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Called a waffle-knit shirt or simply a thermal, these long-sleeve base layers ooze Americana and prove plenty warm.

When the air turns cold and snow starts falling, you're going to need layers to keep warm. Sure, classics like a crisp white tee or an old-but-gold tank work well and fit without fuss beneath sweaters and dress shirts, but if you want to be comfortable outdoors in the winter months you need a thermal base layer — a firm, infallible foundation, if you will.

What is a thermal? And, how is it different from a waffle-knit shirt? Well, they're one and the same. Colloquially, the term waffle-knit refers to the fabric itself, a literal description of how it looks and feels up close. On the contrary, the term thermal refers to the garment's function: to serve as insulation for your body. You see, the style is meant to be thick, a truly hefty option oft cut from mid- to heavyweight cotton. The weight alone warms your skin's surface, but the breathable waffle formation lets air flow freely out but barely in, thus regulating your temperature and wicking unwanted moisture.

This combination of capabilities make it — the waffle-knit or the thermal, whichever name you prefer — the ideal base layer. Put it on first beneath a nylon liner and a wool overcoat? Incredible; you'll be super warm. But paired by itself with faded blue jeans? Stylish in a '90s-era Wrangler commercial — you know, the now questionable one with Brett Favre slinging the pigskin — kind of way.

So, seek out an iteration that feels heavy without being overwhelming, tightly-woven without seeming artificial and, of course, stylish enough to serve as an unidentifiable base layer or a standalone top.

Cream — which is decidedly different from stark white — is certainly the preferred color, but faded black, navy blue, olive and gray iterations are OK too.

The softness and aesthetic of a vintage find without the stretched out sleeves and shoulders. This iteration is modeled after an official military issue, and it comes in nine colors.

Filson's option is overall a bit heftier, which is why it costs more. This one has a straight hem opposed to others (curved or cropped hem), and comes in four colors. Think of this iteration as this list's homage to rugged Americana, or cabin-core, if you're a Gen Zer.

Available in nine colors, Gap's Waffle-Knit is a good option for those not interested in devoting a bankrupting chunk of their budget to base layers. (They're important, but I get the desire to buy something more...affordable, especially if you're looking to order a bunch of them all at once.) 

If you already own anything from Everlane, you know what to expect. If not, here's the TLDR; excellent basics (T-shirts, chinos, sweatshirts) at affordable prices. But they do a lot more, too: tons of types of outerwear, sneakers and even niche undergarments. Case in point? This comfy yet plenty sturdy waffle-knit long-sleeve shirt. 

This is the cheapest thermal here — for sure. But at a cost. Although it's only $8 dollars, you'll definitely notice, and that's not a welcome revelation 10 minutes in an hour-long winter hike. This is a super underwear-ish option, meaning it's thin, tight and shorter in the sleeves as to not peek out from under another shirt. 

A lesser-known fact about thermals? They can be worn year-round. The North Face isn't shy about this feature with their waffle thermal. It's called the "All-Season," and it's made from a cotton-elastane blend, a composition that promises structure with plenty of stretch and warmth without suffocation.

The Real McCoy's strives to reproduce classic garments for modern audiences. (But even the purists, meaning those that shop at supply stores and military surplus shops, appreciate these.) Appreciate this version for its versatile: It's part pajama top, part gym attire and an obvious essential for everyday activities in the dead of winter. 

Because of the pocket and its sort of boxy build, this crew tee from Marine Layer won't look right under a tighter top layer. It's best you wear this as your top layer with, if you need it, another layer beneath it. Or, try wearing it beneath a puffer or another parka — any outerwear that shouldn't look tailored.

When discussing a classically American garment, or at least one deeply embedded in the Americana aesthetic, you can't leave out L.L. Bean. Wearable as a base layer or light crewneck sweatshirt, the Waffle Crew by the Maine 0utfitter eschews extras and adornments in favor of classic, kind of dad-like colors and signature features like a V-stitch and straight hem.

What impresses most about Pact's Thermal Waffle Crewneck is how it fits. If you're sure of your size — trust the measurements — you'll find that it wraps nicely around the shoulders, flatters the chest and falls straight down through the hips, creating a trim yet classically straight silhouette. 

The most fashion-forward here by far, NN07's Clive 3323 T-shirt, as it's called, takes cues from military and old boy school styles but tweaks them for the modern man. It's soft, supple, sort of slouchy through the arms and available in nine colors.

Although thermals aren't what you would've turned to Abercrombie & Fitch for less than 10 years ago, the brand's done a near-180 and filled its online store with solid basics — and almost all of them are free of that big gaudy moose logo.

Cop Up West's Cozy Up Crewneck in one of six colors and trust it to help trap heat underneath other layers, stand alone beneath a lighter jacket (maybe a chore coat?) and even act as lazy PJs for trips or weekends wasted on the couch.

Bonobos built their Waffle Crew with enough heft that it can stand alone as a sweatshirt or a second (not base) layer underneath a lighter jacket. There's obvious versatility to this option, and it's up to you how you wear it, meaning there's no real wrong way to do it. 

RRL is the upper echelon of Ralph Lauren, where the good stuff goes (and for more than three times the Polo stuff). This Textured Crewneck is definitely good, if not great. It comes in an already faded black color, complete with arm-length yoke seams, raw hems and ribbed cuffs.