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.
Low pressure systems are the “engine” that drive weather. They are what interact with warm and cold fronts, and they function to convert and dissipate the energy stored within the fronts. These systems have a “traditional” or “classical” progression that can be observed.
Frontal Wave
The first sign of the development of a low pressure system is the appearance of a frontal wave. This is a slight bend in an area of high thermal contrast. This is the beginnings of the warm and cold fronts.
Surface Low Appearance
As the wave tightens, and the cold front becomes more perpendicular to the warm front, the low pressure center appears on the inflection point of the warm and cold fronts (where the two fronts attach to each other). The low will then move with the mean flow aloft, following the troughs of upper-level waves (more on that later). As the low moves, the fronts move along with it. And naturally, the weather associated with those fronts moves along as well.
Mature Low
A satellite image depicting a mature low pressure system. The cold front is represented by the blue line, the warm front by the red line, and the trowel by the blue/red half-arrows. The center of the low circulation is marked by the large L.
One funny characteristics of cold fronts is that they are often faster moving that warm fronts. This can result in the warm front moving “along” the warm front and lifting the warm air up. This is called an occlusion process. The warm air aloft then is pulled towards the low and around it, rising in height. This warm air aloft is called an occlusion, or more frequently today, a trowel.
A mature low will have 4 distinct areas and kinds of precipitation: warm front precipitation, cold front precipitation, occlusion/trowel precipitation, and “wrap around” precipitation. Wrap-around precipitation is the weather that occurs in extremely close proximity.
Low Dissipation
Eventually, the warm and cold fronts pull themselves off the low pressure system. It can be likened that the “gas” for a low pressure system is the temperature contrast present in the fronts. When the fronts leave the low, warm air wraps around it and soon there is no more sharp temperature contrasts. When this happens, the low will “fill in” and dissipate.
Why Is It Called A Low Pressure System?
When a low pressure system begins to form, air is pulled in towards the center of it. We have discovered that, more or less, air in the atmosphere doesn’t like to compress. So instead of compressing as all this air meets in one place, it pushes air upwards. This creates a circulation where air moves in towards the low at the surface, rises some height, then flows out and away from the low. This results in low pressure near the surface, where the air is rising, and higher pressure somewhere above, where the air is moving outwards. Thus, a low pressure system is called such because the surface pressure is actually lower than the areas around it. As it “dies,” the surface pressure will return to the normal pressure around the low.
I should mention that this is an extremely brief overview of low pressure systems. If you would like to learn more, there are entire books written on the subject, and to this day it is still an area of active research.
Next week, land and sea breezes!
Filed under: Cold Front, Convection, Precipitation Events, Synoptic Features, Thermodynamics
The weather effects each of us, every day. A lot of people have expressed to me interest in learning about the weather, but it’s definitely not easy. I’ve decided that each week, I’ll post a short-ish post explaining the basics of various weather phenomenon; this week: The Cold Front.
For many people, cold fronts are one of the most noticable weather events out there. They are fast moving features that often produce severe weather and result in drastically different weather after they have passed through.
Formation
All over the globe, air has a tendency to form into large areas with similar characteristics. We call these air masses. One example is could be a large area where it is hot and humid. In the wintertime, often there are extremely large areas of cold, dry air. These air masses move through the atmosphere, and fronts are developed. A cold front is the boundary on the leading edge of colder air moving into warmer air. It does not matter how cold the cold air mass is or how hot the hot air mass is, as long as they have a temperature difference, the cold front will form.
Structure
Cold fronts have a very defined structure that is based on a simple physical principle: density.
From high school physics, we know that the density of a gas is defined by it’s temperature and it’s pressure. Similar to how oil and vinegar have different densities, so do warm and cold air. Cold air is more dense than warm air. Because it is heavier, as the cold air (blue) pushes into the warm air (red), it begins to undercut the warm air and push it upwards at a very sharp angle. This causes a very narrow, relatively intense band of precipitation to form along cold fronts. Cold fronts move fairly quickly as the cold air digs underneat the warm air, and the weather associated with the front will be fairly short lived at any one location, usually lasting no longer than a few hours.
Common Weather
In the summer time, the passage of a cold front can often produce thunderstorms, or other significant weather features such as squall lines, supercells, and mesoscale convective complexes. These are all different forms of organized thunderstorms that last for a very long time. Due to the sharp upward push of the warm air, cold fronts can often produce short-lived, severe weather such as heavy rain with potential flooding, thunderstorms with large hail, and even tornadoes. In the summer, cold fronts can offer a reprieve from hot, humid weather. In the winter, the passage of a cold front can result in long stretches of cold weather and clear skies.
Next week, warm fronts. If there are any questions regarding this week, just leave them in the comments!
Filed under: Precipitation Events, Synoptic Features, Thermodynamics, Warm Front
Warm fronts are, in many ways, the complete opposite of cold fronts. They often cover a large area and are slow moving, bringing extended periods of precipitation. They often produce large amounts of rain or snow (depending on season), and may contained a sizable amount of embedded thunderstorms in them.
Formation
A warm front is formed in the same mechanisms as cold fronts; the warm front is the boundary of warmer air moving into cooler air. Again, like a cold front, the actual temperature of the air does not matter, as long as it is warmer than the air it is moving into.
Structure
Warm fronts have a defined structure as well, and it is significantly different than a cold front’s structure.
As the warm air moves into the colder air, it encounters air that is more dense than it. Opposite of the actions of the cold air moving into the warm air, the warm front slopes over top of the cold air, as warm air slowly overtakes the cold air, gently being lifted over top of the cold air at the same time as it moves the cold air out. This produces a front that is very different in appearance when compared to a cold front. Instead of being narrow and extremely sharp, warm fronts are extremely wide features, sometimes in excess of 500km, that feature a very gradual slope to their fronts. The passage of a warm front, due to it’s size, can often take 6-12 hours, and in some cases, as long as a day.
Common Weather
In the wintertime, warm fronts make snow. A lot of snow. A majority of the snowfall in East Coast North America winter storms comes from the warm front. In the summer time, warm fronts often bring prolonged periods of light to moderate rain. Cloudy conditions are widespread with warm fronts.
In fact, a warm front is easily detectible and can be forecast by a few keen observations out your window. The clouds associated with a warm front have a very distinct progression: first high cirrus clouds will move in. They are the ones that are extremely high up and wispy looking. They will gradually get lower and lower until there is some thicker mid-level cloud. This cloud no longer looks wispy and is thick enough to mostly block out the sun. These clouds will continue to get lower and lower until low clouds show up, which are close to the ground, and completely block out anything above them. Often if you spot this progression over several hours, some sort of precipitation is likely on it’s way.
In the summer time, warm fronts can offer the conditions necessary to produce Mesoscale Convective Systems, which are a cluster (often you can draw a circle around them) of organized thunderstorms that are usually severe and can last 12-18 hours (although likely only 2-3 hours in any one given location). As well, warm fronts often have thunderstorms embedded in them in the summer months.
The weather behind a warm front is not as definite as the weather behind a cold front. It may be completely sunny, it may be cloudy, or it really can be anywhere in between. The one thing for sure is that it’s warmer than it was before, and often it’s more humid as well.
Sorry it’s late, but I hope you enjoyed this! Next week we’ll talk about low pressure systems and how these [warm and cold] fronts work in relation to the larger picture!
