For decades, scientists have argued over whether there is a link between cosmic rays and cloud cover, which in turn could affect climate. Now two atmospheric physicists in the UK have discovered that global atmospheric electricity - which itself is altered by cosmic rays, space weather and El Nino - affects the base height of certain types of clouds.

"Electric currents flow continuously throughout much of the atmosphere because of the global atmospheric electric circuit, and these currents sometimes pass through clouds," explains Giles Harrison, who along with Maarten Ambaum did this latest study at the University of Reading.

"Whether these small currents affect the cloud's constituent droplets has proved to be a question that is very difficult to answer because, almost invariably, other much stronger influences on the droplets are present," Harrison adds.

Polar darkness
With that in mind, the pair investigated a common type of cloud called "layer clouds" during polar darkness when many of these other influences are lessened or absent. Measurements with a laser ceilometer - a device that determines the height of a cloud base - done in Sodankyla, Finland and Halley, Antarctica, revealed that the cloud base rises an average of four metres for a 1% increase in fair-weather electric-current density. This means that shifts of up to about 200 m per day are possible.

Global atmospheric electricity exhibits a daily cycle, hitting a minimum at around 03:00 GMT and peaking at roughly 19:00 GMT - when activity is high in thunderstorm hotspots such as Africa and North America. This cycle was discovered in the early 20th century on board a ship operated by the Carnegie Institution of Washington. This variation is known as the Carnegie curve, or as Harrison puts it more poetically, "the fundamental electrical heartbeat of the planet".

Harrison and Ambaum found that the base height of the layer clouds they looked at showed a similar cycle to the Carnegie curve. They believe the effect may be due to the charging of small droplets in the cloud's base, which encourages them to stick together.

Thunderstorms and space weather
"The implications are that factors affecting currents flowing in the atmosphere - such as thunderstorms, cosmic rays and Pacific Ocean temperatures - may have distant effects on droplet properties in cloud bases," says Harrison. "Particularly interesting is the possibility that space weather changes could affect weather in the lower atmosphere."

Harrison stresses that the results say nothing about any long-term effects, as they were found for rapidly occurring changes from hour to hour. He reckons that establishing whether the electric currents influence clouds gives an additional perspective on coupling processes within the atmosphere.

"The realization that the electrical heartbeat of the planet plays a role in the formation of layer clouds indicates that existing models for clouds and climate are still missing potentially important components," adds Ambaum. "Understanding these missing elements is crucial to improve the accuracy of our weather forecasts and predicting changes to our climate."

Trapping and reflecting energy
Layer clouds cover about 40% of the planet, trapping heat at night but reflecting back solar radiation during the day. Unlike thunderclouds, they do not generate strong electrification internally.

The magnitude of cloud effects arising from global-circuit-current changes remains to be quantified, says Harrison. "We plan to make improved weather-balloon measurements of cloud droplets and their electrification, to unravel the detail of the droplet processes concerned and their effect on surface temperatures or rainfall."

Harrison's previous work has developed new experimental methods using weather balloons to detect whether droplets near the top and bottom edges of layer clouds are electrically charged. "Using these techniques, we have shown that droplet charging does occur in layer clouds, as a result of currents flowing in the atmosphere," he says. "We have also shown, theoretically, that the charges generated can affect the behaviour of the cloud droplets. Demonstrating the planet's electrical heartbeat in polar clouds is a further step in establishing whether droplets are actually affected by the currents flowing."

The research is described in Environmental Research Letters and you can view the video abstract above.