Performance Thermal Underwear - What Skiers Should Look For
I grew up in Florida. When I moved to Utah as a teen, I had no idea what how important good thermal underwear was in the winter time. I wore some cotton sweats to keep warm while playing in the snow. I never could stay out long because the snow would quickly soak through and I would be frozen within 10-15 minutes.

Picking the right thermal underwear will keep you more comfortable on the slopes
Once I found out about thermal underwear, I quickly transitioned from surviving winter in Utah to enjoying winter in Utah. Thermal underwear technology has changed since I was a kid. I would never think of wearing the cotton thermals from my youth to go skiing.
Now, on to what you should look for in a good set of performance ski thermals. First, you want something that will pull sweat or moisture away from your body. Sweat is designed to cool your body so to stay warm you need something that will manage this for you. Merino wool, polyester or polypropylene are at the top of my list of fabrics that are ideal for ski thermals. Each of them insulate well, wick sweat, and dry quickly keeping your skin warm and dry. Each of these fabrics will retain your body's natural heat even when wet. Stay away from cotton. Cotton absorbs water and retains it so you end up wet, soggy and cold. You can even get hypothermia if you are exposed long enough because cotton does not retain heat when it is wet.
Performance Thermal Underwear - Different Weights For Different Ski Days
Thermal underwear is typically available in 3 different weights. Lightweight, mid weight, and heavy weight. It is important to know which weight to wear. There are several factors to this, but suffice it to say that lightweight thermals are for mildly cold temperatures, midweight thermals are for freezing temperatures, and heavyweight long underwear is for extremely cold temperatures from freezing to negative temperatures. If you go too light you'll be cold, if you go too heavy you'll end up sweating too much.
Performance Thermal Underwear - On A Tight Budget
If your finances force you to be a little money conscious then I have some good recommendations on high quality performance ski thermals. If money is really not an issue, I can help there too.
By far the most economical thermal underwear is 100% polypropylene thermals. You can typically get a set for under $40. Polypropylene retains more heat than any other fabric. It is much lighter than other fabrics and it dries extremely fast. The US Navy & Coast Guard wear 100% polypropylene for these important features. The next step up from polypropylene would be thermals made by Coldpruf. Coldpruf is very reasonable priced for such high quality. They have products in each weight range and the most you'll spend for a set of their warmest performance thermals is $90. Lastly, for those who just want the best and are not worried about the cost, you'll want to consider 100% merino wool thermals by Minus 33. They too are available in each of the weight ranges. They are made to last and are super comfortable.