Rain, snow and ice

It rains, the roads get wet, water collects in puddles and flows in little streams along the roadside. Raindrops cover plants and everything else nearby. Then the rain stops. A while later, the roads are dry again, and the puddles and raindrops have disappeared. But where does the water go?

Well, some of it flows into the sewer system or the nearest stream, river, or lake. Some seeps into the ground. But the rest of the water simply evaporates into the air. We call this process evaporation. Some of the liquid water turns into a gas. Tiny droplets break away and spread through the air as water vapor. We can’t see it, but it’s still there.

Evaporation does not happen only after rain, but all the time wherever water and air meet—over a pond, a lake, a river, or the sea. We sometimes call this evaporated water in the air humidity.

Warm air can hold more water vapour than cold air. When the air contains a lot of moisture on a warm summer's day, we find the weather muggy. Warm air also rises. This allows cooler air to flow into the space previously occupied by warm air. This is how wind is created. As warm air rises, it cools down at higher altitudes. It can no longer hold as much water vapour. So the water vapour in the air eventually turns back into liquid. We call this process condensation. It usually occurs a few hundred metres above the ground, but sometimes at even greater heights. Tiny water particles cling to other small particles, such as equally tiny dust particles floating in the air. This is how fine water droplets form. They are larger than the particles that make up water vapour. We call a collection of many such water droplets a cloud. Clouds are carried along by the wind. Eventually, rain falls from these clouds onto the Earth, and the whole cycle begins again.

In winter, things are sometimes different. The air is now colder than it is in the warmer seasons. Water still evaporates in winter, and water vapor rises. When the air temperature drops below minus 12°C, the tiny water droplets in the clouds freeze into ice crystals. These crystals are all the same shape and have six sides. Eventually, they fall to the ground. As they fall, they hook together to form snowflakes, which float silently down to Earth. When the temperature is near freezing, large flakes form and tumble to the ground like cotton balls. When it is very cold, only tiny flakes reach the ground. Snowflakes fall slowly, traveling one to seven kilometers in an hour. That is because a lot of air is trapped between the branches of the snow crystals. This makes the flakes very light, so they drift slowly down to Earth. When the temperature is below 0°C, the many snowflakes soon form a layer of loose snow, transforming everything into a white winter landscape. Wet snow is perfect for making snowballs and building snowmen. Fine powder snow is best for sledding and skiing. Snow always looks white because the tiny snow crystals reflect all the light. We see this as the color white. The air trapped between the snow crystals acts as a sound absorber. That is why a snowy winter landscape is so peaceful and quiet. At temperatures above 0°C, snow melts and turns back into liquid water. This often creates unsightly slush.

Water is made up of very small particles called molecules. Each molecule consists of three even smaller atoms: one oxygen atom and two hydrogen atoms. In warm liquid water, these atoms and molecules dart from one place to another. New chains of water molecules are constantly forming. As the temperature drops, the molecules become less mobile. Below 0°C, they eventually become firmly joined together. Water thus turns into solid, hard ice. This can hang from a gutter as an icicle, for example, or cover a pond or lake with a sheet of ice. Ice can become so solid that it can support heavy loads.

In certain weather conditions, hailstones fall from the sky instead of raindrops or snowflakes. This happens more often in summer than in winter. Hailstones are lumps of ice that form in thunderclouds. They sometimes reach several centimetres in diameter and can cause serious damage. When hailstones are smaller than 0.5 cm, they are called soft hail.

Freezing rain is particularly feared by drivers. It forms when rain falls on a frozen surface. The moisture then turns to ice very quickly. This is known as “black ice.” In no time, roads become dangerously slippery. The weight of the ice can break branches and bring them crashing to the ground. - Images: Hamsterkiste

Images: Hamsterkiste