 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. For more weather education topics click here!
© 2019 Meteorologist Gerardo Diaz Jr. Sources: https://www.mountwashington.org/research-and-product-testing/current-projects/cold-air-pools-in-mountain-valleys.aspx https://journals.ametsoc.org/doi/full/10.1175/MWR3180.1 https://www.hko.gov.hk/education/article_e.htm?title=ele_00004
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 ‘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.  Credit: Mentalfloss 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 |
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