Weekend sunshine, midweek rain
Human-made emissions during the business week affect the weather, increasing summer storm activity in the U.S. Southeast.
Researchers using satellite measurements confirm that midweek pollution from cars, industry, and other human activities contributes to the intensity of midweek rainstorms.
Publishing in the Journal of Geophysical Research on January 31 (113, D02209), a team led by Thomas Bell of NASA's Goddard Space Flight Center analyzed data from the Tropical Rainfall Measuring Mission (TRMM) satellite over the southeastern U.S. to understand trends in land- and ocean-based storms.
Over land, the researchers found, workday aerosols seem to suppress rain in the U.S. Southeast, the region visible to the satellite. They suggest that as polluted clouds build up and cloud droplets climb higher and higher, convection over land is increased by the release of latent heat when the droplets freeze—eventually leading to bigger clouds and heavier rainfalls. The authors find an increase in wind velocities to match the increased convection.
The trend is the opposite over the nearby Atlantic Ocean, where stormy weather increases over summer weekends. The authors compare this to a kind of monsoon effect, in which increased convection over land pumps air up into the upper troposphere. The air then falls over the ocean and suppresses convection there.
However, the researchers emphasize, "correlation does not prove causation." They have not determined which pollutant particles might create these effects, or how. Factors other than pollution, such as plane contrails and heat released from cars or industrial sites, might also influence storm behavior.
The drier, western U.S. shows no evidence of this weekly trend. But the team says the trend in the Southeast, taken together with rain gauge data, was probably discernable as far back as the 1940s. Because of differences in pollution composition then (i.e., more black carbon with different radiative properties), storm peaks occurred at the beginning of the week, creating gloomy Sundays and Mondays.
