Published Tuesday, October 8, 2002 in the San Jose Mercury News

Weather Corner

Effects of huge storms off coast of Louisiana are felt worldwide

Special to the Mercury News

In recent weeks, Tropical Storm Isidore and Hurricane Lili have made headlines as they crashed ashore on the south coast of Louisiana. The media have been full of images of crashing waves, flooded roadways and reporters leaning into the wind and rain. These reports have given us the salient facts about the number of people killed, injured or evacuated and the number of counties declared disaster areas.

But the overall toll from a hurricane goes far beyond these facts and figures, and the effect can be felt across the country.

One of the unseen casualties is oil and natural gas production in the Gulf of Mexico. According to the U.S. Department of Interior's Minerals Management Service, every day wells are shut down due to a hurricane, or even the threat of one, production drops by about 1.5 million barrels of oil and 8 billion cubic feet of natural gas. The cost of that loss does not include damage to equipment. More than 4,000 gas and oil platforms in the Gulf of Mexico provide about a quarter of domestic oil and gas production, so the impact is felt in the marketplace worldwide.

Inland, the impact of tropical storms ripples through the agricultural economy of the South as torrential rains halt many activities. According to the U.S. Department of Agriculture, cotton and rice crops in the Mississippi River delta have been particularly hard hit, especially since the fields and paddies did not have a chance to dry out between storms. Mississippi lost an estimated 60 percent of its cotton crop from Isadore alone.

On the positive side, the heavy rain has made at least a dent in the drought that has been lingering across much of the East. In many areas, Tropical Storm Isadore cut the rainfall deficit in half, and when totals are available from Lili, we should see the deficits fall even more. The rain has also helped replenish soil moisture for the planting of the winter wheat crop.

Is it true that precipitation always begins with the formation of ice crystals, which then form snow if the air temperature is cool enough and melt into rain if the air temperature is warmer? If so, does that mean that air temperatures actually drop below freezing in the clouds, and that this even occurs in the tropics? Also, what in the heck is sleet? How does it differ from snow and freezing rain? Tom Pratt - San Jose

A. There are actually two separate processes in the atmosphere that produce precipitation.

In relatively warm clouds, where the coldest temperatures are above zero degrees Fahrenheit, rain forms by the ``collision-coalescence'' process: Small droplets run into other small droplets to form bigger droplets, which run into other drops until they are large enough to fall to the earth as rain. In the tropics and in warmer middle-latitude storms, this is the primary rain-making mechanism.

In the higher latitudes, precipitation is the result of the ice-crystal or Bergeron process. Ice crystals form into snowflakes and grow by accumulating other ice crystals and super-cooled water droplets. As snowflakes fall, they melt into raindrops when the surrounding air temperature is above freezing.

Sometimes there are layers of cold air near the surface that cause the raindrops to refreeze into ice pellets, or sleet. These tend to be small, BB-sized and relatively soft and mushy. If the layer of freezing-cold air is very shallow and right at the surface, then water droplets freeze on contact with objects on the ground; this is what is known as ``freezing rain.''

Q. When a long-range forecast predicts above-average precipitation, does it mean an above-average number of storms with varying amounts of precipitation, or above-average amounts of total rainfall? Dan Carlentine - San Jose

A. This is an important distinction. All of the long-range forecasts I am familiar with deal only with the total amount of rain, not the number of storms. However, long-range forecasts are very general in nature, and even when average conditions are forecast, this does not preclude smaller regions having more or less rain than average.

Q. Could you tell me where the warmest beach in the San Francisco and Monterey bay areas is located, and why it's so warm? Tom Kellond - Mountain View

A. For our section of coast, warm is definitely a relative term. Santa Cruz has the warmest water temperature along the beach but checks in with an average of only 60 degrees for the latter half of August and the first half of September. At the other end of the range is Bodega Bay, with an average of 54 degrees. Compare this to 70 degrees at Newport Beach in Southern California and the 80s for the Gulf Coast and the Atlantic Coast from Florida all the way north to Virginia.

The primary influence on beach temperatures is the direction of the ocean currents in the region. On our coast, the California current brings cool water from the north; this is exacerbated by the upwelling of cool water from beneath the surface. South-facing beaches along the California coast, such as Santa Cruz, are somewhat sheltered from the cold water and upwelling, making them slightly warmer.

In contrast, the Atlantic coast is influenced by warm waters flowing north in the Gulf Stream. But this is also why they have hurricanes and we don't.

To see average monthly beach temperatures around the nation, go to

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to weathercorner@ Please indicate in your e-mail what city you live in.