Table 1. Characteristics of the environmental variables.

Variable	Temporal resolution	Time period	Resolution of grid squares
Interpolated meteorologic station dataa
Average daily mean absolute humidity	Mean monthly	1961–1990	0.5° lat x 0.5° long (nominal 50 km)
Average daily rainfall	Mean monthly	1961–1990	0.5° lat x 0.5° long (nominal 50 km)
Remotely sensed satellite datab
Average daily aerosol index (dust)	Mean monthly	1980–1999	1.0° lat x 1.25° long (nominal 100 km)
Digital maps
Land-cover typec		1992–1993	1 x 1 km
Population densityd		1990	0.042° lat x 0.042° long (nominal 4 km)

^aMean monthly climate averages (1961–1990) for absolute humidity and rainfall (the former derived from vapor pressure and mean temperature) were obtained from the Climate Research Unit, University of East Anglia, UK (available from: URL: http://ipcc-ddc.cru.uea.ac.uk/).
^bDust was obtained as monthly aerosol index coverages from the U.S. National Aeronautics and Space Administration (NASA), Goddard Space Flight Center, Maryland (available from: URL: http://www.gsfc.nasa.gov/), excluding the period May 1993 to June 1996 for which data are not available.
^cLand-cover type was obtained from the USGS NASA PATHFINDER 1km project (available from: URL: http://edcwww.cr.usgs/gov/landdaac/1KM/). 
^dU.S. Geologic Survey 1990 population density forecasts (available from: URL: http://grid2.cr.usgs.gov/globalpop/africa/).