Satellite image of the western United States, taken on Thanksgiving Day. Image: NASA.

The defining sign of a La Niña is cooler-than-average sea surface temperatures in the Pacific Ocean, near the equator. Cooler water evaporates less than warmer water, so there is less moisture in the air. This means that during a La Niña, there is less precipitation in some areas, like California and the southern United States. Other areas, like the Pacific Northwest, get more precipitation during a La Niña. (Book your winter ski trip strategically!)

Sea surface temperature anomalies on November 25, 2010. The equatorial Pacific is cooler than usual (note the blue color), a characteristic of a La Nina event. Image: NOAA.La Niña also affects air temperature. During a La Niña, the South is typically warmer, while Southern California and the Pacific Northwest are cooler. You can see climate predictions for the next three months, from the National Weather Service Climate Prediction Center.

A La Niña is basically the opposite of an El Niño. El Niños are characterized by warmer-than-average sea surface temperatures in the equatorial Pacific. This happens when the atmospheric pressure system gets a bit out of whack. Usually there is high pressure over the Pacific Ocean and low pressure over the Indian Ocean—picture a see-saw anchored over New Zealand, with the heavy kid sitting on the end over Indonesia. For reasons we don’t quite understand, sometimes the see-saw tilts the other way—now the heavy kid is sitting in the southeastern Pacific, and the kid on the Indonesian side is way up in the air.

This swap in atmospheric pressure has quite a few consequences. The trade winds (which typically blow across the Pacific from the east to the west) get weaker. The water that the trade winds usually push westward instead piles up and moves east. This water is warm, and it evaporates, causing more rainfall along the west coast of South America. Meanwhile, places like Indonesia and Australia get less rainfall, along with drought and fire.

The weakening of the trade winds and westward flow of ocean currents in the Pacific has a second effect. Under non-El Niño circumstances, the trade winds carry the top layer of water to the west, and so water from deeper in the ocean moves up to take its place. This is called upwelling. The water from deep in the ocean is cold and full of nutrients, and it drives the fisheries along the coast of Chile and Peru. Without the trade winds and the upwelling, fisheries crash. A strong El Niño has big economic impacts, not all of which are negative; some agricultural areas are benefited by the extra rainfall, and people in places with unusually warm weather can save on heating bills. Of course, La Niña events have economic impacts too.

As scientists learn more about predicting the climate during El Niños and La Niñas, we can plan accordingly and mitigate the economic impacts of these events. My comment about planning your ski vacation according to this year’s La Niña was a little bit serious! Rainfall predictions based on El Niño and La Niña models can help farmers decide which crops to plant. And, here in drought-prone California, La Niña precipitation predictions are influencing water allocation decisions for the coming year.

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More than 20 million gallons of raw sewage spilled into California waterways last year, according to the state Department of Water Resources Control Board. That's not counting the partially treated sewage that makes its way into our water from overflows and sewer system malfunctions.

Many big sewer pipes are old, and many of the sewage treatment plants are antiquated. But the biggest problem faced by sewer systems in California is the tiny pipe called the lateral.

That's the pipe that runs from your home to the street, the small pipes under all of our homes that end up joining the bigger sewer pipes. When those pipes develop cracks, water leaks into them.

Storm water itself would not overwhelm a sewage system, because it's designed to be a closed system. Storm water is not supposed to BE in sewer pipes. So in one way, it shouldn't even matter what the weather is like – storm water shouldn't really mix with sewage at all.

But during a rainstorm, water seeps into your broken lateral pipe, and all your neighbors' pipes, and that rainwater mixes with sewage in the sewer pipes, and the volume of water/sewage can actually build up far beyond the capacity of the sewer pipe. And in the same way, thousands and thousands of gallons of water mixed in with the sewage can swamp a treatment plant during a rainstorm.

That's the number one concern of sewage treatment plants now. And the sewer districts need your help.

Those laterals are owned by homeowners. They're on private land, so the district can't just go in there and tear them up to replace or fix them.

However, most sewer districts offer a service where they will inspect your laterals to check for leaks, and many have started programs where the district will help pay the cost of repairing or replacing those pipes.

Sewer systems are run by local municipalities. Most communities have a local sewer district, and officials at the district can help you inspect and fix your lateral pipes.

Listen to the Sewage Spills Increasing radio report online.

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Riding to work on BART, about a week before I was to begin shooting this story, I ran into a former colleague of mine whom I had not seen since 2001. He and I had worked together for something like 9 years at KUSF, a non-commercial community radio station in San Francisco. The station -and this particular guy- have always had a reputation for being fairly progressive. OK, that might be an understatement: he's a militant vegan, Critical-Mass-bicycle-riding, anti-automobile, bleeding heart liberal who played a lot of 60's Psychedelia and Prog Rock like the Fugs, the 13th Floor Elevators and Frank Zappa on his radio shows.

After explaining to him that I was working on a story about the impact that global warming is having on the Earth’s supply of fresh water, I was shocked to find myself arguing with him about the very existence of climate change. He claimed that it was all a bunch of sensationalism and that the Earth's climate has always had dramatic changes and that what we are experiencing now is anecdotal and has nothing to do with humans. He said that even if it is happening, longer growing seasons in northerly regions would be beneficial to world food supply and that an increase in carbon dioxide in the atmosphere would cause plants to grow even better. Sure, I’ve heard these types of arguments before but usually not in San Francisco. It was definitely a timely reminder to me about why the media should cover the kind of scientific work that’s being done on this issue.

Given my conversation with BART guy, one of the reasons the Keck Hydrowatch Project is so interesting to me is that Inez Fung, Todd Dawson and the rest of the team aren’t actually setting out to prove the existence of climate change. The researchers are embarking from the position that without question, global climate change is happening and what we are experiencing today is just the tip of the rapidly melting iceberg. And you know what? I believe them. So, they are dedicating the next four years of their lives to understanding how these changes will affect the availability of fresh water for use by humans, plants and land animals. So far, their predictions are "rather grim", as Fung says in the story. It will rain but because we've cut down so many forests, altering the natural landscape that allows the water to cycle back into the atmosphere, much of it will fall in the middle of the ocean where we can't access it. This will result in widespread drought and famine in the not so distant future. Yikes. Being an environmental reporter in this day and age can be a bit disheartening.

So, I was curious how scientists like Fung and Dawson, whose research leads to predictions of widespread climatic chaos and environmental meltdown, are able to cope with their frequently depressing findings. And what do they hope to do with their results? Well, according to Todd Dawson and others on the Keck Hydrowatch team, this project can serve as a model for understanding water movement throughout the globe in order to more fairly allocate water for future human use in a dryer world. But also, and perhaps most importantly, this research can drive home the point that because climate change is largely the result of human activities, its solutions also reside with us.

Watch the "Tracking Raindrops" TV Story online, as well as find additional links and resources.

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I was reading out loud as we drove down I-64 towards an ever darkening sky. My friend, Brad shushed me at one point to concentrate on driving. I looked up to see why and was stunned. A sheet of water and staccato pulses of hail and lightning were all I could see. It reminded me of the intensity of being caught in blizzard conditions while driving toward Tahoe in Northern California. But here we were, in Virginia on a very warm and balmy day and the sky had literally opened up with water. What's more, hail was falling. I turned to Brad and asked how could hail exist in such warm conditions? He was stumped and I was fascinated.

With a little detective work, I came up with an answer to the dilemma. Hail is only produced in cumulonimbi clouds (thunderclouds). They usually only occur at the front of a storm system which was what we experienced. The hail hit in the first ten minutes and then was followed by heavy rain. However, the rain was warm to the touch unlike the frozen water making up the hailstones.

The fire in the nearby Great Dismal Swamp National Wildlife Refuge and incredibly hot and humid conditions in Virginia created ideal conditions for hail. Hail is created inside a thunderstorm that has strong updrafts of warm air and downdrafts of cold air. A water droplet with an apex point is picked up by the updrafts and travels into the cooler air and freezes. The apex point known as the condensation nuclei in the water droplet was probably dust from the fire or nearby salt water during this particular hailstorm. (Both Brad and I experienced dry and stinging eyes after going into one of the storms later that weekend, much like the stinging of salt water.) Layers of ice are then accumulated around this nuclei as the droplet goes through a cycle of being caught in an updraft and then carried beyond the freezing level of the atmosphere and then thawing partially in entering the warmer air on a downdraft. This cycle repeats itself creating increasing layers of ice. Then as some point this frozen water droplet with several irregular layers falls to the ground as hail.

Some of the largest hailstones have been recorded during summer storms in humid climates because the warm updrafts and cold downdrafts along with high surface heat create an optimal cycle for large hail. Smaller hailstones can be coupled more easily with larger hailstones in these conditions.

Video of Large Hailstones

Over the course of the weekend, we had three more thunderstorms (one of which broke a car back window nearby) and we kept an eye on a thundercloud that looked like it wanted to become a tornado. I came home very thankful for the fog! NOAA the National and Atmospheric Administration has a National Weather Service. At http://www.weather.gov/ anyone can check weather reports in any given area. To issue proper warnings and forecasts regarding hail, the National Weather Service uses a network of NEXRAD doppler radars to detect it. Hail size and probability can be determined from radar data by a computer by different algorithms and compared to the local atmospheric data to determine the threat level.

It seems the storms have not cleared yet throughout the area in Virginia we visited– severe thunderstorm warnings are still posted on NOAA. So the fireworks might have passed but thunderclouds are still lighting up the sky.

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