It's a common occurrence to see temperatures drop rapidly when clouds dissipate above fresh snow cover, and I got to wondering if this "post-snow" temperature drop could be quantified. For example, in the latest event the temperature dropped from 22° to -20°, a drop of 42 °F, and after the early December snowfall the temperature dropped 38 °F from 25° to -13 °F. Are these numbers typical? To look at this, I searched the winter months (November-March) since 1930 for snowfall events of 4 inches or greater, with two-day totals included. I then removed overlapping events and obtained the largest temperature drop within two days after snow ended in each case (243 cases in 85 years). A histogram of the results is shown below.
The historical data reveal that a drop of 40 °F in one or two days is very typical, with the long-term median being 37 °F. It's not at all surprising to see that large temperature drops are quite common, because the thick cloud cover and warm advection associated with these relatively "heavy" snow events initially brings elevated temperatures, but then clearing skies and cold advection behind the system allow for rapid cooling. Fresh snow cover is also very favorable for radiational cooling.
Out of curiosity I looked quickly at the long-term history of this phenomenon by taking a trailing median of the temperature drop associated with the prior 20 snow events, and then applying an extra 7-event smoother, see below:
There have been some interesting variations in the past two decades, with snow events generally being followed by less cooling in the 1990s but considerably more cooling since 2000. I have no useful suggestions on how to interpret this change, so I mention it merely as a point of curiosity. But it's clear that this weekend's chill was exactly in line with the Fairbanks climate of the past decade.
The temperature did drop. We've all seen it happen before after a snow event followed by clearing and perhaps cold air advection from a previously cleared off area in the direction of wind flow.
ReplyDeleteI keep thinking that the humidity from the snow should slow down the long wave heat loss. But looking at the temp-dew point spread it stayed about 5-7 deg through the snow>cold>recent cloud cover events. Must not work like the classic humid jungle versus dry desert nighttime temp changes here.
Is there a humidity threshold below which the rate of cooling effect is minimal for us in Fairbanks during winter? Maybe any humidity is just limited to a shallow layer near the earth.
Gary
Good question, Gary. This would be worthy of more detailed investigation, but I think the short answer is that winter air in Fairbanks holds so little moisture in absolute terms that the absorptivity/emissivity of the warm(er) near-surface layer is very small, and therefore little warming is possible. In other words, at such low moisture levels a very deep layer of warm air would be required to produce enough downward longwave radiation to stem the energy loss from upward longwave emission. In terms of absolute humidity and longwave transfer, it is just like the desert except colder.
DeleteIt should be possible to look at clear-sky cooling rates for varying levels of humidity and see if there is an obvious threshold...
Thanks for the explanation Richard. I guess what's lacking is...what is the absolute humidity at the temperatures and relative values presented? Maybe there's a conversion table somewhere for reference? The clear sky rate of cooling vs humidity would be interesting to see.
DeleteGary
Gary, it turns out that all you need is dewpoint, because dewpoint is proportional to absolute humidity. You would have to go through a table or equation to get an alternative measure like grams per cubic meter, or the vapor pressure.
DeleteHere's a nice little discussion:
http://www.crh.noaa.gov/lmk/?n=humidity
What was also interesting was the time that the -20°F reading occurred; at 2:00 p.m. People often have a difficult time grasping the fact that the low temperature at this time of year can literally happen at any time of day.
ReplyDeleteGood obs Brian...and BB King would sing..."The thrill is gone"...anytime when there's no insolation.
DeleteGary
I was getting use to the idea of having fairly consistent high temperatures. The sudden drop in temp took me a little by surprise. Luckily we didn't go from 0 to -40. Then I would have to plug in my car.
ReplyDeleteThose 70-89 degree drops appear to only be in the 1930s. Is this true? If so, why? Change in observation times? Is there a reason to not think these observations are suspect?
Eric, the early 1930s showed a high frequency of large temperature drops, but they are by no means absent in later years. Here are the numbers of events for 60+F by decade:
Delete1931-40: 8
1941-50: 0
1951-60: 7
1961-70: 6
1971-80: 2
1981-90: 1
1991-2000: 5
I would be much more suspicious of observations from prior to the Weather Bureau/NWS era, i.e. pre-1930. The 1930's temperatures are credible, I think, and reflect a much smaller urban warming effect in Fairbanks.
Thanks, Richard.
DeleteI think how you calculated the second chart threw me off. You have a very high 30s. But this appears to be more of an artifact of the algorithm used - with it being at one end.
I was going to ask if the official weather station moving around throughout the years (Ladd field, downtown, the Airport) would make a difference. But I think the sample size is too small to tell.
Either way, thanks for answering what was on everyone else's minds.