I came across a real good article about corn snow (and many other snow terms) from Bruce Tremper of the Utah Avalanche Center:
http://www.avalanche.org/~uac/encyclopedia/cornsnow.htmThis gave me the impetus to do a little write up on corn, since the term gets tossed around here incorrectly quite a bit in the spring. True corn is a magical and rare occurrence here in the east; it may happen every single day for a period of weeks, but for just minutes at a time on a trail, and experiencing real corn snow can be elusive in this area due to lower altitude, temperature, and exposure. Western skiers know how precious good corn skiing is, but our geography makes it even harder and more challenging to come by. Timing is everything.
The first requirement for corn is to have consistent thaw-freeze cycles happening so that the snow transforms into large grains (resembling small ice pellets about 2mm in size). Once the snow gets into that form, each day's corn cycle starts with an overnight freeze. It doesn't have to be a deep freeze -- generally it takes a few hours near or below 32F for the snowpack's thermal mass to firm the surface back up into frozen granular. If the overnight temperature was much higher than 35-37F and the snow base is thin, or it has been raining, forget about corn the next day; it's getting physically impossible at that point. Based on experience, I think 35-37F is about the upper limit with a deep base.
With an overnight freeze, another big factor is grooming; if a ski area doesn't groom, or grooms prior to the snow surface freezing up, it sets the stage for corn pretty well. Naturally frozen granular snow and frozen granular corduroy will corn up pretty well. However, if grooming happens during or after the freeze, chances are there will be dry loose granular in the morning and this goes straight to wet granular (slush) as the day goes on, with no chance of corn.
So we need an overnight freeze and some form of frozen granular surface in the morning. It should be fast and hard enough to catch your attention, and rattle your teeth if ungroomed. At this point, it becomes a waiting and watching game, as temperature and sunlight become key factors in the appearance of corn. If you're a serious corn hunter, you want to have relevant details, such as weather, the lay of the land, and exposure, worked out ahead of time so you know where and when to be in the right place. Play the cards right, and you might be able to catch the corn on multiple trails across a mountain as the morning sun works its way up overhead.
The corn will occur briefly when the overnight crust softens, to the point where it gets velvety and breaks into mostly-dry "grain clusters". See the figure below (from the link up above) to understand what I mean by a cluster:
You'll know it when you hit corn, because it will be a sublime experience, like carving on sugary cake frosting. Very smooth, very edgeable, and surprisingly fast.
Once the grain clusters warm up to the point where all the individual grains get lubricated by water and separate, we get to the wet granular stage, AKA slush. This is the primary spring condition here in the mid-Atlantic, and can get pretty sloppy by mid-day. At this point, the corn hunt is over, so move on and look for the next conquest.
To give an idea how quickly corn snow comes and goes on a typical day, let me relate my experience at Snowshoe on Wednesday 3/17. Overnight temperatures were in the upper 20s, and warmed to the mid 40s during the day. I was on the hill at 9am sharp, when frozen granular was the typical condition, though some trails had clearly been groomed late and were in stages of loose granular (true to my expectations, those trails went straight to slush later in the day, though they skied great in the morning).
I started out skiing the basin side, as it faces north-east to east and got the morning sun. Green trails in the PowderRidge area were the first to go through the corn cycle around 9:15, followed by trails in the middle of the mountain. Except for isolated shady spots (which remained great all day long), corn had come and gone in the main basin area by 10:30 or so. By 11:00, the corn cycle had passed Widowmaker, though Camp99 and Sawmill, being on slight shaded aspect below Widowmaker, remained frozen granular a little bit longer. Once they went to slush, I headed over to the Western Territory and enjoyed good corn over there until temperature caught up and kicked the snow into wet granular by about 12:30, the exception being lower Shay's which stayed pretty firm until the afternoon. So in a period of 3.5 hours, the majority of the mountain went through a corn cycle and had transformed into wet granular, with some trails flipping in as little as 5-10 minutes. Except for Shays and shady spots on other trails, wet granular was the condition for the remainder of the day. If you were out for corn that day, the hunt would have been over right around lunch time. If you skied trails in the wrong order, you may have never found corn!
I hope this info is useful as we close out the season. With cooperation from the weather and good planning, I hope you'll be able to hunt down the corn for some sweet runs!
look on my face. Beyond Saturday night, Wednesday looks like the only day without some amount of rain in the forecast, and that's the most optimistic thing I can say right now! I am hoping the forecast goes back to the previous trend, which would have put nightly temps well into the 20s early next week, with less rain. It's not uncommon to see reversals sometimes. Otherwise the corn prospects look dim 
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