#532 Why are clouds at different heights?

Why are clouds at different heights? It depends on the temperature and the pressure of the air.

Clouds start when water vapor evaporates off the surface of the Earth into the air. The sun heats the ground, or the surface of the sea, supplying it with thermal energy. If there is enough thermal energy, the water on the surface of the sea or on the ground can become a gas. That means the water molecules have more kinetic energy than the water molecules that are still a liquid and they can move a lot more. They move so much that they can break their bonds with other water molecules and they rise up into the air. The air at the level of the ground has also been heated by the sun, which means the air molecules also have more energy, and they start to spread out more. When the molecules spread out, the air becomes less dense and buoyant. It starts to rise up through the cooler air, which is more dense.

Air can hold water molecules until it reaches saturation point. This saturation point changes with the temperature of the water. Warm air can hold more water because the air molecules are spread out more and there is more space for the water molecules. As the air cools, the air molecules get closer together and there is less room for the water molecules. The saturation point is the point where the air has cooled to the point where it cannot hold any more water molecules because there is no more space. The temperature at which this happens is called the dew point.

When the air is cooled to the dew point, it cannot hold any more water because it is at 100% capacity. If the air cools ever so slightly more, the air has to start shedding water and the water vapor begins to condense. The air has cooled, and the molecules are closer together. This squeezes the water molecules closer together and they start to join with each other. The water droplets need something to collect around and these are called cloud nuclei. The air is full of particles, such as smoke, soil, salt, and other substances. These have a flat round surface that the water molecules can collect around. The water droplets start to fall to Earth, but they are so small that they fall very slowly. Sometimes, they pick up more water droplets as they go and then fall as rain, but most of the time they fall to the bottom of the cloud where the air is warmer and they evaporate again, getting carried back up into the cloud. The water droplets are close together and they reflect light, which is what makes clouds white. When the water droplets accumulate enough to be raindrops, they are much closer together and there are gaps between them, meaning less light is reflected back and the clouds are darker. The wind blows the water droplets and the clouds start to move across the sky.

So, why are clouds at different heights? The column of rising air has to cool to its dew point before the water vapor it is carrying can start to condense and clouds form. If the day is very hot, the water saturated air can rise much higher than if the day is not so hot. When the air is cool, the clouds will form at a lower level than when the air is warm. The pressure of the air also affects the height of the cloud. When there is low pressure, the air molecules can spread out more and the air can go higher before it reaches its dew point. If there is low pressure, it can’t rise as high. This is why there is sometimes fog or mist in the morning. The air close to the ground is not very warm and the air reaches its dewpoint at ground level. The water condenses and you have fog.

Clouds can be at ground level and then all the way up through the atmosphere, but the maximum height for clouds is about 9,000 m. At this point, the air is too thin to hold the water vapor and any water molecules that are carried this high fall back down to a lower level. The jet stream winds at these heights are also too strong to allow clouds to form. That is why, when you are flying in a plane, you break through the cloud level and it appears like a flat carpet in every direction. That layer is at approximately 9,000 m, and your plane will cruise about 1,500 m above that level.

So, the temperature of the air and the air pressure affect the height of clouds. And this is what I learned today.

Sources

https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/what-are-clouds-and-how-do-they-form

https://education.nationalgeographic.org/resource/condensation/

https://www.usgs.gov/special-topics/water-science-school/science/evaporation-and-water-cycle

https://www.usgs.gov/special-topics/water-science-school/science/atmosphere-and-water-cycle

https://www.lsop.colostate.edu/wp-content/uploads/sites/6/2014/10/WhyDoesWarmAirHoldMoreWater.pdf

https://www.davisinstruments.com/pages/what-is-dew-point

https://www.thenakedscientists.com/forum/index.php?topic=25225.0

https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/cloud_development/clouds.htm

https://en.wikipedia.org/wiki/Cloud

https://education.nationalgeographic.org/resource/cloud/