Most everyone has heard of or has seen some type of fog, but not many know how it forms, or much less that there are different types. In fact, there are seven different types of fog, with major differences in the way they form. For those who don’t know what fog is, it’s a thick cloud of tiny water droplets suspended in the atmosphere near the earth’s surface, restricting visibility. It can be influenced by a variety of reasons including topography, temperature, saturation levels/precipitation patterns, etc. Fog is most commonly seen during the early morning or evening hours but can also be seen overnight or during the middle of the day depending upon the atmospheric conditions at hand.
Ground fog, also known as radiation fog, is the most common type of fog that people experience. This fog is formed generally after sunset, when the Earth is beginning the cool down process and starting to emit the radiation that was absorbed during the day from the Sun. As this process begins, the air layer immediately above the surface cools rapidly and falls below the dewpoint temperature, which means the air has reached its maximum amount of water vapor that it can hold, and the water vapor is now able to be physically seen. This fog will typically dissipate after sunrise when the earth begins absorbing radiation again.
Valley fog is similar to ground fog except the duration of it is much longer. Valley fog forms in a valley as the air cools above the ground beneath it. However, what makes this last so much longer than regular ground fog is that the air gets trapped by the less dense layer above it-- Potentially extending its lifespan to last for days or even weeks.
Evaporation fog is the type typically seen over water and occurs when the cool air right above the surface is heated by the water below it, thus causing that layer of air to be warmer than the air further above it. As the layer of air warms, it begins to rise and mixes with the cooler air above it and, in turn, causes the water vapor in the air to condense to form the fog.
Upslope fog is formed when a warm moist air mass is blown up the windward side of the mountain and begins to cool as it gets higher in elevation. As it begins to cool, it will form fog as the water vapor condenses.
Advection fog is typically seen in areas that border an ocean. This type of fog forms when warm tropical air is blown over the cooler surface of the water and causes condensation by advection. This phenomenon is mostly common on the Pacific Coast of the United States due to the chilly southerly-flowing California Current.
Freezing fog, which was seen in north Mississippi in the fall/early winter of 2018, is formed when the water vapor inside the fog become supercooled due to freezing temperatures and freeze on contact with the surface below it. This kind of fog can be dangerous for reasons other than visibility because it can cause black ice on roadways.
Finally, frozen fog is formed when the air is cooled to temperatures of -40°F or below causing ice crystals to be suspended in the air.
Fog is a very interesting topic. Most people have different schema when they hear about fog. Some relate it with graveyards, some with early morning commutes to work, and even some relate it to lakes. Across the country, the weather patterns that people experience will vary and not everyone will experience every weather pattern, but everyone will experience some sort of fog in their lifetime.
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©2019 Meteorologist Ashley Lennard
DISCUSSION: In any given moment, clouds obscure the usually easy-to-spot mountain ranges in the distance. Suddenly, the clouds part and give way to sunshine. Or say for potential hikers on the trail, they are rushed with a strong flow of air that is much warmer than its surroundings and at times could pose danger simply because of the intensity at which wind the wind is blowing, especially when not known and when caught under the tree line. The phenomenon known as a downslope wind event is defined as a channel of wind that is directed down a mountain slope and . Usually driven by subsidence, or when air parcels in the atmosphere have the tendency to descend or sink towards the surface, downslope flows are responsible for the overall drying and warming of the atmosphere since this air warms adiabatically as it approaches the surface, and thus clearing of most if not all cloud cover if clouds are in the vicinity. Most of these downslope wind events are also referred to as a “föhn” or “chinook” winds and are usually tied to larger dynamical forces that are larger than the slope itself.
In recent literature, there are a couple of proposed mechanisms for the development and sustenance of impactful downslope wind events: An example of this type of event is when flow over the crest of the mountain transitions from subcritical (stable and laminar) to supercritical (turbulent and unstable). This is very similar to what is seen during a hydraulic jump, where potential energy is converted to kinetic energy as the waves ascend then descend through the crest. As an alternative hypothesis, vertically propagating waves that have a larger amplitude undergo a partial reflection especially on the leeward side of the mountain.
Forecasting these downslope wind events become a challenge and return back to understanding the three aforementioned mechanisms that could be driving their development. However, several key ingredients stand out that give forecasters clues they need. For instance, long ridges with gentle (steep) slopes on the windward (leeward) sides of the mountain can alter the effective terrain shape because of upstream blocking. Downslope wind events are also likely to occur in environments where the humidity is lower since it is hypothesized that higher moisture content dampens the gravity wave amplitude. Of course, one of the most notable points of detection would be a sudden drop in barometric pressure matched by an equally rapid rise at the conclusion of the event. It does raise the question, “what are the most critical components that make up the main cause for the high winds?”
It goes without saying that, while these windstorms are spectacular in nature and not all windstorms are of severe levels, they are still capable of producing significant damage. In particular, one case that stands out is the October 1997 blowdown event across the Mount Zirkel Wilderness Area in northwest Colorado. In total, roughly 13,000 total acres of trees were blown down while the maximum winds occurred for roughly ~30 minutes. Closer to the Foothills of Colorado, Boulder and Fort Collins have been subjected to damaging windstorms with winds exceeding 100+ mph on more than three dozen combined occurrences. This is a relatively new field of study for most researchers, but the overall benefits of understanding these windstorms will help in forecasting them in the future.
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© 2019 Meteorologist Brian Matilla
Under Pressure: How Atmospheric Pressure Impacts the Weather (Photo Credit: University of Wisconsin SSEC)
Perhaps you have heard a local meteorologist talk about a low pressure system or an area of high pressure. These are both common terms used to describe meteorological features and weather patterns. The big question: what is atmospheric pressure and why is it such an important factor in determining weather conditions? Let’s take a closer look.
In a general scientific sense, pressure is simply a way to quantify how much force (or in many cases weight) is being applied to a particular area. Think about what happens after diving into a swimming pool. As you move downward from the water’s surface toward the pool floor, the amount of water above you increases and the pressure exerted on you grows larger. This change in pressure is why oftentimes our ears will “pop” underwater. When you return upward to the surface and the mass of water above you decreases to zero, the pressure applied by the pool is much less than it was near the pool floor. As we will see, a similar principle applies in our atmosphere.
First we will consider a hypothetical situation in which a vertical column of air extends from the surface upward. If air within the column is cold, it is denser and sinks from upper-levels toward the ground. The air in this scenario is more compact near the surface, it occupies a larger volume close to the ground. Therefore the weight of this air is larger, corresponding with a high value of surface pressure. The opposite is true within a column containing relatively warm, less dense air. In this case, air is less compact just above the surface which means less weight is applied downward and pressure is fairly low. These opposite situations play a major role in both the overall weather pattern and specific meteorological conditions on any given day.
When we have an area of high pressure in lower-levels of the atmosphere, air sinks from aloft and spreads out horizontally, blowing away from the center of circulation. This situation is usually associated with sunshine, clear skies, and calm winds. During the day in the summertime, this lack of cloud cover leads to high values of solar radiation and thus warmer temperatures. At night, this radiation escapes into the atmosphere and temperatures at the surface can cool dramatically. When pressure at the surface is instead relatively low, air rises, cools, and condenses. This process results in stronger winds, less variable temperatures and the formation of clouds and precipitation.
These pressure patterns continuously change over time and in fact, a shift between high and low pressure over any geographic area can occur on the order of only a few days. However, climatological evidence has shown some common trends in system location and path. For example, you may have heard of the “Alberta clipper”, a low pressure system that tends to form just east of the Rocky Mountains in Canada’s Alberta province. While not always a certainty, historical data can help meteorologists understand where these extreme pressure areas are likely to develop and impact weather conditions!
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©2019 Weather Forecaster Dennis Weaver
Weather has a large influence on many sports- sports like golf, tennis, baseball and football to name a few. This time of year, football season is in full swing and we are closing in on the playoffs. This season we have seen a few match-ups where weather affected how the game was played. With the exception of dome/indoor arenas, weather certainly plays a role in game outcome and implementation in each season. It can pose as a big influence to players in many various ways. Weather like snow, rain, temperature, and wind can handicap different players on the field.
Wind seems to be quite a nuisance when it comes to our kickers and quarterbacks. Strong winds loves to take the ball into its hands and redirect where it ends up. For kickers, high winds make it hard for them to kick a field goal. Especially when they have to kick it at a further yard line. Wind can blow the ball out of range and cost the team possible points. Passes are hard to manage in windy conditions. We see a lot of incomplete passes from our quarterbacks in windy conditions. In order to work around the wind, the team may implement more running plays as opposed to passing plays. A good example of wind influence on games for this current season is the Giants vs. the Patriots in week six. We saw a lot of incomplete passes from Patriots quarterback Tom Brady, including one that landed in the hands of a Giants player resulting in a turn over. It was clear in the recaps that the trajectory of the ball was taken by the wind, an unplanned trajectory by Brady.
Rain can pose as a big issue to almost all players of the game. Pouring rain makes natural fields muddy and turf fields slippery. Running backs, wide receivers and tight ends will find it difficult to run, dodge tackles and block defense on a muddy/slippery field. Rain also makes the ball slippery. Quarterbacks will have a hard time gripping the ball and throwing a passing play while receivers will have a hard time catching the pass. In rainy and slippery conditions, special gloves are worn to increase grip on the ball. Defense also finds it hard to tackle in muddy/slippery conditions on the field. Slipping and falling is a handicap to strategy and the coaches may change up game plays to mitigate the issue.
Temperature can go both ways for football, especially the first half of their season being in the fall where temperature fluctuates significantly on a daily basis. In hot temperatures, defense and running backs suffer the most as they are the players that run, tackle and all around move the most in the game. It’s easy to see why it could slow these players down when making running plays. Heat makes it difficult to breathe and can overwork your body to keep cool, making the body tire quicker. Including the rest of the team, hot conditions increase risk for heat stroke and wearing all that padding makes it hotter. Cold temperatures also seem to slow down the team by making the players who don’t move as much, stiff. Quarterbacks wear special gloves that grip the ball and keep their fingers warm but it affects the feel they have for the ball and could affect their throws. Most of the team can layer up in uniform or cover up with a blanket to keep warm on the bench. Running backs don’t get affected by the cold as much as the rest of the team because they run the most. Running helps them generate heat and keep warm. Moderate temperatures are an ideal condition for football players.
Snow is another form of precipitation that makes the field slippery. It’s also cold while it occurs and if it’s snowing heavily, visibility decreases. This is a triple negative when it comes to football games. It makes it hard to throw and catch the ball, tackle, and make significant runs. In the recent Packers vs. Panthers game in week 10, we saw how snow can affect one of the most significant plays in a game. Lambeau field being an open field in Green Bay northern Wisconsin, home to the Packers, is always open to the elements. Being the home field to the packers, they are accustomed to the cold and snow. This gives them an advantage in every home game where snow falls. The Carolina Panthers have the disadvantage here not being used to playing in cold and wintry conditions. One example of snow playing a significant factor in the outcome of a game was in the second quarter of the same game. The score stood 10-7 with Panthers in the lead. Hoping to continue the score at midfield, Panthers Quarterback, Kyle Allen, took a hand off and slipped, losing control of the ball resulting in a turnover and ultimately allowing the Packers to regain lead and win the game.
It’s interesting to analyze how weather can affect each play of the game and to see it in action in recent games like the Giants vs. Patriots and Packers vs. Panthers. It’s no doubt that weather can ultimately change the outcome of a game if conditions are harsh. It’s possible as we head further into the winter season we will see more games, even playoff games, possibly be faced with cold, snowy and windy conditions.
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© 2019 Meteorologist Alex Maynard
Goss, Nick, et al. “Latest Weather Forecast for Patriots vs. Giants.” NBC Sports Boston, 10 Oct. 2019, https://www.nbcsports.com/boston/patriots/latest-weather-forecast-patriots-vs-giants-week-6-calls-sloppy-conditions.
Haby, Jeff. “WEATHER AND FOOTBALL” The Weather Prediction.com, http://www.theweatherprediction.com/habyhints2/602/.
Johnson, Bailey. “NFL: 4 Epically Awful Bad Weather Football Games.” The Weather Channel, The Weather Channel, 31 Jan. 2014, https://weather.com/sports-recreation/superbowl/news/nfl-football-worst-weather-games-20130906.
Marks, Brendan, and Charlotte Observer. “Panthers Nearly Upset the Packers in the Snow, but Carolina Only Has Itself to Blame.” Omaha.com, 11 Nov. 2019, https://www.omaha.com/sports/national/panthers-nearly-upset-the-packers-in-the-snow-but-carolina/article_1874fd12-b937-556d-b3c6-5fc2ee306f88.html.
Maske, Mark. “Analysis | With a Great Defense and an Average Offense, the Patriots Are Undefeated. Can They Make a Run at 19-0?” The Washington Post, WP Company, 11 Oct. 2019, https://www.washingtonpost.com/sports/2019/10/10/patriots-giants-thursday-night-football/.
Spratt, Scott. “Quantifying Weather's Impact on Fantasy Performance: Fantasy Football News, Rankings and Projections.” PFF, PFF, 7 May 2018, https://www.pff.com/news/fantasy-football-quantifying-weathers-impact-on-fantasy-performance.
The Thanksgiving Week bomb cyclone as seen over the Eastern Pacific. Source: COD Weather
One of the biggest weather headlines out west has been the bomb cyclone that managed to drop surface pressures at stations along the Oregon coast to as low as 975mb (source: Wunderground)! Add to it the strong winds associated with the system and the heavy snow totals expected all over the Southwest through the Thanksgiving holiday and it’s easy to see that this November storm will surely be one for the record books. So how did we get here? And what exactly is an atmospheric river? Let’s dive into the science behind this bomb cyclone!
The Thanksgiving Week bomb cyclone as it inched its way onto the Oregon/California coast during the afternoon of 11/26/2019. Source: COD Weather
While at first it might sound like this system is something out of made-for-TV movie about a radioactive hurricane that’s about to dump sharks all over Oregon and California, the reality is that the naming convention comes from the idea that a low pressure system experiences explosive cyclogenesis, or rather a sharp increase in its intensity as it grows and matures. Generally speaking, atmospheric scientists use barometric pressure as the base for describing the intensity of a low pressure system. Sea-level pressure (SLP) lies around 1013hPa, and is commonly measured in fair weather conditions, i.e. sunny skies. The core of low pressure systems, as the name suggests, will be measured at much lower values than our SLP and are usually below 1000hPa. This was absolutely the case with this bomb cyclone, where the pressure was measured by Gold Beach, OR, to be at 975hPa around 7pm local time on 11/26/2019! As mentioned previously, the use of the meteorological term “bomb cyclone” is reserved for low pressure systems/cyclones that rapidly intensify over a short period of time: usually a decrease of 24hPa in a 24-hour period. Other frequently-used terms include meteorological bombs or just weather bombs, but the overall idea remains the same.
500mb height anomaly map for the CONUS. Note the pinks over the West Coast, indicative of very abnormal pressure signatures that were forecast by the ECMWF model on the morning of 11/25/2019. Source: Pivotal Weather
As such, this system is currently packing a punch on the California and Oregon coast, bringing with it heavy winds and a trail of atmospheric moisture that will produce heavy rains and snow all across the Southwestern United States. This atmospheric moisture transport has been very evident in previous model runs going back several days given the anomalous strength of this system. These include the North American, Global Forecast, and European Models, all of which play an integral role in short, mid, and long-range forecasting (and will be talked about in greater depth in future articles!). What’s interesting about these moisture transports across the Pacific is that in these sorts of instances they almost appear as currents that stretch out for hundreds of miles over much dryer environments. This is evident in model runs (that just came out earlier today!) by the Global Forecast Model (GFS) as seen below:
GFS IVT Map: Initialized at 10am PST on 11/26/2019 and valid for 4am PST on 11/27/2019. Source: Center for Western Weather and Water Extremes
In one of my upcoming pieces I’ll dive into this topic a bit more but for now, integrated water transport vectors (IVTs) are essentially a measure of the amount of moisture being carried from one place to another with the wind. The feature we’re focused on covers a good chunk of the 30 to 35 degree region off the coast of California and snakes its way into the State and as far east as Utah. Generally, values of IVT over 200 suggest that there is a decent amount of moisture that is being carried by this flow and when the flow is as narrow as what this model run is suggesting, meteorologists will often refer to such a flow as an atmospheric river. And that is indeed what we are expecting to see unfold with this bomb cyclone as it continues its trek over the Western US. As it does so, heavy rain and snow will flow into the region, bringing with it conditions that will make holiday travels challenging for millions of people. In part two of this two-part article, we will be going over the aftermath of these sorts of systems with the help of one of my powder(snow) chaser colleagues as we visit some of the heaviest-impacted snow areas from this system!
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© 2019 Meteorologist Gerardo Diaz Jr.
Photo: Average precipitable water for ice-free oceans from 1988 to 2016 worldwide. (Courtesy of Willis Eschenbach of WUWT)
Precipitable water is a term (often stylized as PWAT) that is not often heard through media outlets when referring to the intensity of a storm. This is a term best defined as: if all of the moisture in a specific column of the atmosphere was to be in one container, that amount measures an approximation of how much water can precipitate from a storm. This value could determine the strength of a storm system in accumulation and intensity. For example, desert and high drought areas have low precipitable water rates, while moist environments (generally near the equator or near a body of water) see higher rates. While these are the most common criteria for determining a rate, there can be fluctuations globally and seasonally. The value for precipitable water is read in millimeters or inches and can be shown on a map similar to accumulation forecasts.
To better define the concept of precipitable water, here’s an example provided by the GFS (Global Forecast System) model for Australia. The first example depicts the 12-hour precipitation that fell in comparison to the higher rate for precipitable water:
Photo: This is a precipitable water map for Australia on the morning of 11/3 showing values between 1.5 and 2 inches highlighted in purple. (Courtesy of Pivotal Weather)
Photo: This is a map showing accumulated rain 12 hours following the high PWAT in the area showing a concentration of the highest rainfall in Eastern Australia. (Courtesy of Pivotal Weather)
These photos depict an obvious correlation between the higher precipitable water rates and higher amount of rainfall. A higher influx of moisture from open waters that becomes paired with hot and dense air can hold more water in the atmosphere, thus resulting in a larger chance of accumulating rainfall at the surface.
Since precipitable water is measured in a column of air in a given area, it can change frequently, vary over a short distance and can depend on the current season. The most noticeable difference is during the winter when air is colder and has a decreased ability to hold large amounts of water, thus making the precipitable water rate much lower than summer. While strong snow storms may seem like a significant producer of precipitation, a ratio of 10 inches of snow to 1 inch of water is the average, and this warrants quite a low rate of precipitable water.
The main concerns of focusing on precipitable water in a forecast is the possibility of flooding. Precipitable water rates that are higher than average can cause trouble for those living near areas that do not commonly see high amounts of water. Some examples include those living near rivers that don’t regularly experience water levels rising and farmers with large fields of crops. Overall, we see precipitable water as a tool that can aid in the forecast for any form of precipitation and will give further insight into explaining expected totals. The impacts can be avoided if there is added confidence to a forecast that allows for an audience to prepare.
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© 2019 Meteorologist Jason Maska