Have you ever experienced a period of grey, wet weather that seems to drag on forever? While this can happen for a number of reasons, there is one phenomenon in particular that causes these weather conditions in mountainous areas. Residents of the southeastern US know this locally as “The Wedge”. The Wedge most recently affected areas of Northeast Georgia, near Atlanta and eastward towards South Carolina. It brought days of chilly, dreary, misty conditions. But what exactly causes the Wedge?
The Wedge is more technically known as a Cold Air Damming event. Cold Air Damming, or CAD, happens when cold air gets trapped by mountain ranges. Low-temperature air is denser than air of a higher temperature, which means that cold air is effectively heavier, and is thus unable to rise up over the mountains and dissipate – so it gets stuck there for a few days. However, this layer of cold air is very shallow, confined to the part of the atmosphere that is closest to the surface. Above this layer of cold and misty air, conditions are much warmer.
The arrows on the diagram above depict the direction of the winds coming towards the eastern side of the mountain, and the blue color depicts the cold temperature of the air that gets stuck near the surface.
Because the colder air is very dense and confined to the surface, warmer air can get pushed above the cold air, as shown by the red arrow in the diagram. Rising air, and the energy associated with it, is what causes thunderstorms. As warm air rises and cools, water vapor in the air condenses into clouds. When this warm air rises quickly, all of that energy of motion causes a thunderstorm to develop. In the case of CAD events, warm air is able to rise quickly because it is being pushed against the heavier cold air and forced rapidly upwards.
This is what was seen by residents of northeast Georgia in their most recent CAD event. There were many localized thunderstorms in the area caused by this phenomenon. These thunderstorms were brief, but brought torrential downpours and loud cracking thunder with them. The strength of these thunderstorms is a testament to just how strongly and quickly warm air is forced above the colder air when the Wedge/CAD is present.
While Cold Air Damming can bring about some dreary, dismal conditions, it’s also a pretty interesting weather event. Next time you experience a few days of cold and mist, check your local weather maps for an area of cold temperatures near a local mountain range. You just might be experiencing Cold Air Damming!
©2018 Meteorologist Margaret Orr
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If you are living or spending some time east of the Rocky Mountains, you may experience a certain type of weather that may raise your eyebrows as much as it raises the temperatures. Occurring through the winter months, a warm, dry air mass may descend from the Rockies and blow through your area. Temperatures within this air mass are much warmer than the cold air they displace. Temperatures can rise by 50 degrees Fahrenheit or more from the previous air temperature. This could make the weather change quickly, turning the day from winter in the morning to spring in the afternoon. This type of weather phenomena is called Chinook wind.
Have you ever noticed that cities often feel noticeably hotter than rural places? That is no coincidence! This is a phenomenon called the Urban Heat Island Effect. Although it typically only raises the temperature in the city by a few degrees, those few degrees can have long-lasting and significant impacts on the health of the urban population and the surrounding environment. Such impacts include health hazards, increased energy consumption, and even decreased air quality.
In the summer, when the Urban Heat Island Effect is most prevalent in cities, people tend to turn their air conditioning on maximum power, which leads to increased energy consumption. Consequently, more fossil fuels are used to cool homes, which subtracts from air quality. Finally, a bubble called a temperature inversion forms over the city, trapping smog inside. These effects compound with each other and create a vicious cycle that makes the urban environment harsher.
Since we know that the Urban Heat Island Effect has multiple negative impacts, we should reflect on what causes it in order to explain how it can be dealt with. The first major factor that causes the Urban Heat Island Effect is heat that comes out of our machines and cars. This cause is straightforward: the heat byproduct from our creations leak out into the environment. The second, more obscure major factor that causes the heating is the replacement of natural surfaces with dark and impermeable surfaces. Vegetation has a natural cooling effect because it provides shade and allows for evaporation, so when vegetation gets torn down the temperature tends to rise. Additionally, the dark, manmade surfaces that we use to replace vegetation absorb more heat and don’t allow water to percolate down into the ground, instead being redirected to storm drains. Without the water on the surface, there is no evaporative cooling, which makes cities even hotter.
What can we do to alleviate the heat? One common solution that is being explored is to add more vegetation to the urban environment, sometimes even on a roof or terrace. This way, the evaporative cooling from plants can be used to cool down the surrounding environment. Another more zealous solution is to create permeable and reflective parking lots and sidewalks to have them absorb less heat and, therefore, have more water for evaporation to occur. Of course, these solutions are up to city planners to create and implement, so it may seem hard to have an individual impact. However, there are small ways to help that anyone can do. Things such as using less energy, carpooling, and planting vegetation can help decrease the severity of the heat. It’s important to keep in mind that what we change about the environment will come back to us in the end such as in the case of the Urban Heat Island Effect, so thinking about how we change our world will always be essential to our prosperity.
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© 2019 Weather Forecaster Cole Bristow
Image Source: Meteorologist Gerardo Diaz Jr. Flooding in South-Central Kentucky in February 2019, during a weak El Nino season.
The atmosphere is a dynamic fluid that encompasses the entire planet. As such, it should come to no surprise that any small changes to one section of the fluid can result in exponentially greater changes that can ripple across the entire system. As such, these changes to the system tend to be referred to as atmospheric teleconnections, and can be found all over the world. Scientists have studied these phenomena for decades and while we may not completely understand them, many have been studied to the point where they are routinely used for long-range forecasting. Indeed, they are an integral our understanding of the atmosphere and this article hopes to summarize some of the most commonly-studied teleconnections in the world.
Image Courtesy: NOAA. Normal conditions versus El Nino conditions over the Pacific Basin.
The Madden-Julian Oscillation, or MJO, works in a similar fashion to ENSO but takes its form over the Indian Ocean. However, unlike ENSO, the MJO works at a much more localized scale and a much faster rate than ENSO. In other words, unlike ENSO which occurs over the period of an entire season and requires large-scale alterations to atmospheric wind motions on the order of thousands of miles, the MJO generates a moisture transfer over the entire Indian Ocean, and eventually the entire planet, from west-to-east on the order of every 30 to 60 days.
Its lifespan can be categorized into eight distinct phases. According to the UK MET Office, Phases 1 through 3 revolve around the formation of storms over the far Western Indian Ocean which gradually converge and become one large system. During the middle phases (4-6), the fully-developed MJO makes its way into the eastern Indian Ocean and brings moisture into Oceania and East Asia. And finally, by phases 7 and 8, the cluster of storms gradually weakens and subsides as it enters the Western Pacific. Indeed, while the MJO and ENSO run at different temporal and spatial scales, the MJO’s suppression and introduction of moisture onto opposite ends of the Indian Ocean basin act as a strong analog to ENSO over the Pacific Ocean.
Image Courtesy: MET Office UK. The MJO and its movement across the Indian Ocean and Western Pacific.
While the previously mentioned teleconnections coincide with responses to oceanic conditions as much as atmospheric ones, the North-Atlantic Oscillation, or NAO, is much more atmospheric in its nature in the sense that it is directly related to the formation and suppression or even temporary disappearance of a large-scale atmospheric high pressure system that is commonly referred to as the Azores High. During a positive NAO, this High, which is nestled over the North Atlantic and tends to be centered near or directly over Iceland, is at its strongest intensity. As a result, westerly winds that already normally blow into Europe strengthen significantly, resulting in colder air masses being advected into Europe during the summertime while any hurricanes that make their way across the Atlantic and towards the US Mainland are steered closer to the East and Gulf Coasts of a noticeably warmer-than-average North America. Likewise, a negative NAO results in a much weaker Azores High and causes the exact opposite conditions.
Image Courtesy: NASA. The two phases of the North Atlantic Oscillation
And similarly to the NAO, the Arctic Oscillation, or AO, is yet another teleconnection with stronger atmospheric response conditions than oceanic ones that also comes in two phases. Nevertheless, it revolves entirely around the upper-level wind conditions over the Arctic Ocean and is responsible for the ability of cold, artic air to move into the mid-latitudes. Under a positive phase, the Arctic experiences periods of strong low pressure systems while the middle latitudes tend to experience the movement of high pressure systems across the planet, which in turn allows for a more zonal mid-latitude jet stream. When this phase occurs during the wintertime, the end-result tends to be much warmer winters in the mid-latitudes. This is a sharp contrast to negative AO conditions, in which strong high pressure systems dominate the arctic, resulting in the displacement of cold, arctic air into the mid-latitudes due to a much more zonal mid-latitude jet stream.
Image Courtesy: NOAA. The two phases of the North Atlantic Oscillation
Indeed, these are just some of the few teleconnections that can be found all around the world. And while our understanding about these phenomena has improved greatly in recent decades, there are still a plethora of atmospheric teleconnections that are just now being researched, including the Southern Atlantic Oscillation to name just one. And as we continue to observe and analyze their unique impacts on the atmospheric system as a whole, we can only expect that our long-range forecasts to continue to improve with time.
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© 2019 Meteorologist Gerardo Diaz Jr.
DISCUSSION: Winter is in full swing in the Northern Hemisphere, and while much of the United States mainland is experiencing cold temperatures, or winter storms which encompass snow, there are warm locations within the U.S. that are experiencing winter phenomena not known to its mainland counterparts. Hawaii, the 50th state, experiences 11 of the world’s 13 climate zones, often experiencing various weather on several parts of one island even within several miles. Of notable recent winter conditions in Hawaii, was the widespread impacts of a Kona low.
As recent as the week of February 9th, 2019, did the state of Hawaii feel the effects of a Kona low in the area. Typically, island weather is dominated by the Northeast trade wind flow, at times bringing wet conditions to windward and mauka (mountain) locations of the island chain. A Kona low, Kona in the Hawaiian language having various interpretations, in this context could be determined as “leeward or dry side of the island,” is primarily a subtropical cyclone (counterclockwise inward flow) with a southerly wind component in this case. Often Kona lows bring unprecedented winds, large surf, heavy rains, and other hazardous conditions to the islands. During this most recent storm, surf brought nearly 30-40-foot faces, winds in excess of 45 mph and of note a 191-mph wind reading at the top of Mauna Kea, in addition to snow seen on Mauna Kea, Mauna Loa (on Hawai’i) and Haleakala on Maui, an astounding happenstance on Maui as elevations of snowfall around 6000 feet.
Kona lows while they often disturb the trade wind flow, bringing cooler temperatures, are not uncommon outside of summer, often seen between fall and spring in the Pacific. There is a potential for the low to be subtropical or tropical, often a colder core of air than a tropical storm, mass disorganizations and in general characteristics of several types of systems. While winter is not quite over yet for the islands, chances so see such storms may decrease as temperatures begin to warm waters and the atmosphere in the Pacific region, as we move into spring and summer seasons.
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© 2019 Meteorologist Jessica Olsen
Video: Honolulu, Hawai'i, Courtesy: Meteorologist Jessica Olsen
DISCUSSION: During the heart of any given Winter-time season, there can periodically be rounds of particularly cold weather which impact various parts of North America, Europe and beyond. Depending on the exact severity of any given cold air intrusion, air temperatures can sometimes either fall slightly below the freezing mark (i.e., 32 ° Fahrenheit or 0 ° Celsius) or substantially below the freezing mark to even dangerous levels. One such example of this is what unfolded across a good portion of the north-central and northeastern United States within the past 10 days as a “lobe” of a true Arctic mass associated with the highly-touted “Polar Vortex” planetary circulation feature broke off and descended into the mid-latitudes (i.e., regions which are located between 30°N and 60°N latitude which include but are not limited to a good portion of North America and Europe). When this frigid air mass made its way further into the central and eastern United States (i.e., even at a somewhat modified severity in terms of the extent of the cold air temperatures being measured), this paved the way for some neat associated Winter weather science education opportunities.
One such example of a neat winter weather science education moment was a “case and point” example of how supercooled and fully purified water in a bottle can freeze on command after being left outside during a single overnight period. To be more specific, when a bottle of undisturbed and fully purified water is left outside in very cold (and well-below freezing temperatures) for several hours or even overnight, there is often no freezing which occurs. As also explained by ABC Kansas City Meteorologist Nick Bender, freezing of water molecules within some given confined space requires other a bubble created from some given physical disturbance to set off a chain reaction and/or some given particulate to help initiate the freezing process to get underway at a molecular level.
Thus, this winter weather science education experiment example just goes to show that even during the coldest parts of the year, there can always be a great opportunity to still learn something about how physics affects the natural world. So, the next time you, your friends, and/or your family are lucky or unlucky enough (depending on how you look at it from your perspective) to experience the full-force of a true Arctic air mass intrusion, definitely be sure to bundle up in warm layers and give this simple experiment a try to see for yourself.
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© 2019 Meteorologist Jordan Rabinowitz