Preparation for the VSA Seminar on Weather at WSC on Jan 12, 2019
For over 20 years the USAF paid me to travel around the world, allowing (requiring) me to talk with meteorologists before each of my many flights. The last 15 years of my flying in powered aircraft had very little direct interaction with the weather people, but the commercial aviation dispatchers were always available to explain any challenging weather events that I might encounter on the flight plans. I’ve spent a lot of time looking out the window at the weather. Flying gliders was just a natural transition after all of those years. Before every glider flight I study the atmosphere. It just never gets old.
You want to fly like a bird, but not flap your wings? (or use any means of propulsion) – That doesn’t sound possible.
Travel long distances, stay aloft for many hours at a time, AND return to the place you departed from? – Sounds even more impossible.
Gravity is always pulling you back to the earth’s surface. You can’t defeat it. But, there may be a way to do it. Well, not to defeat gravity, but you could use the natural motions of the air, because we know we have an atmosphere that is always in motion due to the solar radiation striking the earth’s surface.
If we want to try to understand the atmosphere it will take a lot more than a one hour seminar on the weather to discuss even the most basic concepts of how to find that “up air“ *¹ that is so vital to our endeavor. Let’s try to address, or at least identify, some of the basics of our atmosphere, but keep in mind (as you try to forecast what will happen next, be it five minutes from now or 48 hours from now), it’s complicated. You will be on a long journey to places heretofore unknown to you. You will soon realize you are not in Kansas anymore *² . If you thought learning to fly was gong to be hard, you may soon think that being able to forecast the weather is totally impossible, but let me tell you for a fact – it is not impossible to forecast the weather. I can predict the weather with 100 % accuracy in many instances. It just depends on your time frame, and what you know about the atmospheric conditions prior to the time of the forecast.
Let’s review some basic, for those of you that may not have been paying attention in your science classes.
The world is not flat. Every 24 hours the earth, makes one revolution. Spinning on the axis as it does, the speed of rotation is easily measured, and, FYI, here at 40 degrees latitude, we are continuously traveling at a speed of approximately 795 MPH. But what about the air above the surface? Well that too is traveling at a very high speed, but maybe not quite as fast as the surface. Additionally, if it is moving north to south the relative speed is changing as well.
The surface of the earth is heated by the sun, but at various rates – depending on a multitude of factors. Air near the surface will naturally rise (convection), but as it rises it expands, and as it expands, it cools. Ever wonder how the air conditioner works on a jet plane? Compressed air from the engine drives a turbine, which causes rapid expansion of the air and it gets very cold, very fast. Thus warm air rising from the surface cools because it is removed from the heat source, cools, because it is expanding, etc, but, it continues to rise until stability is achieved, but there is so much motion going on stability is a fleeting concept. If some air is rising up in altitude, other air is descending – maybe not near by, but due to the pressure change (Lower pressure) caused by the rising air the replacement air is arriving in the form of wind along the surface. Depending on the time of year, the equator gets the most sun, thus is the hottest, so the lower air rises to the top layer of the lower atmosphere, as it cools off it descends at the mid latitudes, but the surface at mid-latitudes is being heated as well, and thus low pressures form there as well, just like they did near the equator. Rising air from mid latitudes rises and moves towards the poles to form another area of high pressure.
The troposphere contains about 75% of all of the air in the atmosphere, and almost all of the water vapor (which forms clouds and rain).
The Coriolis force is zero at the Equator. The Coriolis effect is a (fictitious) force which acts upon any moving body (an object or an parcel of air) in an independently rotating system, such as the Earth. In meteorology, the horizontal component of the Coriolis force is of primary importance, as the most well known application of the Coriolis force is the movement or flow of air and ocean currents across the Earth. Ref: https://www.weatheronline.co.uk/reports/wxfacts/Coriolis-effect.htm
Atmospheric circulation is the movement of air at all levels of the atmosphere over all parts of the planet. The driving force behind atmospheric circulation is solar energy, which heats the atmosphere with different intensities at the equator, the middle latitudes, and the poles. The rotation of Earth on its axis and the unequal arrangement of land and water masses on the planet also contribute to various features of atmospheric circulation. Ref: http://www.scienceclarified.com/As-Bi/Atmospheric-Circulation.html
There are three wind cells or circulation belts between the equator and each pole: the trade winds (Hadley cells), prevailing westerlies (Ferrell cells), and polar easterlies (polar Hadley cells). Read more: http://www.scienceclarified.com/As-Bi/Atmospheric-Circulation.html There is so much to know about the interaction of the air between the wind cells, it boggles the mind.
So we have regions where the air is constantly going up and other regions where it seems to be constantly descending. etc.
Some graphics to contemplate:
This last graphic is a transition point. It is a screen shot of a Windy.com display. Details are not important to this discussion. The learning point is that the high and low pressures systems that migrate over our soaring area, have complex interactions. They are constantly changing. Having a feel of the dynamics will get you to thinking three dimensionally. Hopefully this topical review will cause you to broaden your perspective and take more factors into account as you plan your flights in the future.
If you have had the patience to read this far, it shows you are hungry for more, but let’s be realistic. You could spend decades of your life studying the atmosphere, but all you really only want to know are two things:
- what day should I go flying and, when I do fly,
- where can I find the up air!
It is undeniably interesting to learn about the atmosphere; to study the laws of physics that cause the upper air to be in motion. These same laws apply as well to the air surrounding you as you thermal above Snow Mountain or Yolla Peak. I suggest you review the the discussion presented by the professional meteorologists employed by the National Weather Service. I often end up checking the glossary to grasp the details of some discussions. The Sacramento Office is sto and the San Fran Office is mtr, as in: https://www.weather.gov/mtr/
FYI: I found “Discussion” by inserting “discussion mtr” into the search window, and came up with this:
Browse those NWS pages, but you may come away with the realization that there is more to this than can be easily assimilated.
Monitor the discussions, and evaluate what the pros are describing. Then, go to Windy.com and vary the display until you can comprehend what they were talking about in “The Discussion”.
You should look at the seven (7) day NorCal RASP if you are planning your work or family activity schedule well ahead, but, if you had studied the Discussions and manipulated Windy.com displays sufficiently, you will have a more valuable estimate of which day later in the week might be your best choice.
Acknowledgement: Thanks to Leon McC. – WSC glider pilot for assistance in the discussion of the upper air.
Footnotes, by Author, Peter Kelly
*1 “up air” – From an entertaining post dated 10/13/2018, by Thomas G:
My version of the story, based upon reading this thread:
Thomas was on a flight out of Hollister, and decided he was going to have a “marginal glide” getting back to Hollister. He had been in contact with NorCal due to the routine need for traffic advisories, and informed the controller he was low before switching over to the Control Tower at Salinas Airport.
Before switching “…the Nor Cal controller was extremely helpful giving me winds on the ground from every airport in the area so that I could look for ‘up winds’ as he called them ;)…” The subsequent dialogue on that thread was a good read, with tower requests for “how many souls on board” and “how much fuel remaining”. The pilots at Hollister post some great email threads! This report was on the post: [hgcgroup] Digest Number 4606.
Very educational and informative as well as entertaining. I strongly recommend that all pilots subscribe and keep abreast of the flying in that part of the world. Join: email@example.com
*2 “not in Kansas anymore” – My paraphrasing of a Google inquiry on that expression:
What does it mean – we are not in Kansas anymore?To be not in Kansas anymore: To no longer be in a place that one knows or where one is comfortable; to be in a completely unfamiliar and/or discomfiting environment. A reference to The Wizard of Oz, in which Dorothy, upon arriving to Oz, says, “Toto, I’ve a feeling we’re not in Kansas anymore.”