Background
As the air begins to cool for the season, the freezing level gradually drops. Occasionally, it drops low enough to cause snow to form at the elevation below the surrounding peaks. Fig 3 shows the percent of the time that the weather balloon launched twice daily in Anchorage is less than or equal to +1°C (34°F) at the 850 mb geopotential height (approximately 4,700' in August). This is a temperature where it's cold enough for precipitation to fall as snow. On July and August days when this happens, the average elevation of the 850 mb height level is 4,600'. When precipitation is falling, that cold air is dragged lower down in the atmosphere hundreds of feet.
If you ask the average Anchorage resident, the most common description of Termination Dust is the snow that marks the beginning of the end of fall. It's a marker for people to do the things necessary to get ready for the upcoming winter.
Criteria
OK, so that's a little of the background. Now, as an Anchorage-based climatologist, EVERY SINGLE YEAR, when snow first hits the peaks, I get asked "is this Termination Dust?" My first response is that there is no one definition. Everyone has their own idea of what it is. Does a coating along the top of the mountains in July count? What about the snow that frequently dots the top of Pioneer Peak above 6,000' as seen during the Alaska State Fair in August? Is that Termination Dust?
Without a consistent definition, there's no way to track how it changes over time. As our climate warms, we'd like to have some data to show how this aspect of the climate responds to increasing temperatures. The Nenana Ice Classic and Fairbanks green-up are some other Alaska-specific seasonal markers that we note the annual trends of because someone took the time to mark down when they occurred.
As a climate scientist and a long-time Anchorage resident, I have given this a fair amount of thought over the years. Here are my criteria for Termination Dust:
1) Must occur on or After August 1st. Period. Since it represents the beginning of the end of summer, it makes no sense to allow a snow that occurs before peak summer temperatures typically occur (mid to late July). Inspection of Fig. 3 shows that early August is when the atmosphere at 4,000' to 5,000' begins to cool off.
2) There must be a solid snow cover above 4,000' as viewed from Midtown Anchorage across most of the front range. There should be no ambiguity. If one person thinks it's Termination Dust and another does not, it probably isn't. I chose 4,000' for the solid snow because most of the tops of the mountains in the Chugach Mountain front range that you see from town go above 4,000'. At this elevation, more that 50% of the mountaintop horizon is covered. Photo 1 has an annotated drone photo showing Front Range peaks with their elevation values and where the 4,000' line is as viewed from south Anchorage. [The perspective from other parts of town will be different.]
Photo 2 is also a drone photo with an approximately 3,800' snowfall from 2020. Photos 3 and 4 show a near Termination Dust right at 4,000'. Photos 5 & 6 show lower elevation snows. These lower elevation snows are too low to be a requirement to count as Termination Dust, but if it's the first event, they obviously count. Remember, this is an Anchorage-specific definition. Many places across Alaska now incorporate the Termination Dust terminology to describe their local first mountain snow.
Alternatively, any accumulating snow down to 3,500' should count. At this elevation, Flattop Mountain gets snow.
3) The snow must hang around for at least 12 hours and still cover most of the areas half a day later. If it snows and then quickly melts due to sun, warm temperatures, or rain, then it doesn't count. You need several inches of snow to survive half a day hours from mid-August to mid-September at 4,000'. Since there are no trees above this elevation, the snow is sitting on bare ground.
4) There is only one Termination Dust event per season. By definition it is a "first" event. If there's a Termination Dust, then it melts away, and another event comes along two weeks later that looks exactly the same, only the first one counts.
Examples: Are the following First Snows Termination Dust?
A) A 4,000' solid snow cover across the horizon on July 15th. Not Termination Dust
B) Snow dots the tops of the 5,000' peaks (e.g., O'Malley Peak, South Suicide Peak) on August 15th. Not Termination Dust
C) Snow covers the peaks in the Eagle River valley on September 1st above 4,000' but not the peaks visible from Midtown above 4,000'. Not Termination Dust (for Anchorage)
D) Heavy snow covers the Front Range at Flattop Mountain elevation and persists until the next day. Termination Dust
E) A light accumulation of snow covers the Front Range at Flattop Mountain elevation but melts out a few hours later. Termination Dust.
F) Heavy snow covers areas above 4,000' for several days and then melts out but no one sees it because it's in the clouds the entire time. Not Termination Dust
G) No snow in the mountains all Fall, but then snow falls everywhere down to sea level on October 1st. Termination Dust
Remarks
Even with a somewhat formal definition outlined above, there will still be some subjectivity. Perhaps a consensus by observers at the National Weather Service in Anchorage can make the "call" just as they do in Fairbanks for green-up.
Photos
Photo 2. Similar perspective as Photo 1, but with actual snow near the 3,800' level. Photo credit: Brian Brettschneider.
Photo 3. Snow level at 3,800'. Snow is covering Kanchee, Konoya, Tikishla, and Wolverine Peaks. Far peak on right (no snow) is Rusty Point. Photo credit: Brian Brettschneider.
Figures
**********************************************
Fig 1. Definition of Termination Dust from the Dictionary of Alaska English (Tabbert 1991). Thanks to Annie Zak for the image ( https://twitter.com/annie_zak/status/779465428217176064 ).
Fig 2. Early reference to Termination Dust from the September 22, 1953, Fairbanks Daily Newsminer. Thanks to Rick Thoman for the image ( https://twitter.com/AlaskaWx/status/779486539923337216 ).
Fig 3. Percentage of the time that the 850 mb temperature in Anchorage is ≤ 1°C (<34°F).
Acknowledgements: A very special thank you to Rick Thoman for the research resulting in Fig. 2, the suggestion for Fig 3., and as a sounding board for ideas.
Termination Frost?
Is there such thing as Termination Frost? The first time of the year there is a little frost on a car windshield, people note it as the first frost of the year. From a growing-season point of view, I would argue that there needs to be significant coverage of frost on grassy (or low-lying) vegetation at a location for it to be considered Termination Frost.
