You hear this a lot in Chicago. “We are having a cold snap because there is a low-pressure system over the Midwest and a high-pressure system to the North. This causes windy conditions which brings cold air down from Canada.”
This sounds better than just saying “it’s cold today” but I can’t tell if it really is saying anything more than that. First of all, as I have said before the following two statements are equivalent, at least empirically:
- The air is colder than usual
- The air was blown here from some place colder than here.
So telling me that the air came from Canada isn’t telling me much more than I already knew, it’s cold. But the extra bit here seems tautological at an even deeper level because these two statements:
- The air is blowing from down Canada
- There is high pressure in the North and low pressure here
appear to be literally the same thing. Why else would the air move from position A to position B if it were not due to pressure imbalances?
Is meteorology really just like finance? (“Stocks fell today because of bearish investors”) Or is there a non-circular way of explaining my frozen toes that just doesn’t fit into a 30 second weather report?
(drawing: glooming from http://www.f1me.net)
Cheap Talk 
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December 10, 2010 at 1:32 am
Andrew
That seems a bit unfair. You could say the same thing about all of science. “He has sickle cell anemia because he has a homozygous mutation in his hemoglobin gene.” “Chlorine is very reactive because it is missing one electron in its outer electron shell.” “The object is moving a constant velocity because no net force has been applied to it.” Aren’t these all “tautological” in the way you describe for meteorology? And yet it’s still an advance over “He has sickle cell anemia because evil spirits have possessed his blood” or whatever.
December 10, 2010 at 4:10 am
ramblingperfectionist
I guess the difference might be that the investors are bearish argument is occasionally an end in itself, and there are no other reasons beyond it, whereas the meteorological phenomena always has some sort of more complex explanation, even though most people reading the weather might not know/can’t explain it?
December 10, 2010 at 7:29 am
Zach
It seems like there are 3 different levels of explanation in your Chicago example:
1. It’s cold today. This is the same as saying the air is cold today
2. The air is cold today because it blew in from somewhere cold. The air could be cold because it lost heat through radiation into space last night.
3. The cold air blew in from Canada. The cold air could have blow in from the midwest, or somewhere else cold.
This leads to the final, unanswered question: why is there high pressure in Canada today, and low pressure in Chicago?
December 10, 2010 at 8:44 am
Nate
a fair comparison to finance. in both cases, it is worthless to write more than 5 words about a 1 day event. but longer term forecasts can benefit from nuance. in particular, stating the mean expected outcome and discussing risks cold or warm to that expectation. then the variables become much more than high pressure in canada.
December 10, 2010 at 8:50 am
tg
Zach, why is there low pressure in Chicago? Easy, because it’s cold!
(PV = nRT)
December 10, 2010 at 9:35 am
Alan Gunn
It is possible to predict the weather with considerably better accuracy than guessing, at least for a couple of days ahead. Furthermore, weather forecasting has improved a lot over time because of better technology like radar and satellites. This shows conclusively that there is a lot more to weather forecasting than to financial forecasting, which is still at the alchemy stage. And at least the forecasters’ explanations of what has happened are true, unlike the financial stuff like “the market dropped because of profit taking by people who made money yesterday.”
December 10, 2010 at 11:31 am
phancy
Well, it’s not tautological in the sense that the pressure differential explanation at least rules out other explanations, e.g., it was blown here by an angry cloud with big cheeks. Or, perhaps more instructive, something like: there have been no clouds, which makes it colder on the ground (although maybe not having clouds is really the same thing as saying high pressure?)
December 10, 2010 at 11:01 pm
Vinnie
You can name names. We know you’re talking about Skilling. 🙂
December 11, 2010 at 1:38 am
itovertakesme
These low pressure explanations are important even if just for the sake of communicating we are having cold weather for the usual reasons. Weathermen are standing by for the day when they have to say something different like “it’s cold today because the earth has spun out of orbit.”
December 26, 2010 at 6:56 pm
Mike from Toronto
Like you, I’ve noticed the tendency for Midwestern TV weatherthingies to talk about “cold air blowing down from Canada”. But I don’t think that’s an attempt to explain the phenomenon: it’s just assigning the blame.
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