How Precipitation Forms

HOW PRECIPITATION FORMS

While everyone has experienced precipitation, few people understand how it forms the different names for precipitation. Let's discuss the different kinds of precipitation and how they happen.

Of course, precipitation falls often in the form of rain. Have you ever wondered why some rain is gentle with small misty raindrops, while other rain is literally a downpour with giant raindrops? Why do you see some clouds that look like rain is coming, but they don't rain at all? Actually, the average raindrop inside a cloud is only 20 micrometers in diameter. This is very small. The raindrops you feel are nowhere near that small; there are reasons for this that we will discuss.

The average raindrop you feel will be about 2 millimeters in diameter. This is literally 10,000 times the size of the average condensation nucleus. If you measure the size of a cloud droplet, a raindrop will be 100 times its size. The size of a raindrop can be as little as 0.2 millimeters or as large as 2.5 millimeters in diameter. This is why you feel such a difference when you feel drizzle as opposed to a downpour. You can imagine that a raindrop will grow in size inside the cloud until it becomes too heavy. Friction caused by the air itself will cause the raindrop to break up, with several smaller droplets falling to the earth. In other words, while cloud droplets have a negligible fall velocity, raindrops do not and they will fall.

The aerosol that causes water vapor to condense must be hygroscopic. The term hygroscopic means it loves to attach to water and absorb it. Salt particles above the ocean are very hygroscopic, so these will attract water vapor at the lowest humidity possible. Other particles are not as hygroscopic as salt.

There is an equation called the Stoke' s drag law, which calculates the drag force and ultimately the velocity of any raindrop. The calculation indicates that the drag force and velocity of a raindrop is directly proportional to the diameter of the sphere, which is essentially the sphere of any raindrop. What you would see from this calculation is that it might take hours or days, if ever, for a cloud droplet to fall from the cloud to the ground. This is because the friction caused by air keeps it suspended within the cloud.

Once the sphere becomes large enough, gravity kicks in and the raindrop has a terminal velocity, allowing it to fall. They simply need to be large enough to accommodate the air friction and the force of gravity on them.

So, how do raindrops get bigger in the first place? There are 2 theories to explain this. The first is called the collision coalescence process. This works only for liquid rain. What happens is that smaller droplets collide and coalesce into a larger droplet. These are heavy and fall, colliding with even more raindrops. It is like a positive feedback loop, where raindrops just get bigger and bigger as they collide with one another, until they overcome the friction of the air that keeps them suspended and start to fall. It actually works best if there are cloud droplets of different sizes. Again, random collision and coalescence increases the raindrop size until it falls.

Interestingly, the very deep cumulus clouds make the biggest raindrops. This is because they have convection updrafts within them, allowing the raindrop to be suspended inside the cloud within the updrafts for a long period of time. These raindrops are the biggest when they fall. Stratus clouds are not very thick, so the droplets do not have the advantage of staying in the cloud very long. This will lead to drizzle or light rain rather than heavy rain.

If you do not live in the tropical areas of the world, you need to assume that a lot of rain clouds have ice in them. Unlike water clouds, clouds with ice in them do not just get bigger through coalescence. They must get bigger through another route.

Water freezes at zero degrees Celsius, however, there is such a thing as supercooled water which is liquid water colder than this freezing point. How can this be? It is similar to what happens with condensation and a lack of a condensation nucleus. Remember, the water must be very prevalent to coalesce without any nucleus. In the same way, cold water in liquid form can exist if there is no obvious ice nucleus to attach to. Liquid water can actually stay liquid at temperatures as low as -40 degrees Celsius. Any temperature below this level of -40 degrees Celsius means that water must be solid. In the range between zero degrees Celsius and -40 degrees Celsius, liquid water can persist as supercooled water.

When it comes to the phase of water in a cloud, altitude is everything. At certain altitudes, water will be ice no matter what the conditions. As you get to lower altitudes, you can get what are called ice phase hydrometeors mixed with water droplets. A hydrometeors is any water droplet in any phase. An ice phase hydrometeors is basically a droplet of water in some solid form.

If you do get a situation where the temperature is below -40 degrees Celsius, you will get what is called homogenous or spontaneous freezing. It is most common above a large body of any kind of freshwater. These tiny droplets start as what are called ice embryos. An ice embryo will by itself get large enough to be an ice nucleus. These lead to homogenous solid clumps of ice. Because it is so cold, these ice embryos do not move very much. If they would move too much, they would potentially break apart. By staying still, they can just get bigger.

Figure 18.

Earlier, we mentioned that ice nuclei are simply cloud condensation nuclei in solid form. They are similar to CCNs, but they need to be a certain geometry to fit into the shape of a proper solid clump of ice. Figure 18 shows you the shape of an ice crystal. It's this shape that ice crystals are more likely to attach to.

It is harder to find a good ice nucleus than it is to find a water vapor nucleus. For example, dust in the atmosphere is one of the best types of ice nuclei you can get. There are certain ice nuclei that allow water vapor to become solid as soon as they touch the nucleus. These are called deposition nuclei, because they allow the phase change to occur at the time they come in contact with the water vapor. This would involve deposition of gas into a solid without entering the liquid phase. Other nuclei attached water in the liquid phase and then freezes.

Small freezing nuclei in the clouds will take supercooled liquid droplets and turn them into solid droplets simply by colliding with them. This is known as contact freezing and the nuclei that do this are called contact nuclei. So, we have 4 different types of ice nucleation:

• Homogeneous freezing – this is freezing without any nucleus at all.

• Deposition nucleation – this is freezing that goes from gaseous to solid without liquid water in between.

• Immersion or condensation freezing – this is freezing on a basic ice nucleus.

• Contact freezing – this is freezing through collision of water on an existing ice embryo.

Because of this super cooling process, you now have a lot of very cold water droplets and a few ice crystals in most clouds. None of these are very heavy by themselves, which means something must happen to allow these to become heavy enough fall to the earth. What you need to understand is that the saturation vapor pressure for the pressure at which the air is saturated with water is different when that water is solid compared to when the water is liquid. To be fair, it isn't very much different. Figure 19 shows you the slight difference.

Figure 19.

The effect of this difference in saturation vapor pressure between ice and liquid water means that at these cold temperatures between zero degrees and -40 degrees Celsius the tendency is for water to escape in liquid form and deposit on ice crystals. This is called the ice phase process, which means that ice crystals will grow bigger even as supersaturated water droplets will get smaller. As you can see, this allows the formation of larger particles of ice or snow in the cloud.

What happens then is that the cloud filled with supercooled liquid droplets provide a ready source for rapidly growing ice crystals. As these ice crystals become larger, they gain mass so they are more likely to fall. Just as is true for liquid water, updrafts in the clouds will cause larger ice particles to grow. Ice crystals can collide with supercooled water droplets, causing these droplets to freeze onto the ice crystal. This is called riming or accretion. The clump that is formed is called a graupel, which is a snow pellet. In