The past couple of days have seen an interesting example of the winter-time effect of clouds on temperatures in Fairbanks. With the noon sun now less than 5° above the horizon, solar heating is very small and there is a more-or-less permanent inversion of temperature near the surface. When clouds are present, the infrared radiation they emit provides a warming influence, especially when the clouds are thick. However, when clouds are thin, high, broken, or absent, then radiation from above is reduced, and surface temperatures can drop - sometimes dramatically.
The chart below shows the ups and downs of temperature in the past three days at Fairbanks airport, and the gray squares show the amount of cloud cover as reported by the ASOS; the cloud cover scale is on the right. I've also added a darker gray shade to denote hours when snow was falling, indicating that the clouds were relatively thick; the light gray squares indicate cloud with no snowfall.
Note the rapid drop in temperature on Thursday afternoon (Nov 23) when snow ended and breaks appeared in the clouds; but then the temperature began a steady climb back up to around 0°F as clouds thickened and snow occurred again on Thursday night and Friday morning.
A similar evolution then happened yesterday afternoon/evening, with thinning and breaking cloud allowing the temperature to drop back to -15°F, but then again snow and clouds pulled the temperature back up to 0°F by mid-morning today. Until this morning winds were calm or light throughout this period, so the temperature changes were not related to changes of air mass.
And for a third time, and even more dramatically, dissipating clouds have once again allowed the temperature to drop very sharply in the past couple of hours. It's a simple energy balance: the surface loses heat by upward radiation at a fairly constant rate, and as soon as the return flow of energy diminishes as clouds disappear, the temperature responds. Without the beneficent presence of clouds, the cold of interior Alaska's winter would be much, much more severe.
Thank you James for explaining in details to us ordinary folks how cloud cover can impact temperatures in high latitudes locations. Having lived in the tropics/subtropics for most of my life, we experience cooling temps during thick cloud cover, and warmer temps with clear skies in the winter and summer alike. I never understood why it would be the opposite for places up north. What is the minimal latitude for inversion to take place?ReplyDelete
Good question; it depends on what sort of duration we're talking about for the inversion conditions. Short-lived surface-based temperature inversions can form overnight almost anywhere on earth under the right circumstances, but at low latitudes they are removed shortly after sunrise. Speaking from experience, inversions can last a few days at latitude 50° in winter without snow on the ground (when strong high pressure is overhead), so I imagine a few days of continuous inversion are possible even at 40-45° if there is snow cover under a cold Arctic air mass (in central Asia, for instance).Delete
Semi-permanent winter inversions must be confined to latitudes closer to 60°, but I can't guess what the limit might be. Perhaps I'll take a look at some data to find out!
Yesterday afternoon Fairbanks was at the northerly edge of a southerly low deck of cloud cover. It was clear in the hills north of town with them well lit by sunlight passing over the clouds, while at the airport snow crystals fell and we were in an overcast. I suspect the clouds blanketed the lower Tanana valley with reduced visibilities in spots.ReplyDelete
When it finally cleared it cooled quickly.
Please explain an “okta”. Eighths of the sky? Sectors? Or weighted toward the zenith? Also during radiation cooling with little wind, (it seems) a downslope breeze develops. How much does this horizontal circulation add to cold temps in valleys?ReplyDelete
Sorry, yes - one okta equals one eighth of the sky covered with cloud. Note that the ASOS ceilometer points vertically up and only reports clouds below 12,000 feet AGL, but nevertheless the data are very useful. Pros and cons of the instrument are discussed hereDelete
The effect of downslope breezes will vary with location, but in general with a shallow inversion even a light breeze will tend to produce surface warming as the vertical gradient is mixed out. In general I think the coldest valley temps develop in-situ rather than draining in from elsewhere; but as you note it is rarely calm on sloping terrain owing to the cold air drainage.
Nice graphs on this site! Are they ACIS-generated?ReplyDelete
Thanks. I may have used ACIS charts on occasion in the past, but this one was made in excel.Delete
What is needed to make such graphs? I'm particularly interested in graphical depiction of cloud cover properties.. Unfortunately, NOWData doesn't provide sky cover values, even though suggestions have been made. Iowa State University has a neat set of graphs, though most cover only MidWest. I have an access to the sky cover data of interest, however, there are ambiguities present in converting FEW, SCT and BKN to a numerical values format. Do you know if it's possible to aggregate the codes into tenths, as is usually done on CF6's? Finally, earlier this year I did an independent research on historical sky cover data in Nashville, TN and came to similar conclusions you did years before (unaware, at the time, of your study). Due to great distances and very different climatic conditions between Alaska and Tennessee, these [conclusions] shouln't be overlooked. The purpose of my research was to validate the perceived change in cloud cover, as well as to support a hypothesis that it may, at least partially, account for increased frequency of very warm years in recent decades. Though the cause and effect aren't clear in this case, the data can hardly be considered a statistical phantom. If interested, I could provide a summary of my findings.Delete
Fred, I simply took the hourly METAR reports and did a crude conversion to oktas, i.e. FEW=1.5, SCT=3.5, BKN=6. The ASOS measures cloud cover in percent, but I doubt those raw values are available anywhere. I don't know how the CF6 values are computed, but note the CF6 data are supposedly only for sunrise-sunset (although Barrow is still reporting daily values).Delete
As for the excel chart it was just a matter of overlaying a couple of series with appropriate formatting of the markers.
Feel free to send your findings to me at richard.james (at) prescientweather (dot) com. I'd be happy to take a look at your methodology and findings.
Hello again, Richard,Delete
Just checking on an email I sent you a few days ago. It was from email@example.com. Not sure if it was delivered properly, or ended up in the Junk folder. Please, let me know if you got it.