 (Diagram showing each stage of twilight with the suns degree below the horizon) Twilight is defined by a period of time after sunset or before sunrise that the sky remains illuminated enough to still be able to carry on outdoor activities without the help of artificial light. It is caused by the refraction of light through the densities of our atmosphere from transmission of sunlight. Dawn is the term for twilight that occurs before the sun rises in the morning and dusk is the term for after sunset. Depending on the season or latitude, twilight can last anywhere from 70 to 100 minutes. In the mid latitudes, twilight tends to be longer during the summer and shorter in the winter months. It isn’t common knowledge, but there are three different stages of twilight. The National Weather Service (NWS) defines these stages as civil, nautical and astronomical twilight. Each stage is determined by the sun’s solar angle when it’s below the horizon. Civil twilight occurs from the time the sun sets or rises and when the sun’s geometric center is at a six-degree angle below the horizon. During this time, twilight is characterized by the appearance of larger bright planets and stars in the sky. Some left over pinks and oranges displayed from the sunset can still paint the sky. Artificial lighting isn’t needed to see your way around outside. The horizon is still able to be seen.  (Civil twilight: March 27, 2020 7:30 PM CDT Louisiana) Nautical twilight is when the sun’s angle is between six and twelve degrees below the horizon. During this time, you will be able to see more light from stars and planets through the naked eye. Most, if not all the displays of color from the sunset have disappeared. The horizon is still visible but artificial lighting may be needed to continue activities. The ability to start seeing stars during this stage of twilight is helpful to sailors who start setting course by the position of the stars. This is how nautical twilight gets its name.  (Nautical twilight: March 27, 2020 7:50 PM CDT Louisiana) Astronomical twilight is the last stage of twilight before it gets completely dark. Here, the sun's angle is between twelve and eighteen degrees below the horizon. Most celestial stars and objects can be seen in the sky at this point. During this final stage, it is hard to distinguish a horizon and artificial light pollution in populated areas tends to block on the remaining light of the day. This stage gets its name from astronomers observing objects in the sky. Brighter objects, such as stars and planets, can begin to be viewed by astronomers at this time. Other objects, such as nebula and galaxies, are easier to see during complete nighttime when the sun goes further below 18 degrees.  (Astronomical twilight: March 27, 2020 8:10 PM CDT Louisiana) There is so much to know about twilight and why it exists. It’s interesting to know there are three stages of twilight that are defined by different sun angles that demonstrate different characteristics before it becomes full night. Next time you find yourself driving to work in the morning or calling the kids inside for the night, reflect on the stage of twilight you are experiencing. It’s good to take a breath and reflect on our surroundings every once in a while.
To continue reading about twilight in part two and for more weather education, click here. © 2020 Meteorologist Alexandria Maynard Sources: Ahrens, C. Donald. Workbook/Study Guide to Accompany Meteorology Today: an Introduction to Weather, Climate, and the Environment. Brooks/Cole, CengageLearning, 2009. US Department of Commerce, and Noaa. “Definitions of Twilight.” National Weather Service, NOAA's National Weather Service, 16 Mar. 2015, www.weather.gov/fsd/twilight. “What Is Astronomical Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/astronomical-twilight.html. “What Is Civil Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/civil-twilight.html. “What Is Nautical Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/nautical-twilight.html. “What Is Refraction of Light?” Timeanddate.com, www.timeanddate.com/astronomy/refraction.html.
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 (The angle of the sun below the horizon and the direction of light as it is refracted through layers of the atmosphere) When the sun is just below the horizon, we often find ourselves immersed in a state of illuminated darkness. It's a part of our day where we can still see our way outside. Many people drive to work in the early morning or come home in the nighttime and children playing outside are called in to dinner or woken up for school the next morning. It’s a part of the day we all find as a comforting end or beginning. A passage from one day to a new day. During the twilight hours, we all go about our business without a single thought of how and why this occurs. Unaware, that if we didn’t have an atmosphere, twilight wouldn’t exist.
A beam of light passing through a substance is called transmitted light. When that light is transmitted through varying densities, it either speeds up or slows down. When light hits a dense substance at an angle, it bends. The bending of light is called refraction. Because our atmosphere is so thick with varying densities, the light that is transmitted by our sun bends, slows down and speeds up as the sun carries out its rotation across the sky. When the sun is at different angles in the sky, it can change in light intensity or directness. At twilight, the sun sinks below the horizon. At this particular angle, the light is no longer being directly transmitted because it is blocked by the horizon. So, how is it we can still see light at this time of day? Due to refraction - the bending of light through our dense atmosphere - the light transmitted from the sun can still reach that part of the world. The more atmosphere the light has to penetrate through, the more it gets refracted. When the sun is below the horizon, light has to penetrate through the atmosphere at it thickest. Therefore by the time the light reaches your area before or after sunset, it has been bent around the circumference of the sun and angled toward you. This is how it still seems like it's partially light out after the sun dips below the horizon. When light travels through space, it is travelling through a vacuum. Space is a vacuum because it is lacking in matter, density, and pressure. When light travels through a vacuum it has nothing to slow it down or bend it. If our atmosphere didn’t exist, it would just be replaced with outer space. There would be no bending of light through different densities. Therefore, once the sun sets behind the horizon, it becomes completely dark. Twilight would not exist. To continue reading about twilight in part two and for more weather education, click here. © 2020 Meteorologist Alexandria Maynard Sources: Ahrens, C. Donald. Workbook/Study Guide to Accompany Meteorology Today: an Introduction to Weather, Climate, and the Environment. Brooks/Cole, CengageLearning, 2009. US Department of Commerce, and Noaa. “Definitions of Twilight.” National Weather Service, NOAA's National Weather Service, 16 Mar. 2015, www.weather.gov/fsd/twilight. “What Is Astronomical Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/astronomical-twilight.html. “What Is Civil Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/civil-twilight.html. “What Is Nautical Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/nautical-twilight.html. “What Is Refraction of Light?” Timeanddate.com, www.timeanddate.com/astronomy/refraction.html.  What is a Cloud?
Fluffy and buoyant, wispy and moody, clouds arise in billions of different forms, each one unique in their characteristics and presence. Though, as beautiful and charismatic as each one is, what exactly is a cloud, and how do they come to be? At its most basic definition, a cloud is a mass collection of water droplets that form from the cooling of water vapor in the atmosphere. Depending on the altitude in the atmosphere, these water droplets cool either into water droplets or ice crystals. The higher in the atmosphere these droplets cool, the more likely they are to form ice crystals. These millions of tiny ice crystals then come together to form various sorts of cirrus clouds, thousands of feet in the air. Though, there is plenty of water vapor in the atmosphere just drifting about, so why is it that only some of this comes together to form a cloud? The answer to this question is CCN, or Cloud Condensation Nuclei. These miniscule particles in the atmosphere, on average only about 2 micrometers in diameter, are the surfaces on which water vapor may condense and become liquid. CCN can be a variety of particles, such as dust, salt, or other atmospheric aerosols. When water vapor collides with these particles, it condenses to form cloud droplets. These cloud droplets then accumulate to form what we know as clouds, be they composed of ice or liquid droplets. From cumulus, to stratus, cirrus and even the most abstract and rare forms of clouds, all undergo some form of this general process. Despite variations in some of the processes for different cloud types, all come to be those beautiful, powerful, and lovely formations that dot our skies. To read more about weather and atmospheric phenomena, click here! https://www.globalweatherclimatecenter.com/weather-education © 2020 Weather Forecaster Alexis Clouser |
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