Rift Valley Fever

Sea surface temperatures can predict the likelihood of the disease because tiny variations in these temperatures cause huge shifts in air circulation patterns - shifts that alter rainfall around the globe. El Niño, for example, happens when a band of warmer-than-average water forms near the Pacific coast of South America. Meanwhile, Pacific waters near Australia and Indonesia become a bit cooler than usual.

A similar type of temperature imbalance can occur in the Indian Ocean, with the western part near Africa becoming warmer than the eastern part near Australia. Indeed, researchers liken this "Indian Ocean Dipole" to El Niño in the Pacific. Both tend to increase rainfall in East Africa.

When the two anomalies occur at the same time, buckets pour.

more This map shows the greening of East Africa during an Indian Ocean warm spell in 1983.

"The year 1997 saw the largest El Niño ever recorded simultaneous with a very large Indian Ocean Dipole (see figure)," says Christina Clark, an atmospheric scientist at the University of Colorado. "East African rainfall was then the highest on record, in many places five times the normal amount." Such floods bring Rift Valley fever because water collects in shallow depressions called "dambos" that punctuate the savannahs, providing mosquito eggs with exactly the nurturing conditions that they need to hatch.

The data about both sea surface temperature and vegetation is provided by NOAA's Advanced Very High-Resolution Radiometer (AVHRR) - a type of visible-light and infrared sensor carried on many polar orbiting weather satellites. "These satellites were not really designed to monitor land surface conditions," says Anyamba. "They were designed to monitor atmospheric conditions - basically, clouds." But Goddard scientist Compton Tucker realized, says Anyamba, that by manipulating the information provided by the AVHRR, he could produce a 'greenness index,' which measured the condition of the vegetation on the ground.