The following is a post I wrote for an “Understanding Weather and Climate” course I teach each semester, online. I’m not sure all will read; it’s about 2000 words. Like I tend to think, “You payz your money, you takes your chances.” If anyone detects any errors, factual or otherwise, please let me know. I appreciate your attention.
I’ve been collecting images over the last couple days. I was hoping to go over these as sort of a postmortem on the changing weather, looking back on the last few days to see how we got to today.
In the image at left I have placed black arrows at the 5400m level (500mb). This is a diagnostic level where moisture above this is frozen, below this level is liquid. By examining this level we can determine who might get snow and who might get rain. In this image, regions to the right of the arrows, in the blues and purples, will be getting snow. Light blues, greens and oranges will be getting rain, if anything.
In this image I have highlighted two areas. The yellow ellipse highlights the transition zone between the amounts of available atmospheric moisture for precipitation. The larger the number the greater the amount of precipitation possible. Light areas have greater moisture than darker areas. The black circle highlights western Kentucky and the Jackson Purchase. You will have to make a mental note of your location in case you are located somewhere else.
One thing to keep in mind as we examine these images is that these maps are based off data collected at a moment in time; they are static images, not dynamic. While informative, they don’t convey the movement of air masses. We will consider that in a moment.
The image is simply upper level (850mb) temperature with western Kentucky highlighted with a black circle. In this case, cool colors represent warmer temps, and warmer colors represent ridiculously cold temperatures. For instance, the light blue areas represent areas with temperatures of about -2°C. As you look to the purples and reds, the temperature decreases. The area of western Kentucky is about -22°C.
These are not surface temperatures, to be clear. These are temperatures aloft, at an altitude of about 5,000ft, maybe a little lower. As water molecules fall if they pass through cold air, they will freeze. Depending on the thickness of the layer they pass through, snow, ice or sleet will form. If the surface layer is warm and thick enough the water droplets will thaw and fall as really cold rain. In the summertime, if the rain feels really cold to you, quite possible the rain droplets initially fell as ice and then melted as they passed through warmer air layers.
Here is the image I was referring to earlier when I mentioned something about these images being static and not properly showing the dynamic nature of the atmosphere.
This image shows a portion of the upper level Jet Stream. Sidebar: When we talk about events in the atmosphere we refer to altitudes. Balloons, aircraft, clouds, etc., have altitude. They not have elevation. Elevation refers to objects on the surface of the Earth and their height above Mean Sea Level. Mountains, buildings, towers, and such objects fixed to the surface have elevation. Buildings do not have altitude – they do not fly through the air. Aircraft do not have elevation – usually, unless they are sitting on the tarmac awaiting permission to depart, or they crash. Clouds can have elevation in special cases. Those cases we call “fog.”
The Jet Stream featured here is at an altitude of about 38,000ft. The arrows show direction; the arrow color indicates wind speed. Notice how all the arrows are mostly parallel? This is what is called “zonal” air movement and is generally not a good thing. In summertime, zonal movement usually brings extensive drought. The “kinks” or “curls” in air movement provide the instability necessary to generate weather systems – in the summer.
In the winter, the same circumstance can occur and create dry and very cold conditions. Zonal winds tend to create a clearing effect moving weather rapidly out of place. Clear skies allow for more heat (long-wave radiation) to radiate back to space and can create very cold temps. And this is sort of what has happened. Except we have had some large weather systems develop over Alaska and the northern Pacific which carry a lot of moisture. The Jet drags this moisture south where it crashes into the really cold air moving south out of eastern Canada, and then we have a real mess on our hands.
I want to take a look at these graphs. I have a mouse-over effect in place to help highlight the graph in the upper left. It may be annoying; not sure.
We are getting a lot of rain on Saturday; lots of precipitable moisture in the atmosphere. The “Temperature, Dewpoint, and Relative Humidity” graph is a pretty powerful graphic demonstrating what happens when the air temp and the dewpoint are almost coincident.
The dewpoint temperature will never be higher than the air temperature. The dewpoint temperature generally increases as air temperature increases, too. The way I think of dewpoint is it is the temperature at which water vapor condenses to form a dew drop, sort of like a rain drop. This condition occurs when the air temp and the dewpoint temp are nearly the same. When that condition exists at a level within a few feet of the ground, we can have dew, or fog. When that condition happens at some altitude, we will have rain, sleet, snow, etc. Depends on the temperature.
Today, temperatures aloft are warm enough that water droplets are warming before hitting the ground and we are getting a lot of rain. As I write this, Calloway Co., Kentucky has received about 3.50 inches of rain since Midnight.
Now, we might be tempted to ask, “What if this fell as snow? How much snow would we get?” The answer to that is not simple. For a long time, I used the ratio 6:1, meaning “6 inches of snow was equivalent to 1 in of rain.” That is no longer true. The ratio could be as high as 20:1; “20 inches of snow is equivalent to 1 inch rain.” The real answer is complicated and requires some moderate amount of statistics training. But, in simple terms, we can examine a certain layer of the atmosphere to figure out the ratio.
If we look at the Earth’s atmosphere from about 850mb to about 700mb, and figure out how thick this layer is, and know something about the temperature, we can determine what the ratio of snow-to-rain is. The thicker the layer, and if it is cold enough, the bigger and fluffier snowflakes can develop. Thus the ratio will be closer to 20:1. If this layer is thinner, the less development time for snowflakes, and thus the ratio is closer to 6:1. The problem for us is this is an introductory course, a survey of weather and climate, and we don’t have time to investigate this.
Some comments directed at people who derive their livelihood from U.S. rivers. All of this precipitation has to go somewhere. The majority of precipitation ends up as run-off. As much as 66% of precipitation runs-off into streams, then rivers, and eventually hits the ocean. People working on rivers to help maintain U.S. barge traffic – a near invisible part of our transportation infrastructure yet one of the most vibrant in the world, by the way – have to pay close attention to these weather events. As run-off heads to rivers, we could expect some flooding. We can expect problems associated with high velocity currents, making navigation tricky, dangerous, and potentially shutting down barge traffic. While droughts can impair barge traffic from the lack of water, too much water can be a bad circumstance, as well. Barge operators and companies not only need to be aware of local weather conditions, but as the saying goes, “[Stuff] flows downstream.] Marine companies have to pay attention to weather systems upstream and days in advance in order to best prepare their crews for changing conditions. The USGS maintains a river monitoring system for these very reasons. In cooperation with USGS, the NOAA/NWS provides weather data and river gauge data for a number of sites throughout the United States.
People can mistakenly think, “Oh, we’ve had a lot of rain. This will really help my aquifer, or our water table, or our groundwater.” No, not generally.
About two-thirds of precipitation (66%) will eventually cycle back to oceans. Another good portion will immediately cycle back into the atmosphere through evaporation. Yet another portion will be taken up by plants and vegetation. While a person might note an immediate improvement in their specific case, the change is generally temporary.
For aquifers and water tables to recover, moderate precipitation needs to occur at regular intervals. Light precipitation tends to evaporate too fast, or is taken up by vegetation, or runs-off. Heavy precipitation can quickly saturate the ground and prevent any more precipitation from penetrating to any depth (hence the term “saturation: the inability to handle any more stuff”.) Lots of rain may help depleted reservoirs which feed communities. But, again, this is usually a temporary circumstance.
What tends to happen is people get excited at seeing water levels return to “normal,” and then people not only return to their usual rates of use, but then build-out and add more subdivisions and such. Then, after a couple years, they wonder why the reservoir is drying again.
Now, climate. Like I mentioned in an earlier post, people may be tempted to think, “Climate change is garbage. Look at this rain/snow/sleet/etc.” I can see this reasoning. The problem with that line of thinking is the use of local conditions, or even regional conditions, and apply those conditions on a global scale. For instance, while we are having some pretty awful weather, so is Australia. The east and north coast of Australia were hit this week not by one hurricane, but by two hurricanes within days of each other. In the Pacific, hurricanes are called “cyclones” and near China, Japan, and Korea they are referred to as “typhoons.” The east coast was hit by Cyclone Marcia, a category 5, the worst. The north coast was hit by Cyclone Lom, a category 4, not the worst but still not pretty.
Climatologists examine at global conditions, how global moisture patterns change, temperature changes, changes in atmospheric chemistry, changes in insolation. Since this is climate and not weather, there may be no apparent and immediate impact. Changes take time to manifest, perhaps years. However, when they do manifest the change can appear quickly.
Let me illustrate. In the summertime, if the weatherperson misses the high temp or low temp by a degree or two no one will really care. If for example, the Weather Channel says, “The high in Murray, KY, will be 85F today,” and the actually high is 83F, no one will shed a tear.
Now let’s say this happens, the Weather Channel predicts, “The high today in Murray, KY., will be 33F,” and the actual high temperature is 31F people might freak-out. Why? The temperature is off by only a couple degrees. What is the big deal?
The big deal is at 33F the temperature is above freezing. At 31F, we are now at below freezing. While at 33F the pavement is wet, at 31F the pavement is now a sheet of ice, and the interstate now has a multi-vehicle pile-up and their are people hurt.
Yes, this is a local example of changing weather conditions, not climate, per se. However, we have to pay attention to changing climate because while 1/10th of a degree in global average temperature may not seem like a lot, this is a global average. Some site, some location, or perhaps a bunch of locations, had to have witnessed a pretty substantial change in order to change a globally measured statistic.
Whew…thanks for reading. Sorry for the length but I hope this helps.