Can Water Naturally Flow Uphill?
Earth’s gravity is strong, but can water ever naturally go against it and flow uphill?
The answer is yes, if the parameters are right. For instance, a wave on a beach can flow uphill, even if it’s for just a moment. Water in a siphon can flow uphill too, as can a puddle of water if it’s moving up a dry paper towel dipped in it.
Waves (powered by wind), tides (primarily caused by the moon’s gravitational forces) and tsunamis (often triggered by earthquakes and underwater landslides or volcanoes) can cause water to go against gravity. The energy and forces produced by these natural phenomena can push water upward, allowing it to naturally rise into a wave or run up a shoreline.
A siphon acts under different pressures. People have used siphons since ancient times; the ancient Egyptians used siphons for irrigation and winemaking. Nowadays, thieves might use siphons to steal gas from cars. However, there is still debate about how siphons work.
You can visualize a siphon by thinking of two cups connected by a tube shaped like an upside-down “U.” The water-filled cup sits on a stair, and an empty cup sits below it. If an experimenter puts one end of the tube into the water-filled cup and sucks the air out of it as you would when using a straw, that will allow the water to flow into the tube.
A siphon is created once the water flows up one side of the tube and down the other, into the empty cup. Gravity accelerates the water through the down part of the tube, into the lower cup. Because water has strong cohesive bonds, these water molecules can pull the water behind them through the uphill portion of the tube. However, many liquids that do not have strong cohesive bonds still work in siphons, so it’s unclear exactly how siphons work in different cases.
What about the paper towel example? This action, called capillary action, allows small volumes of water to flow uphill, against gravity, so long as the water flows through narrow and small spaces. This upward flow happens when a liquid’s adhesion to the walls of a material, such as the paper towel, is stronger than the cohesive forces between its liquid molecules. In plants, water molecules are drawn up capillaries called the xylem, helping the plant to draw in water from the soil.
There are other instances in which water has naturally run uphill. For example, an 8.0-magnitude earthquake shook southeastern Missouri so hard that the Mississippi River temporarily flowed backward. There’s a river that flows uphill beneath one of Antarctica’s ice sheets. In addition, a study in the journal Physical Review Letters showed that small amounts of water put on a hot surface — a scalding pan, for instance — can climb tiny stairs made out of vapor if the water is hot enough.
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