Windshirts, sometimes called windbreakers or wind-shells are one of the most versatile pieces of clothing you can use. A simple, unlined windshirt can ounce per ounce provide more warmth than many other clothing items because it helps prevent convection cooling which increases significantly when windy. In many cases a light wind shirt over an appropriate weight base layer is all you need to keep comfortable when active. Furthermore a good wind shirt can keep you comfortable (but not necessarily dry) in a light drizzle when a full rain shell would be too much. Windshirts can protect you from many biting insects. Finally, wind shirts also slow evaporation a bit (they are not perfectly breathable) which moderates flash-off cooling. Ideally you want slight air permeability to maximize breathability while still providing protection from convection cooling.
I use a 2012 ArcTeryx Squamish windshirt. With an air permeability of around 35 CFM it cuts the wind enough to prevent being chilled while being breathable enough to prevent sweat from accumulated in cool weather. It’s more durable than many of the lighter wind shells, and it’s slight textual feels better against the skin than many other options. A have a Black Diamond Alpine Start which is a ultra-light softshell which is more air permeable (listed as CFM 40, but feels more like 60) which I use when biking and running in the Bay Area winters. It is more durable than most wind shells, the Schoeller NanoSphere treatment lets it shake off light rain (and dry very quickly). Finally I have a I also have an amazing light, breathable, and pricy Gore R7 Trail Running rain shell that I sometimes use instead of a wind shirt when I am minimizing what I carry. I have found the R7 fabric is approaching the breathability of a windshirt.
Classic windshirts are ultra-light, ultra-breathable unlined nylon or polyester jackets which block the wind while being highly breathable. I am particularly fond of windshirts with hoods and full zippers such as the ArcTeryx Squamish, Patagonia Houdini, Montbell Tachyon, and Montane LightSpeed. There are also a number of nice pull over and/or hoodless windshirts such as the Montane JetStream, Outdoor Research Ion (Quantum model), or RAB Quantum Wind Top.
Softshells can be a good alternative to wind shirts. They tend to be more air permeable and more durable than a classic wind shirt. This makes them great for high energy activities and/or when engaged in activities that are hard on clothing like climbing. I like stretch woven soft shells for their breathable, great mobility, and that their air permeability is just about perfect for high output activities. I particularly liked the Black Diamond Alpine Start.
Some people use rain gear for wind protection. In warmer weather, most rain gear is not sufficiently breathable and tends to retain too much heat. They block the wind well, but the wearer will often overheat and get wet from sweat. A counter point is that if you either manage you activity level and/or are in cool weather rain shells can work.
There was a nice discussion about breathability, CFM and wind shirts windproofness, breathability, and air permeability , how breathable a wind shirt is, visual paradigm for wind shirts, on a BPL forum. When backpacking a CFM between 30-40 is considered optimal. There is a thread on reddit about finding 40CFM windshirts. When running or engaged in other high aerobic activities, 70-90 CFM is typically optimal. Most people can’t perceive wind if a material is CFM of <=5. A discussion about older (2012-2014) Squamish wind shirts.
Low Cost: The cheapest solution is a cheap nylon windbreaker found at Target, Walmart, etc. Treat with DWR after-market product.
You’ll wear it when the rain comes down,Ode to a Wind Shirt from Clothing and Sleep Systems for Mountain Hiking
You’ll wear it when the wind blows ’round.
You’ll wear it when the temps are low,
You’ll wear it when it starts to snow.
You’ll wear it while the flies do swarm,
You’ll wear it when the sun is warm.
So skeeters, wind, rain, snow, or sun,
A wind shirt is for everyone.