Almost always, clouds form in the troposphere, the lowest level of the atmosphere where we live and where all weather takes place. The troposphere easily has the most water vapor compared to any other layer of the atmosphere. In fact, the other layers have so little water vapor that it is incredibly difficult for clouds to form anywhere above the troposphere. However, on rare occasions, special mechanisms in the atmosphere can force water vapor to exist in above average quantities in the stratosphere or mesosphere.
The first of the two strange upper atmosphere clouds are called nacreous clouds, also known as mother-of-pearl clouds due to their vibrant colors. These clouds typically form in the polar regions of the stratosphere, the layer of the atmosphere just above the troposphere. Clouds are usually not found in the stratosphere because it is warmer there than at the top of the troposphere. The warmth of the stratosphere prevents air from rising past the troposphere due to convection, which is why clouds usually can’t form there. Nacreous clouds are the exception. Although the exact way they form is not completely understood, there are two major theories as to how they come about. The first method is by lingering updrafts of powerful storms. Storms have a section where they draw up moist, warm air called an updraft, and occasionally they can be so powerful that they drive water vapor into the lower stratosphere. If the temperature is cold enough in the lower stratosphere (about -85 degrees Celsius), spherical ice crystals can form and condense into nacreous clouds. The second method is through lifting from tall mountain chains. When air travels up tall mountain chains, sometimes water vapor from the lower atmosphere can find its way into the stratosphere via complex physical processes. Once again, if it is cold enough, nacreous clouds can form. Since the minimum temperature required for these clouds to form is so low, the only places one can see these clouds are near the poles. Further adding to the rarity of their sightings, the rainbow colors of nacreous clouds can only be seen when the sun is very low in the sky. Otherwise, nacreous clouds strongly resemble wispy cirrus clouds. An example of nacreous clouds at sunset is given below (photo credit: Albert de Nijs).
The second type of upper atmosphere clouds is called noctilucent clouds. These clouds form in the mesosphere, the layer right above the stratosphere. These clouds are known to be made of tiny ice particles, and are even thinner than nacreous clouds. Like nacreous clouds, they are still not completely understood. But, are plausible explanations as to how they get their water vapor. One explanation is that trapped water vapor in small meteors escapes into the mesosphere, where most meteors burn up. Another explanation is that the water vapor comes from chemical changes in methane over time. However these clouds form, one thing is certain: they are truly a sight to behold. These clouds are only seen right after the sun is over the horizon, when most of the sky is dark. The clouds glow as light from the sun strikes their ice particles from beyond the horizon. Sometimes, the clouds are so brilliant that they are mistaken for the aurora borealis. The only areas one can find these clouds are between the latitudes of about 50 to 65 degrees (north or south) during the summer when the mesosphere is coldest. The mesosphere must be quite cold for noctilucent clouds because they only form below -120 degrees Celsius. An example of one of these clouds is included in the thumbnail image of the article, while a diagram of atmospheric layers is included at the bottom (credit: NIWA).
These bizarre, scarce clouds are beautiful atmospheric phenomena that represent the complexity of the atmosphere. Complicated interactions in the upper atmosphere happen often, but it is rare for the average person to see such a clear product of them. Upper atmosphere clouds are truly fascinating formations that add to the allure of the sky.
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© 2019 Weather Forecaster Cole Bristow
Everyone is familiar with cloud-to-ground lightning which occurs during severe thunderstorms and cloud-to-cloud lightning which also is produced during these storms. In rare form, lightning can actually occur high above thunderstorms and give off a rather strange color and shape. These large-scale electrical discharges are called Sprites.
Sprites usually occur 50-90 km (31-56 mi) above cumulonimbus clouds. These cumulonimbus clouds, or simply just thunderstorm clouds, create enough electrical discharge for sprites to form above them. Sprites are usually triggered by the discharge of positive lightning between the underlying cumulonimbus and the ground. As can be seen in the image above, sprites are luminous reddish-orange flashes. They form within clusters above the troposphere, which is what gives them the jumbled or clustered appearance. Cloud-to-ground lightning along with our typical thunderstorms occur in the tropospheric region of the atmosphere. Sprites form within the stratospheric to mesospheric region of the atmosphere and this causes them to have different characteristics compared to lightning. Sprites are technically cold plasma phenomena that lack the hot channel temperatures found in tropospheric lightning. So, these sprites are more akin to fluorescent tube discharges than to lightning discharges. In a fluorescent tube, an electrical current excites a gas present and produces short-wave ultraviolet light which then causes, in the sprites case, light to be produced in the surrounding atmospheric area. As can be seen in the image above and other sprite images, the strands and strokes of sprites are caused by the electrical current. So, the glow of the sprite follows the current from the beginning to the end of its lifespan.
The image above helps visualize the height to which these sprites occur and the wide span of area that they take up. In this example, the sprite is spanning from 40 km to 80 km in the atmosphere. Surely, these electrical phenomena are not measly occurrences in terms of size. In terms of length of time, they are. Through optical imaging, a 10,000 frame-per-second high speed camera conveys that sprites are actually clusters of small, decameter-sized (10-100 m or 33-328 ft) balls of ionization. These are launched high into the atmosphere of about 80 km (50 mi) or so and then move downward at speeds of up to ten percent of the speed of light, which then are followed by separate sets of upward moving balls of ionization milliseconds later. Overall, an entire sprite display only lasts about the same duration of a cloud-to-ground lightning strike which is on average 30 microseconds.
Truly, to see an event like this one will need near-perfect atmospheric conditions along with a watchful eye.
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©2019 Weather Forecaster Alec Kownacki
For those having lived on the United States’ west coast from the era of about 2011-2017, especially those in California, drought was persistent, dangerous, and unprecedented in length and severity. As little came in the way of precipitation, especially the snow that fills the Sierra Nevadas and provides water for millions of Californians, reservoirs sank to record lows in capacity. Little water was available for anything more than necessity. Advisories were sent out to citizens advising and urging water saving strategies such as only watering lawns three times a week and only at night, replacing lawns with drought-tolerant landscaping, not serving water at restaurants unless upon request, installing water-efficient taps, showerheads, and more, and even cutting showers to no more than two minutes in duration. For California agriculture, the drought was arduous with smaller yields, and many farmers were forced to dig wells and tap into groundwater. Though, with all these results and effects, one may wonder: What was the precursor, the cause of the drought? The answer lies in something that climate scientist Dr. Daniel Swain coined: “The Ridiculously Resilient Ridge.”
The Ridiculously Resilient Ridge, or the RRR for short, is an atmospheric blocking phenomena appropriately named for its tenacity and unrelenting presence within the atmosphere off of California’s coast. A ridge is an area within the atmosphere in which there is unusually high pressure. In the case of the RRR, that pressure was ridiculously high. High enough to last for several years and cause the horrific drought conditions that Californians know all too well. But, how exactly does an area of high pressure bring about drought?
The key to answering this question is that areas of high pressure may act as a sort of physical wall to other systems that would otherwise try to move them and push them about. The stronger the wall is, or the higher the pressure is, the harder it is going to be to knock down that wall. As the ridge sat off the California coast, the jet stream and low pressure systems were forced up and above it, that of which brings necessary precipitation, leaving California with little to nothing in the way of water. Instead, California was met with nearly five years of dry, hot weather as the RRR prevailed and refused to subside. As each year went by with little to no precipitation, the effects of drought only compounded.
Come winter 2017, California had officially exited the drought after the RRR had finally weakened. With its downfall, atmospheric rivers of great strength were permitted to flow into California, These atmospheric rivers then brought some of the rainiest seasons on record after just having exited the most intense drought on record. Although California currently enjoys drought free years as a result of these intense atmospheric rivers, the RRR could very well rear its head again as global temperatures continue to warm. In California’s future, there certainly lies drought and lack of water. It simply is a matter of when conditions are right to birth Dr. Daniel Swain’s infamous Ridiculously Resilient Ridge.
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© 2019 Weather Forecaster Alexis Clouser
With the summer months now in full swing, “pop-up” rain showers are beginning to occur with increased regularity. In many portions of the US, these storms can appear almost daily and have strong impacts on outdoor summer festivities. If you open any weather app or tune into any weather station to see the upcoming forecast, you are likely to see the terms “isolated” and “scattered.” Meteorologists often use words such as these to describe storm coverage on any given day. But what do these terms really mean? Let’s explore.
As defined by The National Weather Service (NWS) the term “isolated” is used to describe less than a 10% chance of measurable precipitation in any given location within a forecast area. A forecast area (or zone) is simply the geographic region for which a particular weather forecast is issued. When isolated storms are forecasted, only a small number develop and at a distance from other storms. Therefore, the exact location of an isolated thunderstorm is incredibly difficult to predict. Isolated storms can be intense in nature, often associated with downpours of heavy rain and in some cases even lightning. While they can certainly put a damper on outdoor activities, most isolated storms are short in duration. If you do get caught in a storm, take shelter and wait it out. Chances are the storm won’t be long before you can resume your plans as scheduled.
On the other hand, “scattered” thunderstorms are more widespread in coverage. While varied definitions of the term exist, The NWS defines scattered as “area coverage of convective weather affecting 30 percent to 50 percent of a forecast zone.” In contrast to isolated storms, scattered storms often come one after another or in a multitude of “rounds”. In other words, one specific location may see “on and off” periods of rain and sunshine over the course of an entire day. While it is still difficult to plan for such weather, outdoor plans are more likely to be affected and may even need to be rescheduled if scattered storms persist.
It is important to note that both of these terms describe the coverage of thunderstorms over a given area. However, they say nothing about the intensity of the storms present. Both scattered and isolated storms can become strong or even severe at any given time. While many severe weather days can be forecasted up to days in advance, the spontaneity of these storms make them even more dangerous. In any situation, make sure to follow the old saying “when thunder roars, stay indoors” and take shelter immediately if severe storms roll your way.
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©2019 Weather Forecaster Dennis Weaver
A beach in Santa Monica shrouded in fog and low-lying clouds.
While visiting Santa Monica, CA, this past weekend I experienced rather chilly conditions along the coast; winds gradually picked up throughout the day and temperatures fell rather quickly despite temperatures further inland remaining steady. The locals refer to this feature as the “June Gloom”. It heavily reminded me of lake breezes back in Northern Illinois, which themselves behave in a very similar fashion to their sea counterparts. Moreover, these features are often associated with phenomena that include everything from lake-enhanced clouds to pneumonia fronts. In essence, these breezes derive from the same mesoscale feature which we will be exploring shortly.
Sea/Lake Breezes at their core are essentially the result of differences in surface temperatures between land and a given body of water; because water has a significantly higher heat capacity level than land, meaning that more heat is required to warm it, a sharp temperature gradient develops between their boundaries during the daytime. As air over land warms at a faster rate than the nearby body of water, it rises and cools, leaving behind a void over that is immediately filled in by cooler air over water that rushes in to make up for the change in pressure. This effect is essentially what defines any sea/lake breeze and results in mesoscale frontal boundaries that can travel all along a coastline and even inland under the right conditions.
Source: EA School Today
These features occur very frequently and at considerable intensities over places such as Florida, in which sea breeze-induced fronts tend to allow for enough lift to support convection, especially in the summertime. When warm, moist air rises ahead of cold air, it condenses and allows for the formation of short-lasting but fairly potent air mass thunderstorms. These storms can bring sudden downpours and intense lightning over concentrated areas along the frontal boundary and are a common concern for those who venture outside in the summer months.
Source: The Palm Beach Post
Other examples include the pneumonia front, a colloquial term for when a strong mesoscale cold-front moves ashore from Lake Michigan. As the cold, lake air makes its way inland, the air behind the front becomes stable and cools down significantly, resulting in dramatic drops in surface temperatures across Chicagoland.
Source: The Chicago Tribune
Likewise, the opposite effect can occur at night; because of water’s higher heat capacity when compared to that found over land, it tends to cool at a slower rate as well. This results in cold air rushing in from the cooler land to the void left behind over warmer waters as warm air rises. This leads to mesoscale warm fronts that then move over open water.
Source: EA School Today
Indeed, whether it’s the June Gloom of Southern California, the Pneumonia Fronts of Northern Illinois, or even summertime convection in Florida, these features all play a critical role in the mesoscale dynamics of the areas that they affect and are part of everyday life for those who live at their cross-hairs.
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©2019 Meteorologist Gerardo Diaz Jr.
The term Urban Heat Island gets its name from industrial and urban development that causes the air temperature within the area to be much warmer than its surrounding rural areas. Unlike surrounding rural areas, an urban area lacks vegetation and moist soil. During the summer, the heat radiating off of buildings can be felt more intensely due to a more direct angle from the sun.
In a large city or urban area that is largely congested with buildings and man made materials, you may find yourself really feeling the heat when it starts to warm up for the summer. Warm asphalt roads, tall steel skyscrapers, and cement buildings absorb and hold onto the incoming energy (shortwave radiation) from the sun. They then heat up to a point where they radiate their own heat (longwave radiation) throughout the day and overnight. With tall buildings blocking most of the air passing through the city, this heat gets trapped, unable to filter out as efficiently as it would with less buildings. This congestion of buildings is not the only thing adding to the heat. Running car engines and exhaust pipes give off heat as well as air conditioners, refrigerators and factory machines. Combined, these continue to contribute to the heat surrounding the urban area. Eventually, as the day persists, the pocket of air within the urban area grows significantly warmer than the air outside of that urban development.
A very large difference between an urban heat island and a rural area is the lack of vegetation and moist soil that covers the surface. For an urban heat island, it is mostly asphalt and roads as opposed to soil, grass and vegetation. The soil and vegetation play a vital role in keeping a rural area cool. One of the main contributions to this is the extra moisture the plants and soil release or have stored. Incoming solar radiation is used as energy to evaporate the water or moisture that is in the soil or was absorbed by the vegetation. During evaporation, latent heat, which is the energy used to transform water from a liquid to a vapor, is absorbed. The absorption of latent heat decreases the air temperature surrounding the water vapor. Therefore, leading to a cooler atmosphere.
As we move into summer, it's important to think about how living within a congested city can add to the temperature you are feeling outside. It could help a bit to escape to a rural area and cool off without the urban heat adding to the daily temperature.
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© 2019 Meteorologist Alex Maynard
Fata Morgana is a term given to a type of mirage that occurs over water. This type of mirage is famous for its ability to make ships and islands appear to be floating in thin air. The origin of the term stems back to Italy over the Strait of Messina. It was claimed that castles or buildings would appear out of thin air over the strait and then vanish within seconds. Fata Morgana is Italian for Fairy Morgan whom was a mythical being in the fairy tale of King Arthur. In her tale, she lived in a castle underwater and her mystical powers gave her the ability to build castles out of thin air and destroy them just as quickly. This is exactly how the Fata Morgana mirage is observed and how it came to get its name.
Mirages are defined as an object that appears to be displaced from its original position. Experiencing a mirage feels as if your eyes are playing tricks on you, when in fact, it is the atmosphere that is tricking you. Unlike hallucinations, mirages can be depicted in pictures. This provides proof for people experiencing the phenomenon that the objects being viewed do indeed exist. Many mirages are created by light passing through layers of air that have different densities. Air density is determined by air temperature. Colder air is associated with a higher density as warmer air is associated with lower density. When light hits an area of higher or lower density, it bends at a different angle. This is called refraction of light.
Fata Morgana is created from refraction of light through warm air settling on top of a cold surface such as water. For example, in the diagram below, light reflected off the object passes through cold air settled above that cold surface of water. As that light begins to pass through a warmer pocket of air above that cold surface, the light will start to bend as the air it passes through gets warmer. Eventually, that light would bend in such a way that it reaches the eyes of the observer at a different angle making the object appear to be floating. Now it is pretty clear why so many people claimed to see floating ships or islands over the strait of Messina in Italy. A lot of the light that reflected off ships and the land across the way was refracted into their eyes at a different angle.
Fata Morgana is called the superior mirage. A superior mirage occurs where an object appears to be above its original position. An inferior mirage, the opposite of superior mirages, occurs when the object appears to be below or upside down its original position. This happens when light is bent upward from its original path. Along with Fata Morgana being such a superior mirage, it is also a superior meteorology term with a very fascinating origin from Arthurian legend.
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© 2019 Meteorologist Alex Maynard