NASA - National Aeronautics and Space Administration

The old song, asking rain to "go away" and "come again another day," may get even older for people who live in large coastal cities.

Image of Houston metropolitan area from space. Taken by ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) on the Terra spacecraft.
Credit: NASA

Urban heat islands, created from pavement and buildings in big coastal cities like Houston, cause warm air to rise and interact with sea breezes to create heavier and more frequent rainfall in and downwind of the cities. Analysis of Houston-area rain-gauge data, both prior to and since urbanization, also suggests there have been increases in rainfall as more heat islands were created. The Houston-area study used data from the world's only space-based rain radar on NASA's Tropical Rainfall Measuring Mission (TRMM) satellite, and clusters of ground-based rain gauges.

Buildings, roads and ohter surfaces create urban heat islands that can affect local weather patterns. Also from Terra satellite
Credit: NASA

Scientists believe the impact large coastal cities have on weather, and possibly climate, will become increasingly important as more people move into urban areas, with even greater concentrations in coastal zones. The findings also have significant implications for flood control in Houston.

A recent United Nations report estimates 60 percent of Earth's population will live in cities by 2025. Previous related studies have shown urban heat islands create heavier rainfall in and downwind of cities like Atlanta, St. Louis and Chicago. However, this is one of the first studies to provide evidence of such an effect around a U.S. coastal city. It is also the first to incorporate specific satellite-derived rainfall data for a coastal urban area.

As urban areas grow, it's important to understand their effects and feedbacks on weather & climate. This view from space shows US city lights at night.
Credit: NASA

Urban areas with high concentrations of buildings, roads and other artificial surfaces retain heat, which leads to warmer surrounding temperatures and creates heat islands. Rising warm air, promoted by the increased heat, may help produce clouds that result in more rainfall around cities.

Buildings of different heights cause winds to converge, driving them upward, helping form clouds. The study shows the urban heat island/rain effect may be even more pronounced near coasts. In coastal cities like Houston, sea breezes also create rising air and clouds. The combination of urban converging winds and coastal sea breezes may enhance thunderstorm development.

Krishna Ramanujan
Goddard Space Flight Center