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Wind patterns don't just move air. They move the atmosphere's entire "supply chain". Once we understand how winds transport heat, moisture, and pressure features across the globe, we can start to see why precipitation forms where it does. The same circulations that steer storms also determine where air rises, cools, and condenses into clouds, and where it sinks and suppresses rainfall. In other words, wind is the delivery system whereas precipitation is the outcome. Now that we have explored how air flows and redistributes energy, we can shift our focus on what happens when that moving air becomes saturated, lifts, and transforms invisible water vapor into rain, snow, and ice that shapes our weather.
Raindrops
Raindrops
When an air parcel rises, it cools and expands, reaches saturation at 100% relative humidity, and condenses to form clouds and precipitation. The formation of precipitation begins as very tiny droplets or ice particles inside a cloud on a microscopic scale. Therefore, precipitation formation processes are actually a microscale phenomenon and is why it is very difficult to predict as many weather models do not reach such resolutions. First, we will discuss raindrops, which start off very small at less than 1 millimeter in diameter and are very spherical in shape. The tear-shaped raindrops you see in cartoons is actually a myth. Since these raindrops are very tiny, they are not able to fall to the ground quite yet.
Some droplets grow slightly larger than others. This can happen because of turbulence, different condensation histories, or the presence of larger condensation nuclei, which act as a particle for water vapor to latch onto before water droplets begin growing. Larger droplets fall faster than smaller ones. Gravity pulls them downward, and because they're larger, they have a higher terminal velocity. Terminal velocity is the constant speed an object reaches when the force of gravity pulling it downward is exactly balanced by the drag force pushing it upward. In other words, once an object falls long enough, gravity is equal to air resistance. It stops accelerating and continues falling at a steady speed. That steady speed is terminal velocity. This is critical in droplet formation as it determines how fast they fall, how likely they are to collide with other droplets, and whether they can survive evaporation on the way down.
As the big droplets fall, they collide with smaller droplets. Turbulence, airflow around the droplet, and random motion all increase the odds of collisions. The warm-cloud precipitation process in which larger cloud droplets fall faster, collide with smaller droplets, and merge with them is called collision and coalescence. Collision is when droplets bump into onto another and coalescence is the act of the droplets sticking together after collision. Keep in mind, coalescence is only one outcome. The droplet may bounce off one another, separate, or break apart, so coalesence is not always guaranteed. The process will repeat itself until a droplet reaches greater than 4 mm. As the droplet gets larger, there is more surface tension against the bottom of the trop as terminal velocity increases due to gravity, and therefore, sometimes it splits into two. Secondly, due to the added air resistance, the raindrop flattens and is actually the shape of pancake rather than a tear. Eventually, the droplet becomes heavy enough and survives the fall to the ground without evaporating. However, snowflakes are an entirely different story.
Snowflakes tend to have a lower density due to their intricate structure with mostly empty space of the air trapped between the branches. As a result, snowflakes have a lower terminal velocity than raindrops. Snowflakes are a lot more complicated as they come in many shapes and sizes such as needles, plates, and your typical six-sided dendrites. The shape is entirely dependent on temperature and moisture content in the atmosphere. Large dendrites typically occur below freezing in very cold conditions with really high moisture content whereas plates and columns occur typically in your arctic, dry airmass. However, snow goes through an entirely different process than raindrops.
Explains the science behind the common myth of tear-shaped raindrops.
Dives more into the concept of terminal velocity and how it impacts larger drops.
Types of Precipitation
Types of Precipitation
Before diving into the types of precipitation, there are three important cloud layers to denote. In the previous section, collision and coalescence was explained through a warm-cloud process. This essentially means that all droplets in the cloud are liquid and are typically in clouds closer to the surface up to about 6,500 feet. Above 6,500 feet to about 23,000 feet are middle layered clouds and consist of both ice crystals and supercooled water droplets. Supercooled water droplets are liquid droplets of pure water that can exist below freezing as water doesn't automatically freeze at 32 F (0 C), it needs something to trigger the freezing process, which is usually an ice-nucleating particle. These tiny droplets have a curved surface, which make it harder to freeze. Above 23,000 feet is when you have cold clouds that are entirely made up of ice crystals. These three distinctions are vital not only in the formation of different types of precipitation but also impacts to aircraft.
Snow, Graupel, and Rime Ice
Snow, Graupel, and Rime Ice
All precipitation starts off as snow. If the entire atmospheric column is below freezing, then you will receive snow as expected. The process where individual ice crystals collide and stick together is called aggregation. This forms larger snowflakes and is important in the dendritic growth zone where crystals have lots of branching structure. Essentially, it is collision and coalescence, but with ice crystals and not water droplets. While ice crystals colliding and sticking together is aggregation, supercooled water droplets play a vital role through a process known as accretion. When ice crystals collide with supercooled water droplets, these droplets freeze upon contact, coating the crystal. This produces rimed crystals, and with enough collisions, graupel forms. Graupel forms from the process of accretion, which creates small, soft, opaque, white pellets that look a bit like tiny Styrofoam balls. It is important for aircraft to avoid middle layered clouds due to the presence of supercooled water droplets as they need anything to latch onto to freeze. If the supercooled water droplets don't latch onto an ice crystal, they will certainly latch onto an airplane and freeze upon contact known as the formation of rime ice. Rime ice is dangerous to airplanes as it distorts the shape of the wings and control surfaces, adds drag, and reduces lift, all while forming quickly in the exact conditions where pilots have the least time to react. This is why airplanes are equipped with de-icing in case they run into this issue but is best practice to avoid it.
Freezing Rain
Freezing Rain
Now, let's say a snowflake falls through a deep layer of warm air in the atmosphere. It then melts, but it is below freezing at the surface. This is called freezing rain. Freezing rain is entirely dangerous to travel and infrastructure as it holds a lot of weight and can lead to downed powerlines and power outages. It's very different from rime ice because rime ice forms from very small, supercooled water droplets that freeze upon contact of any object within a cloud or fog whereas freezing rain occurs at the surface from a larger raindrop due to subfreezing surface temperatures. It forms a clear, smooth glassy glaze on the surface. Rime ice is milky, opaque, rough, and feathery in appearance.
Sleet vs. Hail
Sleet vs. Hail
Like before, we start off as snow like all precipitation does. As the snow falls through this atmospheric column, the snowflake hits a shallow warm layer. It begins to melt. It then hits into a deep-freezing layer above the surface. Since this freezing layer is deeper than just at the surface, the raindrop has enough time to refreeze into an ice pellet. These small, translucent ice pellets is known as sleet. Very different from hail! Sleet is a winter season type of precipitation whereas hail typically forms in the warmer season in a thunderstorm updraft. The embryo of a hailstone can be either a frozen raindrop, a graupel pellet, or an ice crystal. The embryo collides with supercooled water droplets and freezes upon contact, adding layers of ice much like an onion. This is the same process that forms graupel, but hail grows much larger because of the storm's strength. The stronger the updraft, the more the hailstone gets lofted back into the cloud over and over until it is too heavy for the updraft to hold and begins to fall to the surface. This is why hailstones can grow up towards the size of baseballs or even grapefruit.
Rain
Rain
If the entire atmospheric column is above freezing, then the snowflake aloft will melt and stay as a raindrop until it reaches the surface. However, as mentioned before, raindrops must survive the process of evaporation from air resistance on the way down in order to fall to the surface.
Dives into the different types of precipitation and the processes of precipitation formation. Secondly, dives into how rime ice can be dangerous to aircraft.
Virga
Virga
A considerable amount of precipitation evaporates between the cloud base and the surface. Streaks of rain evaporating and not reaching the ground is called virga. This is especially common in the desert southwest United States due to low relative humidity. Very hot air would cause intense daytime heating and rising bubbles of air, but since the air is dry, the thunderstorms become more elevated and may produce lightning, with very little rain due to evaporation. These dry thunderstorms pose a risk for wildfires in the west as lightning can ignite dry fuels.
As more evaporation takes place, the air cools and becomes denser. Evaporation is a cooling process because water needs to absorb heat from its surroundings in order to change from a liquid into a vapor. Evaporative cooling can cause a downdraft, winds moving down and out from of a thunderstorm, to strengthen. When rain is heavy enough combined with evaporative cooling and winds aloft reaching the surface, an intense bust of winds can plow to the surface known as a downburst, which is hazardous to aircraft preventing further lift and can cause crashes to be fatal when hitting a tailwind. Two types of a downburst are a microburst, which affects a smaller area than 4 km, and a macroburst, which are larger than an area of 4 km. Both are just different in size, duration, and damage footprint, but both can create very damaging straight-line winds from a few seconds to several minutes in a microburst and anywhere from 5 to 20 minutes in a macroburst. If it is primarily virga within the thunderstorm and no rain makes it to the surface and there are intense, damaging straight line winds, then that is a dry microburst.
Dives into the concept of virga and the consequences it may have out in dry arid regions.
How a downburst forms.
Relative Humidity
Relative Humidity
While precipitation comes in many forms, there must be ample amount of moisture in the atmosphere in order for there to be precipitation. There are several ways to measure the amount of water vapor in the atmosphere, but one of them is relative humidity, which is measured as a percent from 0 to 100 at a given temperature. There are several ways to calculate relative humidity, but here we will discuss one way, which is:
RH = vapor pressure/ saturation vapor pressure *100%
Where vapor pressure is the actual partial pressure of water vapor molecules in the air at a given moment. Vapor pressure can change with how much water vapor is present in the atmosphere. Saturation vapor pressure is the maximum possible water vapor the air can hold at a given temperature. It is determined only by temperature and not by how much water vapor is actually present. As a result, warmer air holds more water vapor and therefore has a higher saturation vapor pressure than cold air. This makes sense as the tropics tend to be warmer and contain more moisture while the poles are a lot colder and drier. It also represents the equilibrium between evaporation and condensation. In other words, the rate of molecules leaving the liquid equals the rate returning. Think of vapor pressure as what you have while saturation vapor pressure as the ceiling, or until a sponge can't soak up anymore water and starts dripping. Once a sponge starts dripping, the atmosphere has reached full saturation and 100% relative humidity and condensation starts to occur to form precipitation. However, precipitation can exist below relative humidities below 100%. Why?
Well, enter the solute effect. The solute effect explains how dissolved substances (like salt) lower the saturation vapor pressure and hence the relative humidity to as low as 70%. Our bodies are made mostly of water, which is why when we eat a lot of salty snacks, we feel thirstier. That is why you should always drink water after consuming a lot of salt to make up for the water lost in the body. It works the same way in the atmosphere. Since precipitation can exist below 100% relative humidity, can it exceed 100%? Yes, and it dives into the concept of equilibrium between condensation and evaporation where both are equal or in balance.
Recall that smaller drops are more curved while larger drops are flatter. A small droplet has a tight radius of curvature, and the surface tension must work harder to hold that curved interface together. Meaning, the internal pressure increases and higher internal pressure gives surface molecules more energy, which escape more easily. Hence, smaller droplets evaporate more easily than flatter drops. This is called the curvature effect, which inhibits the growth of droplets whereas the solute effect enhances growth. This increased evaporation is because curvature raises the saturation vapor pressure. Therefore, the air must be more humid to keep a tiny droplet from evaporating. The droplet would never grow if condensation and evaporation are in equilibrium. Once vapor continues to accumulate through evaporation and cooling, relative humidity can exceed 100% and reach 101% briefly before condensing into precipitation. This is called supersaturation where the condensation rate exceeds the evaporation rate. This matters because the actual vapor pressure exceeds the saturation vapor pressure. Water vapor molecules are more likley to stick to surfaces (condensation) than escape (evaporation). Therefore, cloud droplets can grow rapidly if condensation nuclei are available and go through collision and coalescence and eventually fall as precipitation when the gravity becomes too much for the atmosphere.
Explains why precipitation can exist below 100% relative humidity.
Explains why relative humidity can exceed 100% briefly in order for cloud droplets to grow.
Visual interpretation of the science behind seeing our breath.
A Breath of Fresh Air
A Breath of Fresh Air
Ever wonder why you can see your breath a lot in the winter? Since the air is cold and dry in the winter and your breath is warmer, once you exhale a breath of air, the warm breath cools quickly. If the surrounding air is already saturated, then as you exhale, the added moisture from your breath pushes it past the dewpoint. This allows tiny water droplets to condense just like inside a cloud. As light scatters from the tiny droplets, the mist becomes visible as we call it "seeing our breath".
Dewpoint Temperature
Dewpoint Temperature
Let's imagine you're on the beach on a hot summer afternoon enjoying a glass of lemonade. You many notice liquid forming on the outside of the glass. This is because the lemonade is cooling the glass while the surrounding outside air is warmer. Now, you may not be able to see it, but there is always invisible water vapor molecules in the surrounding air; especially on hot and humid days. The water on the outside of your glass is known as dew. It is based off of another way to measure moisture which is dewpoint temperature. Dewpoint temperature is the temperature to which the air must be cooled (at a constant pressure) to achieve saturation (100% relative humidity). If the temperature and dewpoint temperature are equal, then there is 100% relative humidity. Once the warm air hits the cold glass, the air cools down to its dewpoint and condenses, forming tiny water droplets or dew. This is the same process that forms dew on grassy surfaces on a spring or summer morning from radiational cooling allowing the surface to cool and reach saturation. It also explains why on clear, calm nights where radiational cooling is maximized and there is moisture present in the air, dew forms on your car. When the air temperature is below freezing, water vapor directly deposits as ice known as deposition, going from a gas to solid, and forms frost.
Explains the concnept of dew and how it relates to the dewpoint temperature.
Understanding how precipitation forms and the types of precipitation is fundamental in weather forecasting. Deciphering between cloud heights leads to an idea of the types of precipitation and processes ongoing inside the cloud. Regardless, as long as there is moisture present in the atmosphere, whether it's measured through relative humidity or dewpoint temperature, precipiation is likely to occur for a certain location. It also depends on the temperature for precipitation type as well as wind, allowing that moisture to transport from location to location. The key takeaways are as follows:
Raindrops start off as curved allowing a higher saturation vapor pressure and therefore preventing the droplet from growing due to high evaporation known as the curvature effect. On the other hand, the solute effect, allows droplets to grow more easily due to dissolved substances.
In order for the droplet to grow, condensation must exceed evaporation, disrupting equilibrium, and achieving supersaturation.
Relative humidity is a ratio between vapor pressure, the amount of water vapor in the air, divided by saturation vapor pressure, the maximum amount of water vapor the air can hold. Another way to measure moisture is the dewpoint temperature, which is the temperature at which a parcel of air cools to reach saturation.
Once supersaturation occurs and the droplet is able to grow, as it falls, the droplet becomes flatter from air resistance beneath the drop in combat with gravity forcing it down. When air resistance and the force of gravity eventually equal out, the droplet falls at a constant rate of speed known as terminal velocity.
As the droplet falls, it collides and sticks with other droplets to form larger droplets called collision and coalescence.
There are various types of precipitation:
Rain forms when the entire atmospheric column is above freezing.
Sleet forms when there is a shallow warm layer followed by a deep-freezing layer.
Freezing Rain forms when there is a deep warm layer followed by a shallow freezing layer near or at the surface.
Snow forms when the entire atmospheric column is below freezing. These snowflakes form through a process called aggregation when ice crystals collide.
Graupel forms through accretion where supercooled water droplets freeze upon contact on ice particles.
Rime ice is when supercooled water droplets inside fog or a cloud immediately freeze upon contact with an object like an airplane.
Hail forms when raindrops are lofted high into an updraft, freeze, and are lofted up and down in the updraft adding layers of ice and water.
Virga is when precipitation does not reach the surface.
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