# What disappears when it rains

Well, it doesn't really rain bubbles, because a bubble only arises when a raindrop falls on a surface of water. But bubbles don't always form! Because a raindrop only creates a bubble if it is particularly large. My grandma always says that when it rains bubbles, it rains for a particularly long time. Is there something to it?

### Bubbles in a puddle only appear when there are large drops of water

In order for bubbles to form in a puddle or on a lake when it rains, it has to rain really hard. Because only large drops of water have enough energy to create a bubble. However, bubbles on a puddle say nothing about the duration of the downpour or whether it will rain particularly heavily. Even a strong and short shower can cause bubbles to appear on puddles and lakes. And if it just rains so lightly but constantly, no bubbles will form because the raindrops are simply too small.

### Why do large raindrops form bubbles?

If a raindrop falls into the puddle, its own mass displaces a certain amount of water when it hits the surface of the water. This displaced water escapes upwards, so that a small 'impact crater' is created (Fig. Right). The displaced water is thrown sideways and upwards past the falling water droplets into the air. The drop itself presses a hollow into the smooth surface of the water.

If the raindrop is big and heavy enough, it will displace a corresponding amount of water. Due to the impact of the raindrop, the displaced water receives kinetic energy from the raindrop. This energy determines the height and also the direction in which the water splashes. First the displaced water is lifted upwards and then directed to the center of the impact crater. The result is that most of the water meets exactly in the middle where the raindrop hit before. Because air is trapped in the process, a bubble can form. With large drops, there is more water that splashes upwards. A lot of water can trap a larger amount of air, causing large bubbles to form.

Dirt particles present in puddles or lakes favor the process of bubble formation. If the water is particularly polluted, the bubbles can linger on puddles for a long time before they burst.

Photo water crater: Daniel G. Höck - Freiburg im Breisgau