Meteorology: Frequently, infrequently and never asked questions


TTC = The Teaching Company, now known as TGC (see below)
TGC = The Great Courses

Your course is America-centric. Why such a tight focus on America?

Most of my weather examples do come from North America. A minor reason for this is that I was unaware that TGC had an international presence. The major reasons are that: I live in North America, which has a very rich variety of weather phenomena; I primarily study North American weather; in creating the course, I drew upon a set of examples I collected over the years, which are understandably biased towards the Western Hemisphere owing to my research focus; and that much of my weather data comes from the U.S. Government, which overall has a far more generous attitude towards free dissemination of data than weather agencies elsewhere in the world.

You use many acronyms. Can you define them again?

Some acronyms and their meanings:

acronym   meaning   definition   values
CAPE   convective available potential energy   a measure of environmental instability varies
DALR   dry adiabatic lapse rate   subsaturated air expansion cooling rate   about 10C/km or 30F/mi
ELR   environmental lapse rate   rate at which temperature decreases with elevation   variable in space and time; about 6.5C/km or 20F/mi
MALR   moist adiabatic lapse rate   saturated air expansion cooling rate   variable in space and time; about 5C/km or 15F/mi
PGF   pressure gradient force   driving force for horizontal wind   varies

Why do you use deprecated units (miles, Fahrenheit)? This is out of place in a modern science course.

I do refer to imperial units, not only in Meteorology, but also in the lower division class on which my TGC course is based. The reason is simple: I am primarily teaching to people who were raised in the U.S., which unfortunately is behind the curve when it comes to adopting SI units. For these people, measures like 7 miles or 70F are far more immediately understandable than their metric equivalents, which for most of us has to be converted. The average TGC viewer is likely even farther behind the curve when it comes to the metric system. Indeed, I say this in the second lecture:

I'll start off mainly using familiar old English units, such as inches, miles per hour and Fahrenheit, but I'll provide modern metric equivalents as well. In my classes at UCLA, I often use old-fashioned units, such as Fahrenheit and yards, and a colleague once criticized me for that. He asked me why I wasn't using the language of science and forcing my students to speak my language. But I want you to understand what I'm talking about. I'm willing to speak your language and in return, you learn mine, if you don't know it already. As time goes on, you'll notice that I'll emphasize metric measures more and more.

I am pleased with this compromise.

You propagate the old-fashioned theory of the atmospheric greenhouse effect, and the role of carbon dioxide in determining the temperature of the earth-atmosphere system. Don't you know that this is wrong? See, for example...

I wouldn't touch this one with a 10 foot (3.05 m) pole.

I kept waiting for you to mention that at a given temperature, and pressure, humid air is less dense than dry air. I was amazed when I learned that way back in the sixth grade! So at this point I'm temped to conclude that it must not be very important for weather, if you never mentioned it.

It's difficult to search the transcripts, but it does appear that I never mentioned that humid air is indeed less dense than dry air at the same temperature and pressure. That is an oversight I regret, because it represents one of those learning opportunities that have particular power precisely because it directly contradicts our common experience. Almost everyone thinks that humid air is "heavier" because we generally feel more lethargic when the air has high humidity. Yet, in this situation, we are not responding to the air's weight or density.

In a nutshell, as I said, "less dense air rises, more dense air sinks". One of the things that can make air less dense is to get more water vapor into it. So, the fact that humid air is less dense than dry air is indeed important for weather phenomena such as thunderstorms, and I wish I had worked that in somewhere.

You mentioned that Fahrenheit had mixed three substances together, and that they came to the same temperature, which he used as the zero point on his temperature scale. I'm totally puzzled at the thought that mixing three substances together would "always" produce a mixture of the same temperature -- zero Fahernheit. If those substances were each at 300 degrees Fahrenheit when mixed, how would they get to zero Fahrenheit? Almost instantly?

You are asking about what is termed a "frigorific mixture". This page on Wikipedia discusses how Fahrenheit used such a mixture to create 0F on his scale. I would presume that one should start at a temperature where the substances are in the proper phase (at 300F, ice would not exist and water would not be liquid, so that would not work). I also presume that the length of time needed to attain the equilibrium temperature depends on how far the starting temperatures are from that.

An example I can personally relate better to is this: Ice and liquid water also form a frigorific mixture at 32F, if you start pretty close to that temperature. If it's a little colder than 32F, some liquid will freeze, releasing heat that warms the system (ice and liquid) up, back towards 32. If you start a bit warmer, some ice will melt, taking heat from the liquid and causing the temperature to decrease. If you start too far from 32F, however, ice will melt or liquid will freeze without getting the temperature of the system to 32F, so the experiment would fail.

I installed a weather station which gives barometric pressure. When pressure is reported, is it typically adjusted to sea level? My station is at 1600 feet and the software gives me the option.

We usually adjust pressures to sea-level, so we can more accurately compute pressure differences between places at different elevations. Because pressure varies with height far, far more than it varies with horizontal space or time, the difference in station pressure between two locations is dominated by differences in altitude above sea-level.

I'm guessing the software adjusts to sea-level pressure based on your elevation above sea-level and assumptions regarding the average lapse rate near the ground. You're pretending soil and rock is actually air, so there is no precise value for sea-level pressure at your location -- for weather map analysis, it's a very convenient (and necessary) fiction. If you don't need to compare your station's pressure to any other location, however, you can leave the barometer unadjusted, and just monitor station pressure.

Pronunciation of Buys Ballot's name.

Lacking any guidance, I (along with many other meteorologists) pronounce "Buys Ballot" as the French might. A Dutch viewer sent me an audio clip of how Mr. Buys Ballot would have pronounced his own name. The best I can do for you here is to phoneticize (is that a word?) it as "Bousch Bah-LOT".

I might point out that your explanation of the Green Flash is more properly the Green Rim. The Flash needs a mirage component. The Wikipedia article on this does a good job of telling the difference.

Thanks for the heads-up on the green rim. Unfortunately, I have never personally seen either phenomenon.

Do you know where I can learn more about the Santa Ana winds, which you cover in lecture 6?

These three pages that I created might provide a useful start:

You have "a not-good explanation that the larger an object (raindrop, ice particle) the faster it falls (is Newton spinning in his grave?) without an explanation".

Rest assured that neither Newton (nor, more properly, Galileo) are in the least bit discomfited, at least by me. It is clear that both understood the concept of the "terminal velocity". In a vacuum, all free falling objects do fall at the same rate. However, we are talking about particles like ice crystals, snowflakes and graupel: particles that are fairly light, may have a large cross-sectional areas, and are profoundly impacted by air resistance in a real atmosphere. Hailstones DO fall faster than snowflakes, and our common experience tells us this is true. Indeed, it is so obvious, this is why Aristotle's assertion that heavier objects fall faster by their nature (rather than owing to external forces, like drag) went unchallenged for centuries.

In my course, I adopted Einstein's maxim, "make things a simple as possible, but not simpler". In my judgment, explaining why a hailstone falls faster than a snowflake was not a topic I needed to address, and I didn't have time for it in the relevant lecture (#24), anyway. But, if Galileo were spinning in his grave, would it be clockwise or counterclockwise?

After purchasing your "Great Courses" DVD, I was unable to use it, since I don't want to teach my students that the air "holds" moisture. I wonder why you took this approach?

The short answer is that I do refer to air "holding" water vapor as a "convenient fiction", provide fair warning that it is an oversimplification, and explain why it is a physically incorrect, if eminently practical and useful, description.

Now for the longer response. Very few of us emerge from our middle school years without acquiring the notion that air "holds" water vapor and that condensation occurs when air can "hold" no more. Further, we come to recognize that this "holding" characteristic is temperature-dependent, so that warm air "holds" more water vapor than cold air. An immediate consequence of this is that colder air is easier to saturate than warmer air.

Like two other convenient fictions that are prominent in meteorology -- the Coriolis and centrifugal forces -- the notion that air "holds" water vapor has the important property of conforming with our experiences. Further, it provides us with an empirical tool that allows us to anticipate the consequences of important processes relating to moisture. In meteorology, this convenient fiction is enshrined in the form of the "mixing ratio", which is expressed as vapor mass divided by the mass of (dry) air. The actual mixing ratio tells me how much water vapor is present in a particular air sample, while the saturation mixing ratio tells me the maximum amount of vapor that sample can have -- or "hold". I give these two measures the friendly names of "vapor supply" and "vapor capacity", respectively. Supply cannot exceed capacity (not by too much, anyway) and is a very strong function of temperature (and a weak function of pressure). The primary importance of vapor supply is that it is conserved in the absence of sources or mixing. (The parenthetical details above are not crucial to understanding moisture.)

Speaking of the Coriolis force, one correspondent reported that my explanation made him "cringe".

In non-mathematical classes -- and my TGC "Meteorology" course is one such example -- I explain phenomena such as the Coriolis effect using your everyday experience. Newton already told us than an object put into motion remains moving in a straight line at constant speed, unless other forces are acting. We call this "inertia". So, suppose you watch an object -- which could include a rocket or an air parcel -- curve. You need a force to explain its deviation from straight-line motion, whether it really curved or not. After all, you may have only seen it curve, not because it actually changed direction, but because YOU shifted position while watching the object's motion.

Enter the Coriolis effect, which exists only because you see the direction you call "North" as constant through the day, even though an observer looking down on Earth from space sees as constantly shifting, minute by minute. If a rocket goes off on a straight line, and YOU turn to your left, YOU see the rocket turning to the RIGHT, even though the rocket's trajectory is straight.

I find this explanation far more accessible and illuminating that the usual demonstration of children tossing balls on a merry-go-round, or somewhat messy and incomplete explanations based on angular momentum conservation and the centrifugal force. This (PDF) document has a more complete explanation of the approach I employ in the TGC course. No equations, no appeals to angular momentum conservation or centrifugal forces, and no vector calculus at all. Just a description of what you see, explained with respect to your reference frame.

He repeatedly claimed that a rocket launched from the equator and aimed at the north pole would miss because of the Coriolis effect. This is just plain not true, and the professor should know it. If not, he can check with one of the physics professors at UCLA. If the rocket were launched from the north pole and aimed at a specific location on the equator it would miss because of the Coriolis effect, but not the other way around. Going to the north pole it would appear to follow a curved path rather than a straight one, but that path would definitely cross the north pole.

This is actually a strawman argument, and I'm not revealing the answer here, for two reasons: (1) I have used the correspondent's false assertion as a homework problem; and (2) my students know how to use Google :-).

What is the "shaving cream" guy's beef?

I posted a comment to that review, but it disappeared after a short while. I presume that TGC opted to remove it. Here it is, in its entirety:

The astute person is cognizant of a course's objectives, audience and scope. Making a course like this involves many difficult choices. Simplicity is weighed against complexity. The appropriate level of detail has to be determined. Both breadth and depth cannot commonly be achieved simultaneously. In a science like meteorology, which for majors is a very technical and mathematical subject, it is not possible to create a course that satisfies all comers and accommodates all needs.

My course is based on an introductory survey class, at the general education level, as might be found in many universities. As in many (if not all) such treatments, I cover topics like the greenhouse effect and the ozone hole in a cursory fashion. Yet, this is not a course in atmospheric chemistry, oceanography, or climate and climate change. The missing subjects the reviewer complains about in his second paragraph are simply not germane.

Regarding winds and similar phenomena, I adopted the reference frame of the viewer and, as much and as often as possible, couch explanations and descriptions in terms of common experiences and familiar examples. I know from considerable experience with general education audiences that this is a good way of helping people without hard science backgrounds -- by far, the largest segment of TGC's customers -- assimilate sometimes difficult or abstract concepts as simply and painlessly as possible. The reviewer argues from the point of view of the physics purist, bristling at our "pseudo-forces". He would likely be as disappointed with the most complex dynamic meteorology texts written by my field's greatest scholars. The very first thing we do is take Newton's laws and, quite literally, bring them down to Earth.

The review by dcmitch provides an interesting counterweight against the those that complained that my course is too technical, and that I treat the viewers like "meteorology majors" (as if!). This course is not for everyone, and is certainly not intended for persons who merely want pretty pictures, but nor is it for physics bigots who arrogantly look down on the the rest of us. It is for people who have experienced the wonder and terror of the weather, and want to know a wee bit more about the "how and why".

The very first thing you say in this course is "Nature abhors extremes". Why are you anthropomorphizing nature?

I've been told not to anthropomorphize Nature, because she really hates that.

Seriously, I adopted this theme as a riff off the time-honored maxim, "Nature abhors a vacuum", which probably dates back many centuries. Extremes, stresses, amd imbalances have consequences, and cause circulations, many of which helpful or at least benign, but some of which devastating and extreme in their own right.

The electromagnetic spectrum diagrams shown in Meteorology Lectures 3 and 4 show the visible spectrum as 400-700 microns rather than 400-700 nanometers (or 0.4-0.7 microns, as you say).

That was a mistake made when the TGC art department redrafted my figures. I'm not sure why they did that. Unfortunately, I didn't catch it when I previewed the videos. In my opinion, that's far from the worst blunder, though! See below.


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