The Bergeron Process is an example of the huge variety in the scale of meteorology. It takes place on extremely, extremely small scales, but is absolutely fundamental to mid-latitude weather. Without it, we wouldn’t really have any weather.
So What Is It?
The Bergeron Process is a the collision/coalescence process that produces rain drops and snowflakes. In order for cloud droplets (which are very small) to grow large enough to become raindrops, they need to increase in size by close to a million times. For this to happen, and even for a cloud droplet to form, a complicated process takes place to convert water gasses into liquid water where cloud condensation nuclei (CCN), which are small particles of dust and other solids in the atmosphere, assist in the process. In order to understand why they are important, though, we should look at what would happen if CCN didn’t exist, a process known as homogeneous nucleation.
Homogeneous Nucleation
The process of turning gaseous water into liquid water is known as condensation; this occurs when the relative humidity reaches 100% (which, for the more scientifically minded, is when the vapor pressure equals the saturation vapor pressure). When looking at extremely small scales, though, if the air is completely pure with no solid particles whatsoever (no CCN), condensation will only occur when the relative humidity reaches upwards of 120%. This is known as supersaturation. It is extremely difficult (if not impossible) to naturally get supersaturations this high. The water will not condense before then because of the extremely unstable spherical structure of a water droplet. High levels of supersaturation are required to overcome that resistance to formation. This means that sustained levels of supersaturation, and a lot of time, are required to grow raindrops in this scenario. If this were how weather worked, we would probably never get any precipitation.
Heterogeneous Nucleation
If we were to “pollute” the pure environment with CCN, we would drastically change the process. The presence of CCN allows water to condense at much, much lower values than if they were absent; around a couple tenths of a percent of supersaturation (i.e. 100.2% RH) instead of the extremely high levels involved in homogeneous nucleation (recall: 120% or higher). Supersaturation values of a couple tenths of a percent are extremely common, compared to the near-impossible naturally occurring 120% supersaturation.
Alright, Now Lets Make Raindrops
There are two typical ways to grow a raindrop. The first way is through collision and coalescence. This is purely a physical process. As our little raindrops reside in the cloud, they are blown about by the air currents. In the process, they will bump into other raindrops. If they hit each other and bounce off, this is known as a collision. Often, raindrops can split after a collision, resulting in more (albiet smaller) drops than before the collision. If they stick together and become one larger droplet after the collision, it is known as coalescence. This process is somewhat important at mid-latitudes, and very important in the tropics. They key to creating precipitation is, however, the Bergeron Process.
The Bergeron Process
The long and the short of it: Given the same atmospheric conditions, you need less ice than water to reach 100% RH. Or, if you are at 100% RH with respect to water, then you are supersaturated with respect to ice. And we’ve discussed why being supersaturated is good for droplet formation.
The Bergeron Process relies on two major players in the cloud. First, the presence of supercooled water. This is simply liquid water droplets than exist in a temperature below freezing without turning into ice (of which we won’t get into the physics of). The important thing to know is that they are liquid water, and they have a habit of turning into ice whenever they touch things (airplanes do not like large quantities of supercooled water in clouds). The second player is what is know as freezing nuclei. These are not as common as CCN, as they must have a specific structure similar to that of an ice crystal.
As these two players are blown around, the freezing nuclei bump into supercooled water, which will then freeze onto them, turning into ice crystals. So it all happens as such: the air reaches saturation and some of the droplets will come in contact with freezing nuclei, creating ice crystals. With respect to the supercooled water, the air is in equilibrium, but with respect to the ice crystals, the air is supersaturated. Thus, water vapor will sublimate (turn straight from a gas to a solid without turning into a liquid first) onto the ice crystals. As this occurs, the relative humidity with respect to water will decrease, and the supercooled water droplets will evaporate until the air is saturated with respect to water again. This process continues for the life of the cloud; the ice crystals will grow by sublimating water vapor, which will be replenished by the evaporation of supercooled water droplets.
So What’s The Catch?
The Bergeron Process is how a vast majority of precipitation is made at mid-latitudes. It cannot be understated how important it is. And there are two things that are important to know about it. First, the ice crystals grow at the expense of the supercooled water without any contact between the two. Sure, the ice crystals will occasionally bump into the water and freeze on contact, but in general, it’s required that there are far fewer ice crystals than water droplets. If there were too many ice crystals, the supercooled water would be eliminated very quickly, and huge amounts of moisture would need to be continuously added to sustain the process. For a vast majority of the ice crystal growth, there is no contact between liquid water and ice. Secondly, the process is most effective between the temperatures of -10°C and -20°C. This is the optimal temperature range for both ice crystals and supercooled water to co-exist. This will be an important factor in the next article I will be posting.
The Long and the Short of It
This has been a rather lengthy post about something that might not be considered extremely interesting. It is, however, fundamental to all precipitation processes at the mid-latitudes. Without it, we would hardly ever get any precipitation. And, as a fun fact, you may have noticed I talked about ice crystal growth the whole time, and not about water droplet growth. That’s because that is what happens. Even in the middle of summer, all the precipitation at the mid-latitudes begins as snow. The only question is if it’s warm enough underneath the precipitation generating layer to melt the snowflakes into raindrops.
So, I apologize, but technically for all of Canada and most of the United States, it snows even in August; it’s just not at the surface.
Next week, I’ll actually go through a major weather event I had to deal with recently that shows just how important this process really is and how it can make or break a weather forecast.
