Hats seem to be one of the more personal pieces of gear. More than many items, people seem to care about the style of their hat. There are a huge number of options with a wide variety of styles, colors, materials, etc.
Hats are highly effective at fine tuning thermal comfort because they are easy to put on and take off (provided you aren’t wearing a helmet) which lets you control the venting of a significant amount of heat. Hats have a good warmth / weight ratios because so don’t try to save weight by skimping on your headwear. I would also strongly recommend that at least one of your jackets have a hood.
In hot weather a hat can help you stay cool and protects against sunburn.
I am often not wearing a hat these days, but use the the hood of my shirt / jacket. For sun I haven’t found anything cooler than a Voormi River-Shirt and nothing more versatile than the Patagonia Thermal Weigh Hoody. When I decide to use a hat it is typically a Tilley LTM6 Hat because it provides sun protection for my face and neck while offering decent ventilation. I think it’s one of the more stylish hats which provides full sun protection. My wife says I am wrong, but is willing to put up with me wearing this hat when the sun is big. When it gets cold I sometimes use a PolarBuff that can be configure to suit my needs. In warmer conditions I double the fleece section over my ears with just the light polyester fabric over the top of my head to avoid overheating. When the temperature is down around freezing I reconfigure the buff so the fleece covers all of my head for some extra warmth. When it gets really cold the buff becomes a neck gaiter / face mask and I use a GoLite Snow Cap. I sometimes wear a REI Ball Cap made from waterproof/breathable eVENT.
Many people use baseball caps to keep their hair out of the way, things out of their hair, and shading for their eyes. I don’t wear baseball caps because they don’t provide adequate sun protection for the side of my face and neck.
There are several companies that add a veil to a baseball cap to protect ears and neck from the sun. These hats provide very effective sun protections. The downside is that when using the veil, the wind is blocked which allows heat to build up more than if there was completely free air flow. I think the OR SunRunner Cap is one of the best options with a removal veil that is very packable. Sometimes rather than a baseball cap shape, hats have a higher crown like the hats made famous by the french foreign legion.
I have found the hats from Sunday Afternoon provides more space between the veil and the neck which permits air flow makes it one of the coolest to wear. I used a Sunday Afternoon hat for several years. I haven’t found a better performing hat for hot weather. I think the Ultralight Adventure and the Sport model have the best balance of compactness and coverage. The large models have too much surface area without enough structure to work well in wind. While Sunday Afternoon performance is excellent in hot weather, I decided that the Tilley was more versatile and didn’t look silly on my head.
There are a host of more conventional brimmed hats on the market. Brims for hats are sometimes fabric (like bucket hats), light foam, stiff foam/plastic, or sometimes flexible foam + edge stiffener. I have found that in moderate to high winds brims made from fabric or very light / flexible foam are not very effective. In the summer I would recommend looking at hats which have a full brim made from a light weight supplex (or other woven nylon) material. Dorman-Pacific make a number of nice hats from a very light weight supplex weave which breaths, gives good sun protection, keeps light summer showers off the face and dries quickly. One of their hats hat as an oval shaped foam brim with a plastic stiffener running around the outside of the brim to keep it from flapping in the wind.
Another option are classic Panama straw hat, which actually Ecuadorian. These hats provide sun protection will permitting better air flow than nylon or cotton weave hats. The down side is that they are less durable, and if you are repeatedly caught in rain storms they will start to degrade.
In hot weather a cotton bandana, or better yet, a “tie” filled with poly-crystals such as those made Kooltie can help keep you cool.
In moderate weather having a hat which is wind resistant can be a big aid to regulate your body temperature. In cool or cold weather, hats made from waterproof breathable material can be quite useful. Outdoor Research is well known for making brimmed hats from WP/B materials. These sorts of hats can keep your head dry when you are not wearing a hood which allows more ventilation then wearing a hood which is nice in warmer conditions. There are a number of companies that make baseball style hats out of WP/B materials. Some people use these sorts of hats without a hood, or sometimes as a way to compensate for a hood with lacks an effective brim. Keep in mind that not all WP/B materials are equally breathable. If possible, get a hat made from eVENT. The classic OR Seattle Sombrero is one of the most beloved rain hats. I used one for many years but am back to using a rain jacket which has a hood.
In cold weather you will want a hat which keeps you warm. Winter hats can be caps (typically wool, fleece, or soft shell) though there is the blackrockgear downfill beanie, hunter style (typically shelled fleece), or balaclavas (normal fleece, windstopping fleece, high loft). I typically find a balaclava overkill until it is below 20F and windy at which point it is indispensable. The Mountain Hardware Flex balaclava is my favorite. Don’t underestimate how much a neck gaiter (or scarf) can help you stay warm.
Details and Further Information
There is a wide range of numbers given for how much heat is lost through the head. My investigate of various scholarly articles brought be to the conclusion that somewhere between 35-40% of body heat exits through the head & neck given uniform insulation on the body. This means that in cold weather having a warm hat, or better yet a hood, plays a significant role in keeping you warm.
Get a free hat. Lots of companies give hats away with their logo on them for proportional purposes.