Why Don't Clouds Freeze?
Many clouds do freeze! The misconception that clouds don't freeze stems from a misunderstanding of cloud formation and the phenomenon of supercooling.
Clouds are formed from tiny water droplets or ice crystals suspended in the atmosphere. The process starts with water vapor rising and cooling. At certain altitudes, this vapor condenses around microscopic particles called condensation nuclei, forming clouds.
Crucially, water can exist as a liquid below its freezing point (0°C or 32°F) – a process known as supercooling. This is particularly common in the upper atmosphere where conditions lack the necessary particles (freezing nuclei) to initiate ice crystal formation. Therefore, even at temperatures well below freezing, some cloud droplets remain liquid. Only when sufficient freezing nuclei are present or when the temperature drops extremely low will these supercooled droplets freeze.
Types of Clouds and Freezing
Different types of clouds exhibit different characteristics:
- Cirrus clouds, which are high-altitude clouds, are typically composed of ice crystals because of the extremely low temperatures at these altitudes.
- Lower-altitude clouds, however, might contain supercooled water droplets, meaning liquid water below freezing. These can freeze under certain conditions, like the presence of ice nuclei or a significant drop in temperature.
Why the Misconception Exists
The idea that clouds don't freeze likely stems from the observation of lower-altitude clouds that often appear to contain liquid water even in cold conditions. However, this is due to supercooling, not an inability of water to freeze in clouds. The reality is more nuanced; many clouds, particularly at higher altitudes, are indeed made of ice crystals.
References:
- Several sources mention supercooling as a key factor in the persistence of liquid water in clouds even at sub-zero temperatures. Supercooling, a state where liquids do not solidify even below their normal freezing point, still puzzles scientists today. A good example of this phenomenon is found everyday in meteorology: clouds in high altitude are an accumulation of supercooled droplets of water below their freezing point.
- Multiple sources confirm that many high-altitude clouds are composed of ice crystals: Some clouds do freeze! Most clouds at higher altitudes are made of ice crystals. and In fact, cirrus clouds (those that are usually seen as very thin plumes, up in the sky) are completely made of small ice crystals.
