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The Formation of Rain Droplets Due to Collision and Coalescence Credit: COD Meteorology/Plymouth State Meteorology/ (Wallace & Hobbs, 2006)

4/26/2019

In order for rain to precipitate from warm clouds, individual cloud droplets must first undergo a growth process in order to grow and form rain droplets. There are two processes by which this growth and cloud formation can occur. These two processes include growth by condensation or diffusion, and growth by collection, also known as collision and coalescence.

Collision and coalescence typically occurs in warm clouds with cloud tops reaching temperatures greater than -15 degrees Celsius. In order for this process to occur within the cloud, there must be a high liquid water content, strong and consistent updrafts, a large range of droplet sizes, and a cloud layer that is thick enough for the droplets to have time to grow to larger sizes. Before a cloud droplet can grow to these large sizes and become a rain droplet, the cloud droplet itself must form! This occurs when there are aerosols or other pollutants within the air that can act as cloud condensation nuclei. Cloud condensation nuclei or CCN, act as a particle on which the water can condense onto, as pure water cannot condense and form a cloud droplet on its own.

Credit: Plymouth State Meteorology (Diagram Made Based on (Lutgens, 1992))

The process of collision and coalesce occurs following the formation of the warm cloud. Within the warm cloud, convergence occurs near the surface causing rising motion within the cloud as this air rises in the updraft. Small cloud droplets can be caught within this updraft and rise up into the cloud as larger droplets within the cloud may be falling to the surface. As these water droplets rise and fall within the cloud, some collide into each other due to their different fall speeds. As more collisions occur over time, these droplets can coalesce together forming larger droplets. These larger droplets falling from the cloud are known as “collector drops”. Because larger cloud droplets have a higher terminal velocity, these larger collector drops fall faster all the while coalescing with smaller droplets on the way down. This causes the rain droplets to sometimes become so large that they break apart due to instability as they fall within the cloud.

The formation and growth of rain droplets by collection or collision and coalescence is necessary in order to explain the formation of warm rain in warm clouds, and it is a very efficient way for cloud droplets to become rain drops.

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©2019 Weather Forecaster Christina Talamo

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Interpreting a National Weather Service Area Forecast Discussion (AFD) (Photo Credit: National Weather Service San Francisco Bay Area)

4/26/2019

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DISCUSSION: A few times a day at every National Weather Service (NWS) office across the United States, an Area Forecast Discussion (AFD) is released describing in written form what is discussed during the in-office briefing that took place. The AFD is then used to by students as well as professional meteorologists to help with forecasting such as thinking of possible factors may not be resolved on models. An AFD is usually made of several segments about a paragraph or two each describing the forecast or what happened.
The first section that usually appears on an AFD is a synopsis. The synopsis is usually an overview of the forecast as well as a review of the past few hours. An example of a synopsis would be as follows:

This afternoon will feature cool temperatures and breezy conditions under partly cloudy skies. A warming trend is forecast for Wednesday and Thursday, with temperatures expected to
climb well above normal by Thursday with widespread 80s for inland locations. A cooling trend is forecast to begin on Friday and continue into the weekend. Generally dry conditions are expected over the weekend and early next week.

The next section or sometimes the first section at some NWS offices is the discussion. The discussion talks about what detailed expectations of the forecast such as if there is cold air damming to be expected or if there is a shortwave trough coming through. The discussion is usually split into two or three different parts depending on the NWS to cover different timescales such as the next day, then the next few days, and finally the long term forecast for the rest of the week and into the next week.

The section that follows the discussion is the aviation forecast. The aviation forecast focuses on the visibility, ceiling and other weather factors within the next 24 hours at major airports in the area. A ceiling is the height where the aggregate cloud coverage including the clouds below the ceiling exceeds 51 percent of the sky. After the aviation section in some of the offices are the hydrological forecasts which focuses on how rivers and creeks will be such as whether there is a concern for floods. Some NWS offices have a marine forecast in addition or in place of the hydrological forecast to talk about how a lake or the ocean will be affected by weather patterns such as whether riptides can be expected or waves are expected to be over 20 feet. Also, in some offices, they include fire weather information especially if the forecast area is prone to wildfires.

The final section of the AFD is the warnings and watches. This section covers all active warnings, watches, and advisories that were declared by the specific office. For example, the NWS office in Monterey, CA would have listed a Small Craft Advisory (an advisory about strong winds that are close to but not at gale force speeds) for some areas in the Pacific close to the shore.

At the end of most AFDs after the watches and warnings if any is the social media and contact information for that particular NWS office. The AFD also usually is released with the corresponding hourly forecast that the NWS puts out on its website. The AFD can be a useful tool to help plan for different events such as a weekend excursion to the beach or whether you should prepare by getting food and water for a possible severe weather outbreak.

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©2019 Meteorologist JP Kalb

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Weather Phenomena: What is Kelvin-Helmholtz Instability? (Credit: MET Office)

4/25/2019

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Looking up in the sky and imagining shapes or animals with cloud formations has been with us ever since we could walk. Some of these objects actually hold fluid dynamic science behind their shape. Ever look up and notice a strange cloud that looks very similar to a wave that could be seen on a lake or the ocean? This cloud formation is actually a rare type of cloud that holds some of the more scientifically advanced atmospheric dynamics within it.

Named after William Thomson Kelvin and Hermann von Helmholtz, Kelvin-Helmholtz instability can occur when there is velocity shear in a continuous fluid. This instability can occur when there is a velocity difference across the interface between two fluids. To think about this in a visible sense, think about wind blowing over water. The instability that the wind creates manifests in waves that are produced on the surface of the water. Helmholtz studied the dynamics of two fluids of different densities when a small disturbance, like a wave, was introduced at the boundary of the connecting fluids. Helmholtz noticed how the formation of these wave-like structures were almost always because of different velocities interacting with one another. This can happen with any fluid, most notably, with clouds. When these Kelvin-Helmholtz waves are spotted they tend to look like ocean waves traversing across the sky.

To explain how Kelvin-Helmholtz clouds form one will have to think about shear. When two different layers of air in the atmosphere are moving at different speeds, this is called shear. When the upper layer of air is moving at a higher speed than the low-level air, the air tends to scoop the top of an existing cloud layer into wave-like rolling clouds. Due to the amount of shear and difference between air levels, windy days are often the best days to spot these clouds, and can also indicate aircraft turbulence due to the shear associated with the formation of these clouds.

If one of these clouds ever traverses across the sky above, just know that atmospheric and fluid dynamics is at work with ample amount of shear to create such a cool looking cloud.

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©2019 Weather Forecaster Alec Kownacki

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The Difference Between Astronomical and Meteorological Seasons

4/24/2019

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As the seasons change, you may often hear of the terms “astronomical Spring,” or “meteorological Spring” being used around this time of year. You may then be wondering what the difference between the two are, and which is the correct term to use? The answer is that they’re both correct, but when discussing the different seasons, it’s important to note which definition you’re using and what side of the equator you’re on. Simply, the astronomical seasons are defined by the position of the Earth compared to the sun, and the meteorological seasons are based on the annual cycle of temperatures.
The reason why we have seasons is because the Earth is tilted approximately 23.4 degrees on its axis. As the Earth orbits around the sun, different parts of the Earth receive varying amounts of solar energy at a time. This natural rotation of the Earth around the sun along with its tilt sets the foundation for the astronomical calendar in which we group the seasons into solstices and equinoxes. As you can see in the visual representation of Earth’s seasons below, the tilt of the Earth and alignment of the sun play a key role in defining our seasons.

Image: https://www.oxfordlearnersdictionaries.com/definition/american_english/equinox

The seasons are defined by most cultures as Spring, Summer, Fall (Autumn), and Winter. The astronomical definition uses the dates of solstices and equinoxes to define the start and end of the seasons. Spring and Fall (Autumn) begin on the Spring (vernal) and fall equinoxes, and Summer and Winter begin on the Summer and Winter solstices.
The Summer and Winter Solstices mark the longest and shortest days of the year where the Earth is tilted the most towards the Sun during the Summer solstice, and furthest away from the sun during the Winter Solstice. In the Northern hemisphere the summer solstice occurs around June 21st, the winter solstice around December 22nd, the Spring (Vernal) equinox around March 21st, and the Fall (Autumnal) equinox around September 22nd. In the Southern hemisphere the seasons are reversed but begin on the same dates as shown in the image above. However, the timings of the equinoxes and solstices vary each year, which changes the length of the astronomical seasons.
The Earth technically makes a complete orbit around the sun in 365.24 days, so every four years an extra day is needed to balance this out. The Leap Year was then created to accommodate the extra time it takes Earth to fully orbit around the sun. This in turn makes comparing climatological data for each season difficult from one year to the next. Therefore, the meteorological seasons were created.
Meteorological seasons are separated by Meteorologists and Climatologists into four groups of three months based on the annual temperature cycle. It’s based on the observations that summer is the warmest time of the year, winter is the coldest time of the year, and Spring and Fall are the transition seasons. With that being said, the seasons begin on the first day of each month. In the Northern Hemisphere:
Meteorological Spring starts March 1st and ends May 31st
Meteorological Summer starts June 1st and ends August 31st
Meteorological Fall (Autumn) starts September 1st and ends November 30th
Meteorological Winter starts December 1st and ends February 28th (29th during a leap year)
Below is a full chart showing how the seasons are defined by hemisphere, and which definition is being used:

This is why you often hear your local meteorologist inform you about the first day of the meteorological season at the start of each month that contains an astronomical season.
Defining seasons by the meteorological calendar helps long term forecasting and climatic data easier to calculate seasonal statistics in order to better understand and convey average temperatures to the public. This calendar is also useful for other important purposes such as agriculture and commerce.
Different countries around the world define seasons using multiple definitions and factors as well. For example, Australia and New Zealand rely on the meteorological definition to define their seasons, Ireland uses an ancient Celtic calendar, some cultures in South Asia have six seasons, and in Finland and Sweden seasons are based solely on temperatures above and below freezing, so their seasons begin on different days each year depending on their climate.
In summary, the reason for having two (or multiple) ways to express the start and end of the seasons are because of their different start times and durations and based on the climate and culture of an area.

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@ 2019 Weather Forecaster Christine Gregory

Sources:
https://www.ncei.noaa.gov/news/meteorological-versus-astronomical-seasons
https://www.timeanddate.com/calendar/aboutseasons.html

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Fascinating Meteorology Terms: Doldrums and Horse Latitudes

4/24/2019

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(Photo by Sam Greenfield of Volvo Ocean Race)

The doldrums and the horse latitudes are two interesting terms used by meteorologists and sailors to describe two distinct areas over the ocean. Although they share the calm wind characteristic, they are located on two different degrees of latitude and display different characteristics of climate and weather.
   Calm winds, warm temperatures and very convective thunderstorms are used to describe the doldrums. Located at zero degrees latitude, also known as the equator, you will find this described area over the ocean. Here, there is little change in temperature and pressure in the horizontal direction but vertically is a different story. The warm air at the surface rises and condenses into large towering cumulonimbus clouds that then turn into highly convective thunderstorms. This is an area where the northeast trade winds in the northern hemisphere and the southeast trade winds in the southern hemisphere converge. This converging air at the surface collides and then lifts all the warm, moist air on the surface. That air then condenses and forms a line of strong convection and energy called the Inter Tropical convergence zone. There is so much energy and moisture here despite its relative calmness. Boats travelling in this area may experience bouts of calm weather and then quite the opposite when they are met with loud thunder and lightning.
Moving on up the ladder to an area of even calmer settings; located at 30 degrees north, you will find an area over the ocean that is famously known as the horse latitudes. This area, like that of the doldrums, is characterized by calm wind and small changes in horizontal temperature and pressure. The difference between this area and the doldrums, is that the westerlies and northeast trade winds diverge. This means the winds flow away from this area rather than converge and create thunderstorms. Storms are very scarce here as compared to the doldrums. It’s very calm. For example, if you were sailing to the new world back in the 1400s, you would find that your boat would come to a complete halt. With no winds to push the sails, sailors would be stranded for a long time in this area as they calmly drifted. What made this area take on its famous name was while sailors were stranded, they would run low on supplies and end up having to eat their horses or throw them overboard to make the ship lighter.
The doldrums and horse latitudes are a interesting couple of places to find yourself sailing overseas. Thankfully, today we have advanced in our shipping and travel. Many boats have engines that can tug them along in the horse latitudes and a lot of coverage from heavy rain and storms in the doldrums. Regardless of advances in shipping, these areas are significant when travelling overseas; an experience sailors will never forget.

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© 2019 Meteorologist Alex Maynard

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Fascinating Meteorology Terms: Katabatic Wind

4/20/2019

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(Diagram of Katabatic wind direction from a mountain plateau. Photo from Meteorology Today By C. Ahrens.)

Katabatic wind is a dense cold wind that sinks down from high mountain plateaus to valleys below. It is described by Meteorologists as heavy cold air that practically mimics the movement of mountain water as it flows downslope and waterfalls over steep canyons. Depending on the temperature of the air it displaces during its descent, katabatic wind can become violent, reaching speeds up to 100 mph.
Up on a high plateau of a mountain range, snow tends to be prominent during the winter months and into the spring. This snowpack keeps the air above it cold and dry. Air in the valley tends to be much warmer than the air on the plateau. To understand what makes this air start to move downslope, it is important to know the laws of temperature and pressure. Cold dense air is associated with higher pressure and warmer, less dense air is associated with lower pressure. Air tends to rise when it is warm and sink when it’s cool. Areas of higher pressure will move toward areas of lower pressure to try and replace the air that is lifted. The difference between areas of high pressure and low pressure is called a pressure gradient. The quicker the pressure changes, the stronger the pressure gradient. Therefore, the faster the air from the area of high pressure will move towards the area of low pressure.
The cold, dry air above the surface of the mountain plateau forms a small area of high pressure. As the air in the valley gets warmer, it starts to rise. This causes that high pressure over the snow to move toward that warmer, lifting air. This cold, dense air will eventually move off the plateau. With gravity taking hold, it will start falling downslope, picking up speed as it falls into warmer and warmer air. There is nothing to hold it back as it slips down slopes and plummets over cliffs. Reaching speeds up to 100 mph, this cold air becomes a violent wind that can topple trees and damage crops. Although it can reach such speeds, katabatic wind isn’t always as violent of a wind. It can rage anywhere between 10 mph to 100 mph depending the pressure gradient from the top of the plateau to the bottom of the valley. The stronger the pressure gradient the quicker the air will sink and pick up speed.
Katabatic wind is observed in many parts of the world with high mountain ranges and plateaus. You are most likely to experience katabatic wind in places northeast of the Adriatic sea that reside at the base of the Carpathian mountains, the Rhone Valley that sits at the base of the western mountain range in France, in Columbia at the base of the Cascade mountains, at Yosemite National Park in California and in places like Greenland and Antarctica. All these places where katabatic wind is observed, have other interesting names for the phenomenon. Areas south of the Carpathian mountains call it “Bora wind”, in France they call it “Mistral wind” and in Columbia they call it “Columbia Gorge wind”. Some areas see katabatic wind much stronger than others depending on the climate of the valley versus the elevational climates of the plateau it sits at the foot of.
Although it may become a violent wind, in most cases some natives of areas that see katabatic wind describe their experience as a cold wind sweeping through that forces them to put on a jacket. In another instance, trees toppled over at a campground in Yosemite National Park injuring a park employee that was sleeping in a tent. Wind can be both enjoyable and dangerous depending on the circumstances. It is certainly interesting to learn that there are many different types of wind that have fascinating terminology associated with them.

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© 2019 Meteorologist Alexandria Maynard

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What is a Pneumonia Front?

4/14/2019

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Figure: Graphical explanation of land and lake breezes. (Courtesy of Vaughn Weather)

The Great Lakes in the Midwestern United States go through many phases based on the various season that are experienced. In this part of the country, the calendar tells what season it is, but precipitable phenomenon (i.e. snow) can definitely occur outside of winter months. We will steer our focus on the effects the lakes have on surrounding areas in the early spring. After the lakes have spent the duration of the winter experiencing cold air temperatures and partial freezing, and we progress into spring where air temperatures warm up, the lakes are bound to have an effect on the surrounding land.

The topic of discussion is somewhat of a slang term that came out of the Milwaukee National Weather Service office in the 1960s. The term Pneumonia Front refers to a sharp drop of air temperature on land of 16 degrees Fahrenheit in the period of an hour in the form of a cold front. Often times, the drop of temperature is generalized to 20-30 degree changes over the course of a few hours. This cold front moves inland in the form of a lake breeze and modifies the air around it, thus creating much cooler temperatures inland.

A recent example occurred on March 24, 2017 where a day of strong warm air advection with high winds from the southwest led to temperatures rising to a very warm high temperature for March of 81 degrees. The average high temperature for the O’Hare International Airport station on this date is 50 degrees, according to wunderground.com. Right around noon, local time in Chicago, the winds turned to northeast and temperatures began to drop. Due to the significantly cold lake paired with the northeast orientation of the winds flowing over the lake, the effects of significant cooling were felt inland. Below is the 7-hour period of data that shows a quick temperature drop which drastically changed conditions in a short period.

Figure: Historical Data from wunderground.com of observations on March 24, 2017 at O’Hare International Airport in Chicago, IL showing changes over a 7-hour period.

Lake breezes are not uncommon, but definitely strong during the springtime period. Lake breezes will form due to significant temperature differentials between the land and lake, where air can be easily modified and brought inland. Lake breezes can form and noticeably cool an environment inland in the early spring, but progressing closer towards summer, both the air temperature and the lake will warm up, and the effect of a lake breeze will not be as significant. As summer begins and warmer temperatures are more frequent, a lake breeze can bring decent relief in temperatures during afternoon and evening hours, but is likely not to change an environmental temperature by 30 degrees.

The effect of air modification due to a breeze from a body of water are also possible in other circumstances. For instance, Florida feels sea breeze that can modify weather conditions that usually lead to thunderstorms because of the great convergence over land due to two surrounding bodies of water being the Atlantic Ocean and Gulf of Mexico. Any sizable body of water near land can modify air around it. There have even been reports of breezes from rivers creating temperature fluctuations!

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© 2019 Meteorologist Jason Maska

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Understanding the Difference between Wind Shear Types. (Image Credit: NCAR/UCAR)

4/1/2019

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DISCUSSION: As many parts of North America and Europe begin to prepare and brace in earnest for the onset of the peak severe weather season during a typical calendar year, there a number of premiere issues which are worth understanding. That is, so everyone can have a better idea of how the atmosphere works and why various atmospheric phenomena evolve the way they do. One such example is understanding the different types of wind shear which can occur and their respective relevance.

To get started, wind shear is a naturally occurring aspect of the atmosphere which affects both atmospheric phenomena such as various types of convection as well as all aviation-related concerns. More specifically, wind shear comes in two primary forms which includes speed shear and directional shear. Speed shear is best characterized as any situation in which the wind shear changes in value from two given elevation points. Thus, when speed shear is present, this can often lead to situations in which there are convective storms which do not last for an overly long period as a result of core updrafts being tilted too much. Therefore, with too much speed shear, various thunderstorm forecasts can often end up being a bust in terms of forecast verification.

The other primary type of wind shear (as noted above) is known as directional shear. Directional shear is best characterized as any situation in which the direction of winds at two given points of elevation are different from one another. Often, when there happens to be rather large amounts of directional shear, there are frequent impacts to aviation safety in terms of crosswinds at any point during any flight (regardless of aircraft size) as well as to convective storm development. Impacts to convective storm development are quite often realized by way of there being a greater potential for the development of convective storms with rotating updrafts. Therefore, with greater amounts of directional shear, there can often be a greater threat for supercell thunderstorm assuming all other convective environmental conditions are conducive for supercell thunderstorm development.

Thus, this goes to show that whenever there is a substantial presence of speed and/or directional wind shear in the atmosphere, there a host of different considerations which always need to be made when it comes to both aviation and forecasting interests.

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© 2019 Meteorologist Jordan Rabinowitz

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