Water Cycle Myths and the Real Science

Most of us first meet the water cycle as a neat diagram: evaporation, condensation, precipitation, round and round. NOAA points out that this simple circle is a useful model but that the real cycle is much more complicated. Simplified pictures, plus everyday words such as 'steam', leave many people, adults included, with ideas that are not quite right. The myths on this infographic are among the most common.

Water vapour is an invisible gas. It is always in the air, even on a clear blue day. NASA describes a cloud as a mass of water drops or ice crystals in the atmosphere, so a cloud is liquid or solid water, not vapour. Clouds form when air cools until some of its vapour condenses onto tiny particles of dust, salt or smoke.

The same mix-up happens in the kitchen. UCAR's science educators list it as a common misconception: many people think the white plume above a boiling kettle is water vapour. In fact the plume is made of very small liquid droplets. The vapour itself cannot be seen; it becomes visible only once it has cooled and condensed.

A useful rule: if you can see it, it is not vapour.

Water boils at 100 °C at sea level, and at that temperature it evaporates very quickly. But evaporation happens at any temperature. The USGS explains that it simply happens much more slowly when water is near freezing, because the molecules have less energy to break free. Even ice can turn directly into vapour, which is why frozen washing on a line slowly dries.

Evaporation from oceans, seas, lakes and rivers supplies about 90 per cent of the moisture in the atmosphere, and almost none of that water is anywhere near boiling.

It is easy to imagine groundwater as hidden lakes and rivers. A few streams do disappear into caves, but most groundwater is stored very differently. Below the water table is the saturated zone, where water fills all the pores, cracks and spaces between rock particles. A layer of rock or sediment that holds enough water to be pumped from wells is an aquifer.

The USGS suggests digging a hole in the sand at the beach: water seeps in from the wet sand around it. That is groundwater in miniature. It moves too, but slowly: several metres a day in very permeable rock, and only a few centimetres a century in clay or shale.

A water molecule does not visit every stage in order. It might fall back into the sea straight away (most water that evaporates from the oceans does), soak into the ground, be taken up by a plant, or be frozen into an ice sheet. The time it spends in each store varies enormously. National Geographic gives about 9 days in the atmosphere, about 3,200 years in the ocean and about 17,000 years in the Antarctic ice sheet.

Raindrops are drawn as teardrops, but NASA's Global Precipitation Measurement mission explains what they really look like. Small drops, under about 1 mm across, stay roughly round because surface tension pulls them into a sphere. Drops of 2 to 3 mm are pushed by the air underneath as they fall, so they flatten into a shape like the top of a hamburger bun. Drops larger than about 4.5 mm stretch into a parachute shape and then break up into smaller drops.

Water used by people is not destroyed. It drains, evaporates and falls again as rain, and NASA notes that without this natural recycling we would run out of clean water. The real problem is fresh water in the right place. Only about 2.5 per cent of Earth's water is fresh, and most of that is frozen or underground. Where wells pump water out of an aquifer faster than rain refills it, the water table drops and wells can run dry. Some aquifers in dry regions would take centuries to refill.