Wednesday, January 18, 2012

Relative Humidity, Dew Point, Storms, and Avalanches

It's been cold up here. The past few mornings have been -15°F in my part of Anchorage. Other parts of town have hit -25°F this week. But that is much better than Fairbanks which was dealt a 39 below this morning. Fortunately, we are in an inversion which means "warmer temps" in the mountains. Ridge tops from Hatcher to Turnagain have been in 5-15 above range. Skiing conditions have been great and the cold blue sky has allowed the sun to light the way. CNFAIC forecast for the last few days has been "LOW" and many big lines have been going down.

Unfortunately, I've been shame spiraling pretty hard the last 2 weeks soloing to low angle powder and wooded terrain. Fortunately the fresh cold air and bright sunshine has been exactly what I needed: peaceful, serene, with plenty of time for reflection.

My thoughts drift to recent events and events long ago. I smile, I laugh, I tear up. But being an engineer, thoughts eventually turn to more practical matters like: "Why is cold air so dry even when the humidity is 90%?" Well, it turns out that "relative humidity" is a very poor indication of how much moisture is in the air.

Relative humidity is in most every weather observation, but it can be very confusing. Warm air can hold more moisture than cold air. Here is the maximum amount of water that air can hold at different temperatures:
  • -4°F 0.9 g/m³ (100% relative humidity)
  • 14°F 2.1 g/m³ (100% relative humidity)
  • 32°F 4.8 g/m³ (100% relative humidity)
  • 68°F 17.3 g/m³ (100% relative humidity)

Now, I'm not exactly sure what the Hell g/m³ means, but the warm air can hold more of 'em. This is why you want storms to come in at 32°F versus -4°F. 32° air can hold 5x more moisture which means higher snowfall. Snow at 0° in the forecast does not get me excited.

OK, we realize that "Relative Humidity" has its limits, but fear not, "Dew Point" will help. Dew Point is another number that is in most weather observations. When Dew Point equals the current temperature water will "come out" of the air in the form of rain, snow, frost, or dew.

Dew Point is a much better indicator of air moisture. Say Dew Point = 14°F. No matter what the temperature the air will contain 2.1 g/m³ of moisture. At 6PM the temp is 30°F, frost will not form until the temp = dew point (in this case 14°F).


  • Right now in Anchorage it is -12°F with 73% relative humidity. You may think 73% would mean moist air, but since it is so cold, the air can hold very little moisture. The dew point is -18°F. So even though the RH is high, moisture in the air is very low. If for some reason the temp dropped to -18, then frost would start to form.

Now you may be wondering how this applies to avalanches. Frost = Surface Hoar. Surface Hoar is a familiar term to those who read the daily avalanche bulletin. A detailed description of the how's and why's is given here. I'll try to give you the quick and dirty version.

Surface hoar forms on cold, clear nights on top of the snow. And now you can predict on which nights it is likely to form based on overnight lows and the dew point. Good job!

Hoar frost is not dangerous by itself and is fun to ski, but it becomes very dangerous when snow falls on top of it. Buried hoar frost is no joke and may take until April to heal. This may be the case in Utah this year (along with a multitude of other problems).

So when you are out enjoying those low avalanche blue bird days in the back country, take note of the surface hoar that exists on top of the snow. Stop on the skinner and scoop up some surface snow and look for the signs. Listen as you ski, surface hoar makes a unique "shattering" sound as you float effortless turns through the "potato chips."

Remember surface hoar on the surface is not a big deal, but once it gets buried then it will be an issue. If you know that there is buried surface hoar (from pre-storm tours, avi bulletins, friends that were out yesterday) it is your job is to find it and interrogate it. When you dig a pit, target that buried surface hoar. Again here is the link for more detailed info.

Have fun. Sorry for the "geeked out" rant. I promise photos next time!