Strong_Extratropical_Cyclone_Over_the_US_Photo by NASA, Public Domain
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I was reading a short article in Wikipedia about the climate here in Greeley when I can across this passage,
“Extratropical cyclones which disrupt the weather for the eastern two-thirds of the US often originate in or near Colorado, which means Greeley does not experience many fully developed storm systems. Warm fronts, sleet, and freezing rain are practically nonexistent here.”
Not knowing what an “extratropical cyclone” was made me dig a little deeper. I then found this definition,
“Extratropical cyclones …. are low-pressure areas which, along with the anti-cyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards and tornadoes. These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth.”
All of this made me understand that extratropical cyclones were very important, indeed, but the more I dug into more detailed descriptions of them, the less I was sure that I knew what exactly they were! The idea, though, that weather systems that slam the eastern part of the U.S. actually originate right here in Northern Colorado, was extremely compelling.
So I emailed my brother-in-law, Sanford, a highly respected atmospheric scientist and life-long weather aficionado, and asked him what an extratropical cyclone was. Here is his answer (with my editing and rearrangements):
Extratropical cyclone. Photo by NASA, Public Domain
“(Extratropical) cyclones form and/or strengthen …. in locations where, naturally, there is warm air to the east and cold air to the west. All this (especially occurs) in the winter half of year – when you talk about extratropical cyclones you are almost always talking about late fall, winter or early spring. Extratropical cyclones always have a specific (surface) form, with warm moist air flowing up from the south ahead (east) of the cyclone and cold air flowing down from the north behind (west of) the cyclone. The difference in temperature and moisture content across the cyclone provides the energy that drives the cyclone. Formation of extratropical cyclones … occurs frequently in Texas, Arkansas, etc. which …. often have warm, moist Gulf of Mexico air to the east and drier continental air to the west. You can (also) see these systems in seasonal climate patterns in Denver and Greeley and the rest of eastern Colorado …
The east coast of continents, with warmer ocean air to the east and colder continental air to the west is also a site where these cyclones can form. That’s why on the east coast of North America you always hear of lows forming off North Carolina, strengthening as they head northeast and becoming big snowstorms.”
So, in winter, cold, dry air from the north and west pouring down or over the Rocky Mountains hits warm, wetter air to the east that originated in the Gulf of Mexico. The mixing and interaction of these air masses sets up a great, spinning cyclone of air that then races up the frontal line between the cold air to the north and the warm air to the south. The weather along these lines can generate clouds, light rain and showers, intense thunderstorms, snow storms, blizzards and tornadoes.
I like to think of these Rocky Mountain cyclones as our winter-time gift to the rest of the country. Let it snow, let it snow, let it snow!
Antero Reservoir. Photo by J. Beale. Wikimedia Commons
An amazing weather phenomenon occurred the other night. A site over near Boulder (Antero Reservoir) reached -50 degrees F overnight! The rest of us in this area were down in the positive single digits, but the area around the reservoir was over fifty degrees colder!
The explanation was a “cold sink.” We had just had a pretty good snow fall so the ground surface was white and extremely reflective. Like I talked about a few weeks ago, that shiny, new snow can grab onto and reflect and re-reflect any artificial light that enters the landscape. That new snow also reflects incoming sunlight during the day and sends that light and much of its heat energy off into space. This shiny snow surface, then, is said to have a high “albedo.” Add to this a nearly cloudless night and the reflective (“radiational”) heat loss from these snow fields is considerable. The air over the snow, then, gets very, very cold!
Cold air is quite dense and can flow like water down a gravitational gradient. This cold, flowing air gathers in surrounding depressions in the landscape and can generate quite cold nighttime temperatures.
Antero Reservoir is a low area surrounded by mountains. The cold air flows down into the high mountain valley (or “sink”) and accumulates generating colder and colder local temperatures. When conditions are just right, sinks like these can reach astonishingly low temperatures during the night!
Blowing wind, Photo by HoremWeb, Wikimedia Commons
An old, meteorological acquaintance of mine showed up a few weeks ago and stuck around for quite a while: wind! For three days we had a steady 30 mph wind with gusts up to 60 mph. It sounded like a large animal clawing at our doors and windows and tossing the deck furniture around! Stepping outside to take the dog for a walk was a challenge! Poor Izzy, even though she is low to the ground, she was not prepared for the constant pushing and pummeling. Facing toward or facing away from the wind had their own unique difficulties!
Like I said, though, the wind was an old acquaintance returned. When I went to Texas Tech and lived in Lubbock we called the wind “the invisible hills.” When we went off on bike rides into the countryside we would take into account the wind and tack out slightly against it on our outbound trips and make sure that we would come home with its push at our backs. Sides of buildings on campus would get covered with piles of wind-blown debris and dry, uprooted plants. Once we made a Christmas tree out of a tumble weed that we dug out of one of these debris piles.
Tumble weeds piled around a house. Photo by R. Stanek, USAF
Lubbock is one of the ten most windy cities in the United States! Along with Amarillo and Abilene in Texas and Cheyenne, Wyoming (which is an hour to our north from here in Greeley) you have four places that almost always occupy slots on the top-ten “windiest” list.
Why so much wind? Air is fluid. It flows from areas of higher pressures into areas of lower pressures. The Great Plains has powerful high pressure systems in the north that come in from Canada and similarly strong low pressure systems in the south that originate in the Gulf of Mexico. The differential between these pressures sets up a powerful, wind-generating system even if they don’t get close enough to make a cyclone. Add a flat, relatively tree-less landscape in between these opposing pressure systems and then the winds just blow unopposed and unbroken.
Winter is the season for big winds here in Colorado. These winds can be cold winds (“boras”) racing straight down the east face of the front range driven by large, winter high pressure systems in the north. These are what we have been feeling in this most recent wind outbreak! Or, the winds can be the katabatic, warm winds (“chinooks”) pouring over the mountains from the west. According to the National Weather Service winds of 100 mph are not uncommon from either of these phenomena.
So, for three days we hunkered down in the house surrounded by and battered by the howling wind, and then, quite suddenly, it stopped!