24 July 2014

Dropped cell phone calls become rain gauges in West Africa

Posted by abranscombe

By Alexandra Branscombe

WASHINGTON, DC – A shaky cell phone connection during a rainstorm can be an annoying nuisance. But now scientists are showing that these weakened signals can be used to monitor rainfall in West Africa, a technique that could help cities in the region better prepare for floods and combat weather-related diseases.

Scientists have demonstrated in a new study that cell phone towers can be used as modern-day rain gauges to accurately collect rainfall data during the monsoon season in West Africa. The new research in Ouagadougou, a medium-sized city in the nation of Burkina Faso, shows that the change in cell phone signals caused by heavy rains can be used to calculate the amount of rain that’s fallen in the city, where there are few traditional rain gauges.

The information collected through cell phone networks can be used to build a local rainfall record that could help the city predict if a large storm could trigger a flood, said Marielle Gosset, a hydro-meteorologist at Geosciences Environment Toulouse in Toulouse, France and a co-author of the new study, that has been accepted for publication earlier this month in Geophysical Research Letters, a journal of the American Geophysical Union.

Cell tower in Burkina Faso

Winds carry dust and sand ahead of a heavy storm about to hit the African city of Ouagadougo in Burkina Faso. Scientists have demonstrated there that cell phone towers (like the one in this photo) can be used instead of rain gauges to measure rainfall.
Credit: Frederic Cazenave

“In the last 10 years there has been an increase of floods in many African cities,” she said. “In order to monitor floods we need to know the rainfall history and have rain maps.”

Based on the results in Ouagadougou, the study’s authors are planning to test the cell phone rain monitoring technique in other cities in West Africa. Besides Africa, the technique might also benefit areas in South America and Asia, where flash floods and landslides occur frequently but rain gauge data and weather radar is limited, said Remko Uijlenhoet, a hydrology professor at Wageningen University in the Netherlands who is not connected to the study.

Gosset said eventually the technique – which now only measures the amount of rain that’s fallen over a certain period of time—could be used to monitor rainfall in real time, which could provide early warnings of flash floods. Combining the information from cell phones with satellite data that shows the path and intensity of rainfall across the continent could paint a more complete picture of rainfall in the region and pinpoint which communities are at risk of floods, Gosset said.

For the Ouagadougou study, a telecommunications company in the city, called Telecel Faso, provided to the study’s authors for free the data on cell-phone signal quality needed to measure rainfall. However, cell-phone signal data is not usually publicly available. To achieve cell phone rain monitoring elsewhere, scientists will need to find ways to coax the same sorts of data from telecommunications companies in those areas, Gosset said.

“The major bottleneck to expanding this technique worldwide is access to the data,” said Uijlenhoet. “There is no direct commercial interest for [telecommunication] companies to provide data for research purposes, or societal purposes, but it may help raise the companies’ profiles as being environmentally and socially friendly.”

“We have to convince them that helping monitor the weather is also important for [public] health,” said Gosset. A disease, like malaria, for example, spreads because mosquitos breed in rain-filled ponds. So, keeping track of rainfall can also help communities battle this disease and other water-borne threats, she explained.

Much as a glass of water can absorb, scatter, and refract light passing through it, raindrops falling between cell phone towers can both absorb the energy of a microwave link and scatter it in different directions, altering the signal that gets through. Scientists use records of how much and when the microwave signals change to calculate how much rain fell and for how long.

The technique, which has previously been tested in the Netherlands and Israel, could be of great benefit in West Africa, where rain gauge networks are sparse, while the number of cell phone towers is increasing as more people in the region acquire cell phones, the authors said. Although installing new rain gauges can be costly, the cell phone rain-monitoring method takes advantage of a network that already exists and costs much less than maintaining traditional rain gauges, Gosset said.

Furthermore, most of the rain that falls in West Africa is considered heavy rain – rain that falls at a rate exceeding 7 millimeters (about a third of an inch) per hour. This type of rain reduces the strength of the microwave signal coming from cell phone towers, which is easy to detect, said Gosset.

The region where Ougadougou is located receives about 800 millimeters (31 inches) of rain per year. During the monsoon season, rain storms are relatively short but very heavy, which can create conditions for flash floods, according to the study.

Telecel Faso provided the new study’s research team with a signal record for a network of 10 cell phone towers from June 26 to Sept. 4, 2012. The scientists used the records to look at every time the microwave signal between two towers was altered. There are 14 microwave links between the towers running at a low frequency, with the longest link 29 kilometers (18 miles) long.

To test the accuracy of the cell phone rain monitoring system, the researchers compared their rainfall results with 17 traditional rain gauges and one weather radar, all of which had been put into place to validate the results of the study. The comparison showed that the cell phone method could accurately detect 95 percent of the heavy rain events that occurred between late June and early September, and could also correctly measure the amount of rain that fell during that time period, according to the study.

Uijlenhoet, who was one of the scientists to first demonstrate, in separate research, the capability of the cell phone method in the Netherlands, said the technique works best in urban areas where there are more cell phone towers and more links between those towers. But, he added, the experiment in Burkina Faso—where there are few towers and long distances between them—shows that the cell phone method is still reliable in areas with sparse towers and lower frequency connections.

 - Alexandra Branscombe is a science writing intern in AGU’s Public Information department