When a garment is windproof, it simply means that winds cannot cut through the fabric, which would otherwise lead to convective heat loss. Wind-resistance is measured in terms of Cubic Feet per Minute (CFM); that is, a fabric is rated by how many cubic feet of air (“wind”) can pass through the material per minute. Sometimes, for more technical fleeces and softshells, a number will be given to indicate the CFM rating (we are assuming that all hardshells are windproof; only a very few, low quality rain jackets have been found to be merely wind-resistant). The lower the number, the more wind-resistant the fabric is, with 0 being completely windproof. For example, a typical fleece has a CFM rating of about 60. The North Face makes a membrane for their fleeces, called Windwall. This is kind of deceptive, though, because it’s not windproof, as one might assume with the name “Windwall”; it’s just more wind-resistant than regular fleeces. Windwall garments have a rating of 20 CFM. Softshells are going to have a lower CFM rating than that; some fabrics have a rating of 10 or 5. Polartec (a company similar to W.L. Gore and Associates, in that it lets qualified companies use its fabrics) has a fabric called Windbloc-ACT, which is rated at 2 CFM; it blocks 98% of the wind, allowing the other 2% to help the material circulate air and increase breathability. This fabric reportedly increases sweat vapor transmission, two times faster than other windproof fleeces. Then, there are the windproof fabrics, which are rated at 0 CFM. Windstopper is a well-known windproof membrane, made by Gore. Other fabrics are windproof as well, like Cabela’s proprietary Windshear membrane. Due to strong performance, windproof jackets are inherently water resistant, as well, but, as always, a good DWR will help the jacket perform its best.