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
Looking eastward into Salt Lake Valley on a snowy morning.
I’ve lived in a mountain valley for a few months now, and I’ve recently started noticing an interesting weather phenomena; I begin my morning commutes on the valley floor, where it can be rather chilly well into the late-morning hours. However, by the time I make it up to my office at the top of the valley, I notice it usually feels warmer. At first I thought maybe it had to do with surface heating, but it seemed a bit unlikely that the ground would warm by that much after a mere 10 minute drive. Instead what I’ve been experiencing is a phenomenon that’s known as a valley cold air pool, or (VCP), for short.
The effects of nighttime radiative cooling on the environment. Source: Hong Kong Observatory
Essentially, longwave radiation after sunset carries warm air from the surface up into space, cooling the surface and lowest layers of the atmosphere. This process is accelerated on nights with clear skies given that there’s even less of a barrier stopping the radiation from escaping out into space. As a result, the pre-dawn hours of the day tend to also be the coldest, and it’s not until sunrise that the re-introduction of solar radiation allows for temperatures to bounce back up. And while this process is fairly straight-forward in areas like the Great Plains, they tend to get a little more interesting once we get into more mountainous areas.
In places like the Rockies, the radiative cooling processes that occur at night also come into contact with topographical hurdles, including high elevation and valleys. Given that cold air is denser than warm air, air that cools at higher elevations will become heavy and naturally sink to lower elevations. In cities like Salt Lake and Portland, this sinking patterns leads to cold air descending from the mountains and into a valley floor that’s already radiatively cooling. By pre-dawn hours, the end-result is colder temperatures within the valley when compared to temperatures along its ring. And this is exactly what I’ve been experiencing since moving to this kind of environment!
Certain areas of the country experience more notable cold pools than others, and are therefore studied in greater depth by atmospheric scientists. In some cases, research has been done that tries to measure the height of cold pools and even their evolution, as has been the case by researchers up in Mt. Washington, New Hampshire, and down in the Yampa valley of northwestern Colorado.
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© 2019 Meteorologist Gerardo Diaz Jr.
‘Tis the season to travel north and bask in nature’s glory upon viewing the changing colors in the forests. Peak fall foliage season is roughly from late September to end of October, depending on the location. Year-to-year variability definitely plays a role in the exact timing of peak colors. Much like any other planned event, it is always dependent upon the weather. Whether it be a warm or cold summer, a wet or dry summer, plays a key role in how quickly or slow the leaves change color. Here, we will discuss how weather affects fall foliage.
First off, what changes leaf color overall? There are three main factors: leaf pigments, length of night, and of course weather. Leaf pigments such as carotenoids and anthocyanin present leaves with colors other than green. Chlorophyll, which is most familiar, presents leaves with their classic green color. As the year goes on and night time hours drift longer and longer, chlorophyll production slows down and eventually stops. This opens the door for carotenoids and anthocyanin to be unmasked and show their colors. Each tree has their own particular color: Oaks tend to have red or brown leaves, Hickories tend to have golden bronze, Aspen have golden yellow and Dogwood have a purplish red, to name a few. With longer and cooler nights, these colors begin to be more prominent in leaves, thus presenting us with these vibrant fall colors.
Where does weather come in? Temperature and moisture are the main influences on fall foliage. The perfect “recipe” for most prominent color displays have been found to be warm, sunny days and cool, crisp nights. During these days, sugars within the leaves are produced which help anthocyanin to form. If nights become too cold and near freezing temperatures, then these sugars cannot be produced and the veins within the leaves start to close off. Sugar and light spur production of anthocyanin pigments, which tend to produce the red, purples and crimson pigments. Carotenoids are more so unfazed by temperatures because they appear in leaves year round. So yellow and gold colors, commonly produced by carotenoids, are seen fairly constant year-to-year.
Soil moisture also plays a role in these colors as well. A late spring, for instance, can cause fall foliage to be pushed back a few weeks due to the water budget in the ground to be thrown off. A severe summer drought with essentially choke the soil and cause the fall foliage, later on in the year, to be not as vibrant. Also, drought may cause an abscission layer, reduction in photosynthesis, to form in leaves which will cause them to fall earlier than usual.
Truly, there is nothing like the vibrant colors that autumn presents us. The changing of colors are here to stay, but the timing? That’s up to the weather.
©2019 Weather Forecaster Alec Kownacki
As we are now in October, plants in the mid-latitudes will start their gradual process from having leaves full of green color to having no leaves at all. Depending where you live, this process may start as early as September or as late as December. Colors of many shades from yellow to orange to red will start making up the landscape. As beautiful as this is, many people will not stop to think about the important transitional phases these plants must go through before winter arrives.
This transition from leaves to no leaves is necessary for plants to store their energy when winter arrives. It would take a lot of energy for plants to keep their leaves healthy because of the cold and dry winter months. There’s also less direct sunlight which gives plants their energy. When the leaves fall, the holes where the leaves once were, close up enabling the plant to store water. In essence deciduous trees (oak, maple, walnut, etc.) go into hibernation mode.
Before this period, is autumn. This is when trees start to change colors. This change is more noticeable the further north you travel, especially during the first part of the season. As temperatures start to cool off and the days become shorter, a process known as photosynthesis becomes less frequent. This process uses chlorophyll to absorb energy (sunlight) and to convert it into chemical energy for the plants. Carbon dioxide is absorbed by the plant and oxygen is then released. Physically, chlorophyll is the green pigment within the plant itself. During autumn, less and less chlorophyll is produced, resulting in leaves changing their colors. This is because chlorophyll is broken down into smaller molecules allowing for other pigments to show their colors. The diagram below shows a more detailed view of the process of photosynthesis.
Not all trees shed their leaves. You may have noticed that up in the mountainous regions, some conifers (pine, spruce, cedars, etc.) never lose their leaves, despite it being much colder. This is because they are able to retain water much more easily than deciduous trees. They are able to store water because of their waxy coating.
The weather also affects the intensity of the colors we see. Low temperatures that are above freezing will favor bright red colors in maple trees. If there is an early frost, on the other hand, the red color will be less vibrant. Increased rainfall and cloudy weather will also produce better colors.
To see when your areas may be seeing the change in the seasons, you can access the following link https://smokymountains.com/fall-foliage-map/.
Credit: Arizona State University Biology Department, SUNY College of Environmental Sciences and Forestry Department
© 2019 Meteorologist Corey Clay
For precipitation to form, we need four key ‘ingredients.’ First, we need a lifting mechanism. This will create saturation. There are three commonly known ways to create this uplift.
The most common one is a front, also known as a cyclone. . These are large scale weather patterns that can last from hours to even days. An everyday example of this type of weather pattern is when a warm front moves into a surface cold front. The warm air will try and stay on top creating a ‘lift’ for saturation to form. The next common way is through convection. This happens in deep cumulonimbus clouds and usually only lasts for an hour or two. These will form mostly in the summer, and are known as thunderstorms (like the ones we see in the summertime in Albany, New York.) Lastly, orographic lifting is another way to get lift and saturation. This happens when there is a mountain range, on the windward side of the mountain. The air will rise on the windward side, and, while it rises, it results in adiabatic cooling and condensation. Which is a “condition is which heat does not enter or leave the system.”
Now after we have the lifting part, we need the next few steps: water vapor to condense and grow. We’ll want the air to reach 100% relative humidity, and from here, it depends if it is a clean water particle, or not. If it is a clean water particle, then it will have to find some cloud condensation nuclei to form onto, and then it can grow. Cloud condensation nuclei (or CCN for short) are particles such as sea salt, dirt, aerosols, or minerals and they serve as a base for water droplets to form onto. A clean water droplet will have a hard time forming to another water droplet, unless it has a CCN, which will allow for it to initiate condensation. (If you have a “dirty” water particle, that just means it’s already mixed with a CCN and doesn’t need to find one to form!) Usually, if it is “dirty” air, it will have lots of small cloud droplets (water droplets) within. But if it is “clean” air, then it will have a few large cloud droplets within it. For the droplet to fall, it needs to grow, as they fall dependent on their size and weight. A droplet will collide and coalescence (the joining or merging of two elements) with other droplets as it is falling through the sky growing in size during its fall. The larger the droplet gets, the faster the fall, and the more it will collide with other droplets. This will create a supersaturated cloud and once it has a lifting mechanism, rain will fall.
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© 2019 Weather Forecaster Allison Finch
As September comes to an end and we enter October, many people are excited about the transition into fall and all of the joys that come with this season and its cooler weather. Pumpkin flavored everything has already made its appearance, stores are filled with Halloween-related decorations and costumes, and many people are already longing for the weather to cool down enough for their cozy fall sweaters. One other major fall highlight that many are looking forward to? The spectacular colors brought on by the leaves changing. So why do leaves change colors in the fall and how might variations in the weather alter how they change?
The coloring in leaves is determined by the amount of chlorophyll within the leaf. Chlorophyll is used by the plant to turn the sun’s rays into energy and is what gives leaves their green color. In the fall, leaves produce less chlorophyll as the days get shorter and cooler, causing them to lose that green color and change into the bright orange, red, and yellow leaves that paint fall landscapes. Weather can be used to estimate when the peak “leaf season” will be in various areas since temperatures and rainfall amounts can play a role in when leaves turn and how quickly they will fall from the trees. Drought can cause leaves to turn more quickly and appear less vibrant. On the other hand, too much rain in the summer can also bring about the changing in colors more quickly as excessive rainfall can be a stressor to the trees. Moderately warmer temperatures can cause a delay in the changing colors, however, excessively warm temperatures could cause the leaves to change colors rapidly and fall sooner, causing the beautiful fall colors to disappear more quickly. An overall increase in temperatures could also cause more variability in when the leaves change, making it harder to predict when they will reach their peak colors.
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©2019 Meteorologist Stephanie Edwards
What Kinds of Weather Do Different Cloud Types Indicate?
Looking at the Difference Between Astronomical Seasons and Meteorological Seasons! (Credit: NOAA NCEI)
Discussion: The calendar has just passed the start of astronomical fall, also known as the fall equinox. For meteorologists and climatologists, they identify each season differently. Due to the position of the Earth in relationship to that of the sun, the date for astronomical seasons can vary by a few days; compared to meteorological seasons. Instead, meteorological seasons are based upon the annual temperature cycles of a season.
Meteorologists and Climatologists prefer to distinguish the seasons by the annual temperature cycles of each season. Each season is divided into its respective group by month. Meteorological winter starts with December and ends in February, spring starts in March and ends in May, summer starts in June and ends in August, and Fall starts in September and ends in November. This method allows for more consistent records with each season being approximately 90 days long.
Astronomical seasons have two solstices and two equinoxes which are marked by the point at which the earth’s tilt and the sun’s alignment are over the equator. The equinoxes occur when the sun passes directly overhead of the equator. The summer solstice falls on or around June 21st while the Winter Solstice falls on or around December 22nd. The Spring equinox falls on or around March 21st and the fall equinox occurs on or around September 22nd. These dates for the astronomical seasons are similar for both the Northern and Southern Hemispheres, however; the seasons are reversed in the Southern Hemisphere. According to the National Centers for Environmental Information, the length of an astronomical season can vary in length between 89-93 days. Due to this variation, climate information for each season would be hard to compare year-to-year.
Both Meteorologists’ and Climatologists’ methods of determining the seasons are useful and important to many different occupations and fields of interest. The next time an equinox or solstice is coming up, remember that the particular season it is occurring in actually started a little earlier for your fellow meteorologist or climatologist!
Photo Credit: National Weather Service
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© 2019 Meteorologist Shannon Scully
Everyone has at some point been tricked by the optical illusion of water on a road in front of them or the appearance of a lake in the middle of the desert. Known as mirages, these phenomena represent one interesting example of important scientific principles at work. While these features don’t exist, our brain fools us into believing they are. So what’s really happening? Is there a reason our eyes play these tricks on us? Let’s find out.
To understand the physical process behind this illusion, we must start with an understanding of how temperature behaves in the atmosphere. As you move upward from the surface, air temperature decreases with height. Therefore under average atmospheric conditions, the higher above the ground, the colder the air. The rate at which temperature decreases is known as the lapse rate and is a very useful value within meteorology. Lapse rates help us identify how air will move vertically, analyze the potential for thunderstorms, and, in this case, understand how features such as mirages work.
The strength of lapse rates vary greatly throughout different sections of the Earth’s atmosphere. For example, in the stratosphere (the second lowest layer of the atmosphere), the temperature actually increases with height. This situation is called an inversion and is the opposite of what we typically observe in the troposphere (the lowest level of the atmosphere where our weather forms). If the lapse rate is small, meaning that the decrease happens gradually as you move vertically upward, air is not likely to rise and weather conditions are typically much calmer. When the lapse rate is a high value (i.e. the temperature decreases very quickly with height), air rises rapidly and the potential for clouds and precipitation increases. It is important to note that while lapse rates are important, they are only one of several factors that contribute to the potential for active weather.
In the case of a mirage, we have a very high lapse rate really close to the surface. A hot road for example, underneath cooler air directly above it can create an extreme difference in temperature over a very short distance. Since light travels through warm, less dense air more quickly and wants to take the shortest path possible to any particular point, light travelling near the ground will bend upward in a U-shape through the warm air. To a viewer, this makes light from the sun appear as though it traveled in a straight line directly from the surface. We see what appears to be water because water and this large temperature difference bend light in a very similar fashion. Thus our brains misinterpret this bending of light as the presence of water on the ground. While the water itself does not really exist, mirages are interesting optical illusions that truly trick our brains into seeing something that differs from reality!
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©2019 Weather Forecaster Dennis Weaver
Source: USA Today
Since moving out west I’ve started hearing a lot more about some really informal names for weather features from locals, ranging from the dreaded inversion in Salt Lake Valley to the potent chinook winds of the high plains. And for the most part, most of these names make sense; the inversion refers to a shift in vertical temperatures that result in temperatures increasing with height and the chinook is named after the Native American tribe and its legends about the warm winds. And then there’s the haboob. A name with no immediate bearing to dust storms and whose origins don’t appear to be linked to any Native American legends whatsoever.
Haboob is Arabic for the word “blown”, and it refers to a unique type of dust storm that is common in the Mountain West but especially in the Southwest. During the summer monsoon, isolated and scattered thunderstorms dot the desert and arid areas of the West and as these storms release cold air from their bodies, the air accelerates and sinks back down in the form of downdrafts. These winds strike the surface and then spread out in all directions, kicking up dust in the process. If enough of a column is kicked up from these straight-line winds, the end result can be large, potent dust storms that can reach vertical heights of over 1,000 meters and stretch over 150 kilometers in length, blinding large swaths of land for as far as the eye can see.
Phoenix, AZ, is no stranger to these dust storms. On July 2011, the city experienced one of the largest haboob events ever to happen within the metropolitan area. Indeed, the entire city shut down as skies darkened and visibility levels tanked to less than a meter, making driving nearly impossible across the entire region.
Bystanders on the streets also experienced negative effects, given that air quality almost immediately becomes hazardous as particles can freely enter sensitive parts of the human body, including your eyes, ears, nose, and mouth. So what do you if you’re caught in one of these events? If you’re a driver, your first priority is to immediately get off the road before your visibility is gone. If you’re on a freeway or a road outside of a major city/town, safely pull off the roads and turn your lights off, and wait it out. The only exception to keeping you lights off is if you’re unable to get off the road, in which case you should immediately slow down and have them on while you sound your horn to alert others that you’re driving by. As for those who are outdoors, immediately seek shelter indoors. If you can’t immediately go indoors, cover your mouth and slowly make your way to an indoor shelter so as to avoid having contaminants enter your body. Indeed, haboobs are just one of many unique weather features in the Western US that may back east don’t experience very often. And despite the silly name, these events should be taken very seriously, as they are some of the most impressive but terrifying meteorological forces in desert regions.
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© 2019 Meteorologist Gerardo Diaz Jr