Monday, January 26, 2015

Really Cold Snow

The other day Rick noted that snow was accumulating on Keystone Ridge with a temperature of -25°F. That got me wondering how low the temperature can go and still receive accumulating snow around Fairbanks. With the help of daily climate summaries and hourly observations, we can identify the very coldest days with measurable snow. A key part of this analysis is that we eliminated all observations with fog in the wx code. The reason for this is because I am using visibility as a proxy measure for accumulating snow. I sometimes wonder if dense ice fog creates a rime layer that is counted as new snow or if ice crystals can pile up into a minor accumulation. I expect the Fairbanksans reading this to chime in right about now.

In conversations with Rick and Richard, they both agree that a snow observation with a visibility of 3 miles or less will start to accumulate (actually they suggested 4 or 5 miles). Since ice fog nearly always forms when the temperature is colder than -35°F, for practical purposes, this means true snow events with temperatures under this value are not captured in this analysis.

Hourly observations that are coded to identify snow go back to 1960. I paired all of the hourly observations with the daily climate summary to see which days had accumulating snow. The reason we do this is because a day might have a high of -10°F and a low of -35°F with 0.3" of snow. But did the snow fall when the temperature was -10°F, -20°F, or -35°F? That is why pairing the daily and hourly observations makes a huge difference.

Table 1 shows the dates since 1960 (54 years) that observed accumulating snow (snow wx observation, visibility of 3 miles or less, and no fog or drizzle) with a temperature of -25°F or colder. In the table, the column labelled "Warm Snow Obs" is the warmest hourly temperature that met the selection criteria on that date and the column labelled "Cold Snow Obs" is the coldest hourly temperature that met the selection criteria on that date.

Table 1. Top 25 coldest "pure" snow observations (no fog) since 1960 at the Fairbanks International Airport.

Remember that the idea here is to set practical bounds on accumulating snow and not to identify extremes. In Table 2, you will see how fog plays havoc with the analysis. On December 28, 1964, one observation met the aforementioned selection criteria (temperature was -35°F) but other observations with fog probably has a snow intensity sufficient to accumulate.

Table 2. All snow observations on December 28, 1964 at the Fairbanks International Airport.

It seems that snow can occur at, and just under, -30°F every once in a while in Fairbanks. At that temperature, there isn't very much moisture in the air (mixing ratio about 0.2 g/kg). Unfortunately the hourly observations and the daily summaries leave much of the story untold. This is a great example of where local knowledge can supplement technical documentation.

1. For the benefit of other readers, here's a post on cold snow at Keystone about 3 years ago: http://ak-wx.blogspot.com/2012/01/cold-snow.html?m=1. And another post one year ago about extreme snow and extremely cold temps: http://ak-wx.blogspot.com/2014/01/cold-snow.html?m=1.

As about the ice fog - It's common to walk out in the morning to your car at -40℉ and find a thin layer of dirty snow on your car. The ice smog slowly accumulates over time. But I don't think I've seen it be thicker than 1/8". And at even colder temps the ice fog starts to dry out a little since when you get that cold it's been below -30 a little while and some of the moisture has condensed out. Gary has a few more decades on me and can probably fix my observations.

I want to point out also that ice fog can start to form at -30℉, not just -35℉. It's usually light if it does but it might make a difference.

1. Your obs are good Eric...unfortunately for some the longer we linger the less we observe.

A couple of opinions regarding cold and the potential for snow:

http://www.theweatherprediction.com/habyhints/222/
http://www.weatherimagery.com/blog/too-cold-to-snow/

I believe any situation that involves warmer moist air overrunning a cold layer at the surface can create ice crystals and what we may commonly call snow. It takes the right incoming direction for the overrunning layer for Fairbanks. SW is best as that avoids the drying effect of the Alaska Range to the S/SE. Nothing much comes from the North but cold.

Another contributor to cold wx precip are the three nearby coal fired power plants and their exhaust plumes. Depending upon air flow and the strength of the inversion, they can create the accumulation Eric describes. Is it snow?

Observe them here during cold weather events. The picture changes with time and may not show the exhaust on warm days at the surface:

http://co.fairbanks.ak.us/airquality/CRCurrentPhoto.jpg

Gary

2. The local observation station (PAFA) is located at Fairbanks International Airport which is mid-picture at the extreme right of the image linked above. Typically it's covered with a layer of white ice fog during cold events, and can be visible at night via surface lighting.

Gary

3. I'll like to add that besides the University, downtown, and Ft. Wainwright power plants, there is also the smoke stacks of some houses and businesses that can cause the ice smog. Some areas are infamous for smoke and the extra particulates increase the "snow".

If we want to separate real snow from the effects of human-induced "snow" then observations from a smoke-less area is needed. But those would be difficult to find.

2. Thank Eric and Gary. What about frost buildup on an ice fog day? Here in Anchorage, a thick frost layer builds up when we have day-after-day of freezing fog. My snow depth measurement increases by a few tenths. Do you think that happens with ice fog in Fairbanks?

1. Yes. My deck, protected from snow by the house overhang, typically is covered with a very fine layer (<1/8") of ice crystals and particulates after an event of ice fog. Every time.

The stuff drifts about with whatever air currents are prevalent. If gathered and melted it forms a brown/gray solution of fine suspended particles in a glass.

And Eric's obs are good regarding home and business emission of particulates...instead of condemning only wood stove burners, the local air police would do well to require (or better yet offer an incentive for) periodic maintenance of oil burning furnaces.

Gary

2. Interesting effects at those low temperatures to be sure.

3. The hoarfrost is thick this winter on vegetation and wires at valley level. I wonder if anyone measures the extent of cover? It surely indicates some atypical condition as not every winter develops as much. Is that what you're seeing in Anchorage?

Gary

4. It seems like there's a 30-day period around the solstice where everything gets coated in thick frost. Once the wind kicks up the frost blows off or sublimates. Of course we don't have ice fog in Anchorage (last occurrence was Dec. 1961) – just freezing fog.

5. Wind....maybe that's the secret? We've had none all winter unlike last. Hmmmm. Why no wind in Fairbanks? My radio antennas are covered with frost. Very unusual.

Gary

3. A little off topic but another idea related to cold temps. I've noticed that when we get a cold snap that goes colder than -30 for more than two days, the forecast almost always underestimates how long the coldness will last and how deep. Perhaps we can find how the temperature of a cold snap relates to the persistents of the coldness in relation to forecast.

1. I think you are correct Eric. In the Area Forecast Discussions, the Fairbanks and Anchorage offices often note that models try to break down patterns too fast.

In addition, as noted in earlier posts, long range forecasts always trend toward climatology, which, by definition, show the breakdown of stable patterns.

2. Eric, you raise an interesting point - is there any potential for improving medium-range forecasts by taking account of "persistence climatology", i.e. the average persistence (or perhaps recurrence) of an anomaly? Generally MOS forecasts should beat persistence climatology, but it's conceivable that in low-skill regions like Alaska, the historical statistics may have useful information to add.