Monday, January 13, 2014

Cold and Visibility Trends

In a continued quest to fully understand the long-term decline in extremely cold days in Fairbanks (see previous posts here and here), I decided to look at historical visibility observations to see whether any notable changes have taken place in tandem with the warming that has been observed.  Part of the motivation here is that I have long entertained a hypothesis that increased human emissions of water vapor on extremely cold days might be allowing ice fog to form more frequently and at higher temperatures than in earlier years, and that this could slow or prevent the drop of temperatures to extremely low levels (from latent heat release or changes to the radiation balance).  I came across this idea in a blog post by Haines, AK resident Jim Green a couple of years ago:


An initial examination of the data fails to support the hypothesis, because it appears that low visibility conditions have actually become less common in Fairbanks over the years.  The chart below shows the annual (seasonal) fraction of hourly observations in December through February for which the visibility was 1/4 mile or less (red columns), 1/2 mile or less (blue columns), and 1 mile or less (green columns); the purple line is the 15-year trailing average of the 1/2 mile or less frequency.  There was a rather pronounced increase in low visibility conditions from the late 1960s to the mid 1980s, but in recent decades there has been an overall slow decline in the frequency of fog conditions.


Since I looked at McGrath in the previous post, and Jim Green's article made a point of comparing between McGrath and Fairbanks, below is the corresponding chart for McGrath (no hourly observations prior to 1948). Note that the vertical scale is much different from the Fairbanks chart, because low visibility is much less common in McGrath in winter.  McGrath also shows a decline in frequency of 1/2 mile or less visibility in recent years, with the 15-year average of 1/2 mile frequency reaching its lowest level in the past few years.


The following histograms illustrate the multi-decadal changes in the winter frequency distribution for visibility at both locations.  In Fairbanks, the frequency of sub-1 mile visibility dropped from 9.3% to 5.8% between 1951-1980 and 1981-2010, while in McGrath the rare cases of less than 1/2 mile visibility became even less common.




Assuming that the historical data are reliable, then, it seems clear that low visibility conditions have become less common in Fairbanks in winter.  This rules out the basic initial hypothesis, but still leaves the question of whether fog frequency has become more or less common at comparable (low) temperatures.  In other words, we know that ice fog is a common occurrence when temperatures dip to very low levels, and we know that long-term warming has occurred; so do we still see as much fog at the same temperatures (e.g. -40°), or is fog less common even at the same temperatures?  To answer this we need to look at the joint distribution of temperature and visibility.

The next two histograms show the visibility distribution for only temperatures between -50 °F and -40 °F inclusive.  We see that there has been a modest increase in the lowest visibility (less than 1/4 mile) category for Fairbanks at these cold temperatures, and a slight decrease in the 2-5 mile category, but overall the distribution has not changed a great deal.  The changes are actually more dramatic at McGrath, where visibility less than 1/2 mile has all but disappeared in recent decades under these cold conditions.




The next chart shows how the mean visibility varied with temperature for the two locations and in the two 30-year periods (red and blue for Fairbanks, purple and green for McGrath).  [Update: note that an earlier version of this chart failed to account for the change to automated visibility measurements, which do not report values greater than 10 miles; so I have updated the chart to show mean visibility after truncating all values to 10 miles.]  Again we see that the Fairbanks visibility distribution is not greatly different between the two periods; there is a modest decrease at the lowest temperatures but an increase between -40°F and -25 °F.



Given that visibility and temperature are still related in much the same way at Fairbanks, it seems we can conclude that the decline in low visibility conditions is largely attributable to the declining frequency of very cold conditions; ice fog is not as common as it used to be, because it's not as cold as it used to be.  However, in McGrath there has been an interesting decrease in the frequency of ice fog that can't be explained by the warming trend.

In relation to the initial hypothesis for Fairbanks warming, there seems to be no evidence that changes in fog frequency have contributed to the pronounced lack of extremely cold temperatures in recent decades.

Finally, for those who find this as interesting as I do, I'll include a couple more charts I made during the investigation; these show the temperature at which 1/2 mile or less visibility was observed, for both sites and both periods.  Fog in winter in Fairbanks is observed mostly but not exclusively at very low temperatures, but in McGrath it occurs mostly at relatively high temperatures associated with frontal zones and mixing of contrasting air masses.  Both locations have seen a sharp drop in the proportion of low visibility events at very low temperatures: fog is now more often associated with warm conditions than it used to be.



16 comments:

  1. Very interesting Richard. Anecdotally, the ice fog tends to form in Fairbanks around -30°F while it needs to be -40°F or colder in less populated areas.

    I think that the visibility observation technique went all automated in 1998. Before 1998, there were visibility observations of 60 miles or more on occasion. After 1998, 10 miles seems to be the max. That would certainly affect the averages. It would be interesting to have 1998 as a breakpoint to see how the visibilities changed with the change in observation technique.

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    1. Brian,

      Thanks for pointing that out - indeed it was an oversight and makes a big difference to the calculations of average visibility. I updated the histograms to combine the 10-20 mile and 20+ categories, and re-did the calculations for the averages. No more strange decline in average visibility in McGrath!

      I agree it would interesting to look at the bias in visibility values pre- and post-1998; I'll leave that for another time.

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  3. Other variables need to be taken into account before your hypothesis is really null. Ice fog does requires cold temps. But also water vapor and particulates. How have those changed in the last few years? Also, the airport measurements have changed location. Basic observation today tells me that this should matter. I think you picked a really interesting thing to look at. Unfortunely, might not have what is needed to answer it fully.

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    1. Eric,

      Thanks- you're undoubtedly right that many factors have changed that might influence fog formation. However, the hypothesis that fog is forming more frequently is clearly not supported (if the visibility data are to be believed) - fog has become less common, not more common, in the last few decades in Fairbanks. This doesn't mean we affirm the null hypothesis, i.e. that fog processes are not changing; I'm sure they are in fact changing in various complex ways. We simply reject the notion that I had pondered for the past two years.

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  4. I deleted my first post because of many typos and no way to edit. My phone is a pain....

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  5. Good topic and yes, ice fog and associated low visibility is less frequently observed in Fairbanks in my experience. It may not be as cold as long as it was for some periods, but there've been other changes.

    From a weather standpoint it seems like there's more surface wind flow during the recent cold snaps. The last few days, while not below -40F much, there has been a slight flow through town (<5 mph I'd guess) based upon visual obs of heating and vehicle exhaust, and felt cold while out walking my dog. It was clear last eve where I live at -38F with a slight felt breeze from the north. I'm not sure of the source of the flow, but will pay attention through the coming winter to see if it continues.

    Since the Clean Air Act of the early 1970's, automotive and commercial vehicle emissions have declined per vehicle. That's due to changes in engine design, that while of great interest to me, are beyond the scope of this post. Fuels have been refined to meet emission standards. CO2 and H2O are still produced, but *may* be reduced compared with vehicles older than the mid-'70's which are becoming extinct. They simply run better now in the cold as well.

    Home heating systems and fuels have been engineered to be more efficient and cost effective. Along with that homes have been designed to require fewer BTU's per heating area to maintain a level of interior comfort. By design today most homes likely emit fewer combustion byproducts than previously.

    We don't seem to be having the extended cold spells of some winters. It takes days of deep cold to form ice fog. After another brief cold snap it's forecast to warm again. Good deal, I hope the pattern continues this year.

    Gary

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    1. Gary,

      Wind speed and direction will be the next topic in relation to extreme cold frequency...

      Good point about cleaner combustion - I suppose it's quite possible the air has actually gotten cleaner in some respects despite larger population and larger total energy usage. Perhaps in the "old days" the anthropogenic effect involved more particulates, less direct warming, but now it's more direct warming and less particulates.

      I see the forecast is showing deep southerly flow and another big warm-up. Let's hope the hydrometeors all stay in the solid phase this time.

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    2. Hello Richard. My earliest weather comments and questions to Rick, Martha Shulski, and now here were regarding perceived changes to wind and Fairbanks/Interior Alaska weather. They may be cyclic over decades, but with some observed certainty they have varied in my residency since 1965.

      Whether it's been driven by temperature or pressure gradients in Interior Alaska, and at what level of the atmosphere, I don't know.

      Below I mused about humidity (not sure if RH or dew point are the proper parameters) and any correlation with reported visibility at the surface. Assuming they are related in extreme cold (?), has 'humidity' tracked historic cold and visibility trends, your initial subject? The database may be driven to ambiguity by sensor type and sampling location.

      Wind and duration of cold may be stronger influences forcing local weather.

      Gary

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    3. Add cloud cover to the last comment above.

      Gary

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  6. Here's some questions that've bothered me for some time. Are relative humidity and dew point related to observed surface temps and visibility? I assume they are, but for the last 24 hrs the reported values for each have remained stable, while both the vis and temp have varied some. I suppose at some point 100% RH and DP become fixed, but how can we have 100% humidity and a dew point still below the reported temp?

    How does the NWS measure those parameters? If it's a consistent technique over time, what's the long term relationship between them and reported surface temp/visibility/ice fog?

    Gary

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    1. Gary, I'm not sure I have all the answers, and Rick will have more definitive info on the measurement practices. I believe modern ASOS platforms measure temperature and dew point (frost point below freezing) and then relative humidity can be computed by the end user or software; and if dew point is below temperature, then yes, RH should be below 100%.

      I see the METARs from PAFA were missing the dewpoint measurement for most of the past day or so. Thus if you were looking at airport data that included RH, could it have been outdated or erroneous?

      I imagine that manual measurements of humidity, e.g. with a sling psychrometer or wet (ice) bulb, would be very challenging or impossible at extremely low temperatures, so I would have little faith in any older historical dew point data.

      Concerning the relationship between ambient humidity and ice fog, I would have to read up on the physics of ice fog before I could comment usefully in that regard.

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    2. http://journals.ametsoc.org/doi/pdf/10.1175/1520-0450(1965)004%3C0446%3ATEOSIC%3E2.0.CO%3B2

      A search of "ice fog ambient humidity" detects several others subsequent that that seminal paper, including several EPA documents of some magnitude (APTD0626,
      600379100 for example) .

      I've not read these but will do so soon. Humidity initially appears to not track ice fog due to the assumed physics of formation. After all this speculation on my part maybe I will learn something about ice fog and associated factors.

      PAFA obs seem intact via these links. Might be in error:

      http://aawu.arh.noaa.gov/newobs.php
      http://pafg.arh.noaa.gov/obs.php

      Gary


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    3. Thanks Gary, that is rather an interesting paper. It's fun that cold weather brings out so much interesting physics that does not pertain to most of the world.

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    4. Forgot to mention... the DP and RH values from early yesterday are definitely erroneous. The sensor was not reporting DP until 18Z.

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  7. I'm sorry, Richard. I misunderstood what you were doing; too early in the morning. One posdible way to quantify how particles have changed over time to see if there is a ice fog correlation is to use the borough's air quality measurements. I don't know how far back they go but they might be useful.

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