Every glass of water you drink has been somewhere before. It may have been part of a cloud, a river, a glacier, or an ocean current. Earth's water keeps moving in a giant loop called the water cycle. So how long does one drop take to make the trip?
Here's the short answer: it depends entirely on where the drop goes. Water that evaporates into the air usually comes back down as rain or snow in about nine days. But a drop that sinks deep into the ocean can take around a thousand years to resurface. Water that seeps deep underground can stay there for more than 10,000 years, and some ice in Antarctica is hundreds of thousands of years old.
The Fast Lane: The Sky
The quickest part of the cycle is the trip through the atmosphere. The Sun heats water in oceans, lakes, and rivers, and some of it turns into vapor and rises into the air. On average, a water molecule spends only about nine days in the atmosphere before it condenses and falls back to Earth.
Evaporation is also nature's water purifier. When water evaporates, it leaves behind salts and other particles, which is how salty seawater ends up falling as fresh rain.
Most of the action happens over the ocean. The ocean holds about 97 percent of all the water on the planet. It's the source of 86 percent of the world's evaporation, and about 78 percent of all rain and snow falls right back into it.
The Slow Lane: The Deep Ocean
A drop that reaches the ocean may take a much longer route. Ocean currents carry water around the globe in a vast system often called the thermohaline circulation, driven by differences in temperature and saltiness.
Surface water flowing toward the poles cools, sinks, and spreads through the deep ocean basins. The oldest of this deep water rises back to the surface in the North Pacific after a journey of about a thousand years. That means some of the water in the deep sea today last saw sunlight around the time of the Vikings.
The Waiting Room: Underground
Some rain soaks into the soil. Water in the top layer of soil doesn't stay long, since it's quickly taken up by plants, evaporates, or drains away. But water that trickles deeper can fill underground layers of rock and sand called aquifers.
Groundwater moves slowly, and it can spend more than 10,000 years beneath the surface before it comes back up. Especially old groundwater is called fossil water. Some of it eventually bubbles up in freshwater springs, and some is pumped up from wells.
The Deep Freeze: Ice Sheets
The longest stays of all happen in the great ice sheets of Antarctica and Greenland. Snow piles up, gets pressed into ice, and can stay locked away for ages. Ice from Antarctica has been reliably dated to about 800,000 years old, though most ice doesn't stay that long.
Through it all, the total amount of water on Earth stays fairly constant. It just keeps changing places and forms, from liquid to vapor to ice and back. So the next time you hear rain on the roof, enjoy the thought: some of those drops were clouds last week, and some may carry water that has been traveling for longer than any of us can imagine.
