Dew point

Johnson Pass Trail – Aside from the noisy crunch of fat tires on rough, white pavement and the Paris-Roubaix-like bouncing of the bike at speed, the riding here Saturday on one of the most popular mountain bike trails within an easy drive of  Alaska’s largest city was phenomenal.

And we had it all to ourselves.

Not a single motor vehicle occupied the north trailhead just off the Seward Highway when we pulled in at about 8:30 a.m., and only our vehicle was there three hours later as we wrapped the day’s ride nervous as always that the white pavement could turn into a mushy trap.

All one could surmise after reading a Facebook page popular with the small army of fat-tired cyclists now inhabiting the Anchorage metropolitan area – home to more than half the Alaska population – was that others might have stayed away unaware there is temperature, and then again there is temperature.

Strangely enough, Alaskans who travel spring snows – whether by fat bike, snowmachine, skis, snowshoes or even on foot – might be able to learn a think or two from Georgia farmers who use frost-preventing irrigation systems to try to keep their crops from freezing when temperatures dip.

What the Georgians have learned is the importance of wet-bulb temperature.

“If the humidity and dew point temperature are low, the wet bulb temperature can be quite a bit lower than the ambient air temperature,” the University of George notes. “This is important for farmers because the wet bulb temperature is the temperature that irrigated fruit will reach when it is continuously sprayed with water. If the wet bulb temperature is too low, the fruit will freeze and quality will suffer, or the crop could be wiped out.”

Georgia farmers want to avoid the freeze to save their crops, and Alaska backcountry travelers want to find the freeze to avoid wallowing, sometimes even getting trapped, in spring slush.

The same physics apply in Alaska as in Georgia, and they’re worth understanding in years when the weather proves especially fickle.

Strange March

It has been an unusual March in and around Alaska’s urban core. Temperatures have mimicked those of April when most people are off the snow and moving on to soon-to-be summer activities.

Some appear unaware that the winter’s lingering snowpack can freeze hard overnight even if air temperatures remain above freezing, and overnight air temperatures in the Anchorage area last week were regularly above freezing.

Air temperature, however, is not the critical factor for snow. That wet bulb temperature – a measure of evaporative heat loss similar but different from dew point – is what matters.

Evaporate heat loss can help freeze the snowpack firm even when the air temperature is above freezing. And the firmness of the snowpack means everything for travel in the Alaska wilds no matter how you are traveling.

A frozen snowpack in above freezing temperatures might sound strange, but it can happen. One of our party of Saturday riders almost begged off the outing because overnight temperatures had been so warm he was skeptical there would be crust, and he’d spent enough time wallowing in spring snow to know that you don’t want to be there if it is mush.

As it turned out, however, the crust was almost too hard. Snowmachines with paddle tracks, and most snowmachines these days have paddle tracks, leave the trail behind them churned instead of packed, and the churn turns to cobbles when it freezes.

None of which is a complaint about snowmachines. In general in Alaska – a state with few trails – any trail is a good trail, and where we could get off the snowmachine pack through the trees to cruise open meadows the snow was much smoother.

The hiking conditions would have been equally as good. On a few short, very steep drifts that the bikes had to be walked up, the snow was like a sidewalk.

How could this be if night-time temperatures stayed above freezing?

The answer has a lot to do with evaporation. Here’s an explanation from meterologist Jeff Haby:

“When water evaporates, the evaporation process requires taking heat from the environment in order for the evaporation to occur. With the removal of heat from the air, the air cools….initially dry and warm air will produce the greatest amount of evaporate cooling when this air is saturated through the evaporation process.”

What regularly happens in Southcentral Alaska this time of year is that the snowpack warms up greatly during the day and starts to disappear both by melting and by sublimation. Sublimation is the process whereby water – which can exist as a solid, liquid or gas – goes straight from its solid state, snow or ice, to its gaseous state, vapor.

When sublimation gets cooking during the day and continues into the night, it can pull copious amounts of water vapor out of the snow. That drives the evaporation that cools the snowpack the same way sweat cools your skin.

If you doubt the power of evaporation, go for a run to work up a good sweat on 40-degree day and then take off your shirt. Your 98 degree body will start cooling down right quick, and it won’t take long before your shivering as your body tries to produce more heat to make up for the heat loss.

The snowpack has no heat source to compensate for this sort of cooling. So when it is in this situation it starts to freeze even though the air temperature might be 35 or even 40 degrees.

34-degree icing?

Haby again:

“(One) reason that evaporative cooling is a significant weather changer is that it can cause a cold rain to turn into wintery precipitation. Evaporative cooling can be enough to cause the ground surface temperature to drop below freezing which leads to ice on the ground and travel problems. For example, the temperature could be 34 F with a dew point of 10 F. When rain falls into this air, evaporative cooling will cause the dew point to increase and the temperature to decrease. After saturation, the new temperature will be below freezing.”

This also explains those snowstorms that happen in temperatures above freezing. Meteorologist Chris Robbins at iWeathernet.com has a great quiz about a record snow storm that buried Atlanta, Ga. in 8 inches of snow on March 23, 1983. 

Suffice to say, it all began with the air temperature way, way above freezing, although as it continued the accumulating snow eventually chilled the air to near freezing.

Once you began to understand the oddities of snow, you will recognize the temperature you need to know when hunting crust is not just air temperature but dew point/wt bulb.

There was regularly a firm crust in the Chugach Mountain Front Range in the mornings last week even when overnight air temperatures were significantly above freezing, and the crust just got better on the weekend when air temperatures as well as the dew point started plunging into the 20s at night.

After a cold Saturday night, a weather station near 1,000 feet on the Anchorage Hillside reported the air temperature at 40 degrees at 11 a.m. today, but with the dew point still at 30.

It is a good idea to pay attention to the dew point on the warm side, too, because it’s amazing how fast snow can go from pavement to mush once the dew point climbs above freezing.

Snow bonded by ice below the dew-point freezing level quickly falls apart once those icy bonds are broken. When that happens and the snow is deep, the white sidewalk that was your friend can become a white enemy.

Getting caught out when the snow turns to mush can leave you a long wallow from anywhere.

A variety of automated weather stations monitored by the National Weather Service provide air and dew-point temperatures around the state and can be found on a clickable map here: https://www.wrh.noaa.gov/map/?&zoom=8&scroll_zoom=true&center=62.35215840901419,-146.513671875&basemap=OpenStreetMap&boundaries=true,false&obs=true&obs_type=weather&elements=temp,wind,gust&obs_popup=true&obs_density=1

The site at Turnagain Pass on the Seward Highway showed  the 11:30 a.m. air temperature today already up to 38 degrees, but the dew point lingering at 24. Those are near ideal conditions for spring crust skiing, and if today is like yesterday – when the temperature peaked at 44 degrees but the dew point never passed 30 at Turnagain, it would be a great day for skiing the Pass in glorious March sunshine.

Welcome to spring in Alaska where winter’s snowpack promises the best of times or the worst of times with the difference depending to a significant degree on one’s knowledge of the physics of snow.


4 replies »

  1. Out Funny River way, Browns Lake has been firm and crusty until the evening. No mush at all the past few days at 6+ pm even though the sun has been blazing. Fast fun skiing from morning to night. Get it while it lasts.

  2. The latent heat of fusion is another one of the fun oddities of science. Here is a video of water below the freezing point 30 degrees F that heats up 8-9 degrees when it actually freezes…frozen water at 38-39 degrees. The frozen water bit starts around 1:05 in the video https://www.youtube.com/watch?v=njUzTe5xRH0

  3. Fantastic info!,Still been fairly great x country skiing in Hatcher pass over the past few weekends.Perhaps ill head south or east next weekend.

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