San Francisco Weather

The Golden Gate Bridge is seen through fog from Land's End in San Francisco, Wednesday, Sept. 29, 2010. (AP Photo/Jeff Chiu)

Q: This time of year, you hear about Lake Effect Snowfall over the Great Lakes. How does a large body of water affect the weather?

A: A large body of water directly affects weather patterns, such as rainfall and temperatures, says Brent McRoberts of Texas A&M University. “Consider Miami and Omaha. One is near a large body of water – Miami is by the Atlantic Ocean – and the other is at least a thousand miles away from a large area of water,” he explains. “A large body of water has a higher heat capacity than land, meaning it takes more energy to warm and cool the temperature of water. Therefore, cities close to water tend to have a narrower range of temperatures throughout the year. For example, average July high temperatures in Miami and Omaha are both near 90 degrees, but average January highs are mild in Miami (70s) and frigid in Omaha (30s). Coastal cities, such as Seattle and New Orleans, also tend to get a lot of rainfall. In winter, storms that pass over the Great Lakes absorb moisture from the water and this tends to increase snowfall amounts near the shoreline.”

Q: How else does a large body of water affect weather?

A: On the West Coast, the waters are very cool, and this directly affects the weather, McRoberts adds. “The cool water near land can create some of the most intense fog in the world,” he says. “The cool water also moderates temperatures inland, which is why California has one of the mildest climates in the U.S. Along the East Coast, the Gulf Stream, with its warmer waters, can influence weather patterns from Florida to Maine, while in the Gulf of Mexico, waters stay warm much of the year and really heat up in summer. During the warm season, both the Gulf Stream and Gulf of Mexico provide fuel for some of the strongest hurricanes on Earth. We are still learning a lot about water and how it impacts weather systems.

Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.

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