get into clouds, to make further measurements and undertake experiments on the effects of introducing charge. As well as the established Reading techniques exploiting modified meteorological balloons, we are using Unmanned Aerial Vehicles (UAVs) for this, designed specially by our collaborators at the Engineering Department at the University of Bath (figure 4).

Figure 4. UAVs developed by the Engineering Department at the University of Bath. (a) launch system and (b) the airframe planned to carry the meteorological instrumentation for the field experiments. Test flights can be viewed here.

This combination of new technologies, surface monitoring equipment and numerical modelling is allowing direct exploration of charge effects in non-thunderstorm clouds. In this, we are conscious that the often neglected electrostatic image force between water droplets seems likely to play a central role.

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