Ice Growth: Bergeron Process Quiz Challenge

In cold clouds, ice crystals and tiny water droplets exist together. Because of how moisture moves, the water droplets actually evaporate and turn into vapor, which then sticks to the ice crystals. This makes the ice crystals get bigger and heavier while the liquid droplets disappear. It is a key way that snow and rain start.

It might seem strange, but water in clouds can stay liquid even at temperatures below 32 degrees Fahrenheit. This is called supercooled water. These tiny droplets are very important because they provide the moisture that ice crystals need to grow large enough to eventually fall as snow or rain to the ground.

There is a physical difference in how water vapor reacts to solid ice versus liquid water. The air around the ice crystals becomes "full" of moisture more slowly, so the vapor naturally moves toward the ice. This allows the ice to pull moisture away from the liquid droplets, causing the crystals to expand rapidly.

For this specific process to work, the cloud must be cold enough for ice to exist. It requires a mix of both solid ice and liquid water. Without this mixture, the transfer of moisture cannot happen. Sunlight is not a requirement for the crystals to grow, as this process can happen even in the dark.

As ice crystals continue to steal moisture from nearby water droplets, they gain mass and weight. Gravity eventually pulls them down. Depending on the temperature of the air they pass through on their way down, they might reach the ground as fluffy snowflakes or melt into raindrops before they land.

Even on a hot summer day, the air high up in the sky is very cold. In many areas, rain actually begins its journey as ice crystals growing through the Bergeron process. As these crystals fall into warmer air near the Earth's surface, they melt and become the liquid raindrops that we see during a storm.

Deposition occurs when water in its gas form, known as vapor, skips the liquid stage and turns straight into solid ice. This is how ice crystals grow in a cloud. The vapor molecules find an ice crystal and lock onto it, building the beautiful and complex shapes we recognize as snowflakes during the winter season.

Because the Bergeron process is a primary way that moisture gathers into large enough pieces to fall from the sky, it is responsible for many types of weather. Whether it is snow, rain, or hail, the initial growth of those particles often starts with ice crystals growing at the expense of liquid water droplets.

This process requires freezing temperatures, which are found high up in the atmosphere. The middle and top sections of large clouds are where you find the perfect mix of supercooled water and ice. This "cold cloud" environment is the factory where most of our planet's precipitation is manufactured before it falls.

While the Bergeron process is very common, there is another way rain forms called "collision-coalescence." In warm tropical clouds, tiny water droplets simply bump into each other and stick together until they are big enough to fall. Therefore, not all rain starts as ice, though much of it does in cooler regions of the world.

The liquid water droplets act like a fuel source for the ice crystals. They evaporate to provide the extra vapor that the ice needs to get bigger. If there are no liquid droplets to provide that extra moisture, the ice crystals have nothing to "steal" from, and they will not grow large enough to fall.

The shape of an ice crystal depends on the temperature and how much moisture is in the air. They usually have six sides because of how water molecules lock together, but they can look like needles, plates, or classic stars. They are almost never perfect circles because crystals grow in specific geometric patterns.

The water droplet is losing its molecules to the air as vapor. As those molecules leave the droplet and move over to the ice crystal, the liquid droplet loses mass. Eventually, the droplet may disappear completely as all of its water is transferred to the growing ice crystal nearby.

This process strictly requires ice to be present. Ice cannot exist or grow in a cloud that is completely warm. For the moisture transfer to happen, there must be a difference between the ice and the liquid water, which only occurs when the temperature is low enough for both to exist simultaneously in the air.

It is often called the ice crystal process because the growth of ice is the central part of the theory. Scientists named it after Tor Bergeron, who was one of the first people to describe how ice and water interact in clouds to create the rain and snow that we experience on the ground.