Most riders understand that you shouldn't ride the trails when they're muddy. Most riders understand acceptable riding conditions as well. Sunny... dry... go ride! Bueno. But what about this time of year? When is it ok to ride without damaging the trail? How do you know if it's OK to ride before you get to the trail. Understanding freeze/thaw cycle can help you make that decision of when to ride trail, or when it's time to stay home and play Scrabble, watch QVC, etc.
HOW FREEZE/THAW WORKS
As the temperature drops, moisture in the soil freezes. As most of you know from that forgetful moment when you leave that “I need this hot beer to get cold quick” bottle in the freezer, it expands as it freezes. If you don’t know, here’s an example.
When this scenario happens on a trail, the composition of the soil is blown apart by the expansion of the frozen water molecules. As the temperatures rise, the 'blown up' soil compresses and, with water, makes... MUD. Nasty, slimy, slippery mud.
FACTORS IMPACTING FREEZE/THAW
The real issue is moisture in the soil. Rain from a week or so ago can impact the moisture level in the soil & start the F/T process. If the soil is dry, freezing temps won't have much of an impact on the soil at all.
Air temps & soil temps are two very different things. Say it's 40 degrees for a few days and then the overnight temperature dips to 28 or 29F for a couple of hours. Even though air temp is below freezing, the ground temperature probably won't go below freezing since dirt's a pretty good insulator. The opposite can be true as well. If the ground has been frozen solid for a few days, having air temperatures above freezing for a few hours won't be enough to thaw the ground. Snow can also be a factor in that it has a relationship to the soil temperature. Snow can either buffer the warmer soil from colder air temperatures OR buffer the colder soil temperatures from warmer air temperatures.
A very common situation here in Tennessee is where the trails are frozen in the AM & by mid-morning the increasing air temp and direct sunshine are given enough time to start the thawing process. Two hours can literally change the trail from a perfect, frozen, hard-pack to a sloppy, slushy mess. PSA: If you're going to ride during this time of year, early is usually better.
No matter how much you think you understand the combination of moisture, temperature & time, there's no substitute for making good choices when you get to the trail. If you see that you're leaving deep ruts & mud is sticking to your bike, it's not OK to ride. Yes, it’s frustrating when you've checked the weather forecast, packed up all your stuff, made it to the trail and then realize that it's not in great shape. No weather forecast makes up for good choices on the trail. Be prepared to bail and help preserve the trails for everyone.