Ever wonder why it always seems hotter in the city? Blame the urban-heat-island effect. Big cities create their own microclimates, and with all the buildings providing surfaces to absorb and radiate heat, urban areas turn into enormous ovens that can be up to 15 degrees Fahrenheit warmer than their outer suburbs. The temperature difference not only can alter local wind and rain patterns, but it can also create public-health concerns about heat stroke, organ damage, and the subsequent effect on the aging community. For a primer on the causes and consequences of urban heat islands, the relationship between architecture and climate, and also ways to use architecture to mitigate the environmental footprint of metropolises, head to “Green Design: Rooftop Gardens for a Sustainable City,” which features University of Maryland landscape architect Jack Sullivan and Columbia University climate researcher Stuart Gaffin. Quaff a libation, nosh on pizza and sandwiches, and chat with scientists in an informal setting from 6 p.m. to 8 p.m. at the Marian Koshland Science Museum, 500 5th St. NW. $15. (202) 334-1201. (Huan Hsu)