September 22nd marks the start of autumn, or autumnal equinox every year. Autumn is known as the third season in the year, the transitional period from summer to winter. Autumn also marks the start of daytime and nighttime temperatures steadily decreasing, which leads to leaves changing colors and falling off of trees. Some people have an aesthetic appreciation for the fall transition, but may wonder the scientific process behind this and how leaves fall off of the trees.
First off, this process begins in the spring and summer months. As trees grow throughout these seasons, chlorophyll (or in other words, the green pigment present in all green plants responsible for the absorption of light to provide energy for photosynthesis) is constantly replaced in the leaves. This process with the leaves converting sunlight into energy is known as photosynthesis. This process makes trees lose a lot of water, to the point when winter arrives, the trees are no longer able to get enough water to replace it. As the nights start to grow shorter in the early fall, the cells near the center of the leaf and stem divide rapidly but do not expand. This process of the cells forming a layer is called the abscission layer, which blocks the transportation of materials from the leaf to the branch, and then from the roots to the leaves. As the chlorophyll is blocked from the leaves, it disappears completely from them. The lack of chlorophyll allows the yellow (xanthophylls) and orange (carotenoids) pigments to become visible. The red and purple pigments (anthocyanins) are created from the sugars that are trapped in the leaf. These pigments in leaves are responsible for the vivid color changes in the fall.
As fall continues forward, and leaves start to peak their colors, all good things must come to an end as the leaves will start to fall off the tree. However, the term “fall” is a bit misleading, as this implies that the trees are submissive this time of year when in fact, they are actively pushing the leaves off their branches. The changes in temperature and daylight trigger a hormone that releases a chemical message to each leaf that it is time to prepare for winter. Over the next few weeks, abscission cells form a bumpy line at the place where the leaf stem meets the branch. This process goes at a minuscule pace, as the leaf is pushed from the tree branch. This happens as a means for a tree’s survival.
This process is a gentle but seasonal cycle, however, with changes in global climate variability such as the warming trends that have been noticeable throughout the years will noticeably impact the natural cycles within this process. The peaking of the leaves will start later than it has throughout the past, as well as leaves not becoming as vibrant in color and brittle due to the lack of moisture present and general heating throughout the summer and spring months. Fall is one of nature’s greatest beauties that unfortunately may be impacted throughout the years to come if these trends continue.
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©2018 Weather Forecaster Michael Ames
Now that fall has begun in the Northern Hemisphere, we find ourselves turning our attention to the foliage riot of colors. However, the simple transition from summer’s lush, leafy greens to fall’s bold reds, yellows, and oranges can be impacted in complex ways via weather conditions – temperature, precipitation, and amount of sunlight – as well as climate.
As most of us know, leaves serve a functional purpose for trees, producing energy for the entire plant. Their broad shape makes them perfect for absorbing sunlight, which after absorption, interacts with carbon dioxide and water within the leaves, producing sugars and oxygen in a process known as photosynthesis. The plant molecule responsible for photosynthesis is called chlorophyll – giving leaves their trademark green color. But chlorophyll isn’t the only pigment that resides within leaves! Orange (carotenoids) and yellow (xanthophylls) pigments are also present. However, they remain hidden for most of the year due to chlorophyll’s masking capabilities. Throughout the spring and summer months, chlorophyll is continually depleted by sunlight, only replenished during the growing season. Once chlorophyll levels subside, other pigments are able to shine through.
The most brilliant leaf displays tend to follow a summer filled with warm, sunny days and cool, crisp nights. During this weather cycle, the sunny days allow for leaves to produce an overabundance of sugars, while the cool evenings allow for the narrowing of leaf veins, trapping the majority of the sugars within. An overabundance of sugars and light within the leaves leads to the production of vivid anthocyanin pigments – which produce purple, red, and crimson colors. Soil moisture also plays a crucial role in the timing and brilliance of leaf foliage. The healthiest displays are produced when the soil has been adequately moist throughout the year, teamed with the aforementioned late summer weather. A severe late spring or summer drought can delay the onset of colors. A warm period during the beginning of fall can also decrease the intensity of fall colors by triggering early leaf drop before the colors have had a chance to fully develop.
In the future, these temperatures could potentially increase throughout all seasons due to climate change. However, the effect on fall foliage will depend on just how much the temperatures change. For example, New England is projected to have moderate temperatures – potentially leading to a later emergence of fall colors – while in southern New England, trees may be subjected to heat stress – leading to earlier coloration and faster leaf drop. Future precipitation trends in the Northeast aren’t clear, though there are indications that rain could potentially become more intense in episodic bursts, leaving enough time between these episodes for trees to experience drought stress. These warmer temperatures and droughts could also affect the brilliance of red leaves in the future due to nighttime temperatures – which help stimulate red pigments – potentially rising faster than day time temperatures. Local colors may also shift in the future as tree species migrate upwards in elevation and further north to stay in their preferred temperature range.
While the future remains uncertain, autumn is the one season that allows people to go out and connect with nature, possibly influencing people to pay more attention to climate and weather. In that way, fall foliage is a subtle, kind way to increase environmental awareness.
Curious to see how your local weather will affect the fall foliage in your area? The variability of the conditions involved ensures us that no two fall foliage displays will be identical year to year. However, scientists make predictions about the timing of fall colors, and they even created an app that allows you to view their predictions for your region.
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© 2018 Meteorologist Ash Bray
Different Types of Lightning (credit: The National Severe Storm Laboratory & The National Weather Service)
DISCUSSION: Lightning is one of the most dangerous types of weather phenomena. As of September 3rd, 2018, according to the National Weather Service, in 2018 alone there have been 20 deaths from lightning strikes in the United States so far. It is recommended to stay inside when there is an upcoming thunderstorm.
The National Severe Storms Laboratory (NSSL) has a page that lists and describes the different types of lightning. Lightning is formed by energy transferred from positive and negative charges in clouds or the ground. There are three primary types of lightning which include: cloud-to-ground (the most commonly known type), cloud-to-air, and cloud-to-cloud.
With cloud-to-ground lightning, the rapid discharge of lightning is a channel of negative charge that is attracted to the positively charged ground. Once the two charges are connected by the stepped leader (the initial invisible connection between the two charges) and a positive return stroke from the positively charged ground, the electrical current that is seen as lightning forms. Sometimes, the cloud can be positively charged, and the ground negatively charged, but this doesn't occur as often. The NSSL states that cloud-to-air and cloud-to-cloud lightning have 5 to 10 times more lightning strikes than cloud-to-ground. (Below is a picture of cloud-to-ground lightning.)
Cloud-to-air lightning is described by the National Weather Service (NWS) as “lightning that occurs when the air around a positively charged cloud top reaches out to the negatively charged air around it.” In other words, these lightning strikes are an attraction between clouds and air that are opposite charges and never reach the ground. Most of the time though, the positive charge forms atop of a storm cloud and is attracted towards a negative charge in the air nearby. (Below is a picture of cloud-to-air lightning.)
Lastly, there is cloud-to-cloud lightning. The NWS describes this type of lightning as “lightning that occurs between two or more separate clouds.” This lightning forms by one cloud being a mostly negative charge and another being a mostly positive charge, causing the attraction between the two. The NSSL explains that a type of cloud-to-cloud lightning called a “spider lightning” are formed underneath stratiform clouds (low-level, thin clouds that may produce a light drizzle) and flashes travel horizontally. (Below is a picture of cloud-to-cloud lightning.)
There are a few more lightning types that aren’t necessarily categorized as part of the three main types of lightning. These lightning types include: red sprites, blue jets, and elves. Red sprites usually occur above a large thunderstorm and are normally a red color. Red sprites usually occur during a cloud-to-ground lightning strike. They can be seen from space as they extend up to 60 miles above a cloud top. They can reach all the way to the mesosphere! Blue jets also form above a storm cloud but, unlike red sprites, they are not associated with cloud-to-ground lightning. They reach up to 22-35 miles above a cloud and can be seen by aircraft. They can sometimes just reach the stratosphere. Lastly, elves are described as a glowing disk that can extend up to 300 miles. This upper-atmospheric lightning can reach the ionosphere. The Space Shuttle discovered elves in 1992. (Below is a diagram provided by the NSSL of red sprites, blue jets, and elves.)
Lightning is important to understand and a weather phenomenon that happens constantly on this planet. Knowing what types of lightning occur at what height in the atmosphere is important to assure aircraft and spacecrafts have safe travel paths. Lower-atmospheric lightning is a life-threatening danger to humans and being more cautious when lightning is forecasted could save lives.
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© 2018 Weather Forecaster Brittany Connelly
DISCUSSION: As the National Weather Service's National Hurricane Center office in Miami, Florida continues to work on the track as well as intensity forecasts for a number of current as well as potential storms in both the tropical Eastern and tropical Atlantic Ocean basins, there are still people who have their doubts about the rest of the respective seasons. Having said that, there are a few things that everyone should be aware of when it comes to tropical cyclone impact potential. First and foremost, tropical cyclones are a unique danger to both life and property and often across very large regions due to their size and power.
On that note, as a direct result of their often larger size (i.e., a couple hundred to sometimes several hundreds of miles across), tropical cyclones have the ability to churn up a ton of upper-ocean water content both beneath and just out ahead of their forward track. Therefore, one of the most prolific impacts from a tropical cyclone are those impacts which are delivered by a given tropical cyclone's storm surge. It is very well-known around the world that the most powerful force on Earth is indeed the power of water. Thus, it is no surprise that when a tropical cyclone drags an overstock of ocean water into shallower bays and inlets prior to and during its landfall, this can and often does have disastrous consequences for anyone and anything in its path. This is due to the fact that when water quickly piles up in shallower coastal regions, the water has no other place to go other than inland at that point and this often occurs quite violently.
More specifically, it has been found that even just 6 inches of rapidly moving water can often sweep someone off their feet. Moreover, by the time you reach 1 to 2 feet of quickly moving water, the associated force can often be powerful enough to move most automotive vehicles and sweep them away. However, when it comes to a tropical cyclone's storm surge, you often are dealing with a violent rushing wall of ocean water which can be 5 feet high and counting. Hence, the reason for why inland moving storm surge impacts are often even more destructive than the strong onshore winds associated with a landfalling tropical cyclone.
The points noted above or a big reason for why anyone who is ever positioned in the cross-hairs of an approaching tropical cyclone (regardless of its intensity near the time of landfall) should always treat a tropical cyclone with the utmost amount of respect. Attached above is YouTube footage from Super Typhoon Haiyan and it was captured by Nickson Genesis, a Plan Philippines Community Development Worker at 6 am on the morning of 8 November 2013. Note the massive storm surge was that moved inland in this footage at a location which was at least 1 to 2 miles away from the nearest coastline. This footage just goes to show the natural ferocity and power of an intense tropical cyclone's storm surge.
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© 2018 Meteorologist Jordan Rabinowitz