This week's cold snap that brought rarely seen flurries of snow to San Francisco and California's Central Valley isn't necessarily the work of La Nina, an oceanic phenomenon that can affect worldwide weather, according to a UC Davis atmospheric scientist. "At this point, we can't say that this cold weather is due to La Nina," says professor Terry Nathan. "Months after it's all over, atmospheric scientists can go back, analyze the data and see whether this cold weather was associated with La Nina or just part of the natural variability of weather." La Nina is the flip side of El Nino, both of which originate in the Pacific Ocean. While El Nino is typified by unusually warm waters in the eastern Pacific, La Nina years find those same waters markedly colder than normal. The name El Nino -- signifying the Christ child in Spanish -- was the label put on the warm-water phenomenon because it historically manifests itself near the Peruvian coast around Christmas. La Nina years tend to bring wetter than normal conditions to the Pacific Northwest and warmer, dryer weather to the southern tier of the United States. "To officially be a La Nina, the water temperatures in a sector of the equatorial eastern Pacific have to remain below a certain threshold for at least six months, and we're not quite there yet," says Nathan. "The current forecast for this winter by the National Oceanic and Atmospheric Administration (NOAA) is for a moderate to strong La Nina continuing through May." Nathan, who uses numerical models to better understand the underlying mechanisms that determine how storm tracks and global weather patterns respond to El Nino and La Nina events, is studying the historical aspects of both phenomena. He is particularly interested in how these phenomena have impacted exploration. Nathan is currently studying the weather journals of the Lewis and Clark expedition and hypothesizes that the extraordinarily wet winter of 1805-1806, which the explorers spent at Fort Clatsop, Ore., was a La Nina year. To test this hypothesis, Nathan is collecting and analyzing historical data from around the world.