We saw widespread surface hoar (frozen dew) on the snow surface. This potential weak layer is rarely a problem because it gets destroyed. It was already destroyed on south and west aspects due to warmth sunshine.

The reason I am noting it is that it tells me that the snow surface was much colder than the air last night. When this happens, moisture in the air condenses on the snow surface and forms little surface hoar crystals (photo). I have been watching snow surface temperature at the Atwater Plot in LCC. At night or early evening they have been as much as 10-20 degrees F colder than the air temps.

Why it's important? Seeing surface hoar confirms what the weather starting is showing. It also means that each clear night when the snow surface loses lots of heat to the clear sky, it is experiencing a strong temperature gradient in the top 10 cm of snow. This temp gradient is weakening and faceting the snow surface. Often people say that the old powder has "dried out" but what has happened is that it has faceted and lost any bonding between crystals. When we get more snow on Monday, the current snow surface may have faceted enough to allow the new snow to sluff easily. If the new snow has any cohesion from wind or other factors, then it may fracture as a slab. We'll see.