For me layers are the best way to dress when walking, no matter what time of the year as they allow you to regulate your temperature taking off and putting on as you need to.
Even when the weather is good, you can still get caught in a breezy place when you stop for refreshment or you arrive on the top of a hill/mountain; it can start off cold and turn into a hot day and temperatures drop again at the end. Sometimes a light layer on a hot day can protect your skin from too much sun exposure.
Of course, in winter layers are important to keep us warm. Layers trap the air in-between each layer so wearing multiple thin layers is much more effective in keeping you warm than a single thicker one.
There is a 3 layer principle generally…
The first layer is your Base Layer, worn next to your skin. It keeps the skin dry by wicking away moisture. Ideally it should be tight fitting so that it traps the air next to your skin, therefore, insulating your body and also lightweight and dries quickly. Merino wool is very effective here working across temperatures and activities. It is also antimicrobial, which means it needs to be washed far less often and this can come in useful if you are on a few days trek camping and with little washing opportunities.
The next layer is the Mid Layer, sometimes also called the insulation layer. It provides the warmth and is often a fleece or insulated jacket. There will be various different weights of fleeces and jackets here so what you choose will be very personal as well as affected by where you are walking. This mid layer should also be breathable and able to retain heat. Down insulated jackets are packable with an excellent warmth-to-weight ratio, whereas synthetic insulated jackets are able to retain heat even when wet.
The third layer is your Outer Layer or Top Layer and is the one protecting you from wind, rain and in some cases snow. When you are walking in better weather you may not need this at all or perhaps a very light showerproof or waterproof jacket but when the weather closes in, you will be grateful for a quality and efficient outer layer. This layer too should be breathable allowing moisture and heat to escape from your body so you don’t overheat.
The waterproof hard shell is the classic outer layer that will protect you against the most adverse of conditions. Look out for taped seams and weatherproof zips to ensure it will keep the rain out. Durable water repellent (DWR) coated hard shells can also work well as a light outer layer in warm conditions and for low intensity activity.
Depending on weather conditions, the outer layer does not necessarily need to be fully waterproof. Soft shell jackets offer excellent comfort as they are stretchy and flexible, provide good wind resistance and are more breathable than waterproof hard shells. Soft shells can also work as an excellent mid layer in cooler weather conditions, with a waterproof hard shell on top.
Bottom layers also needs some consideration. In light rain, shorts may mean your legs will get wet but in warm weather they will also dry quickly too as will sports leggings. In heavier rain, waterproof trousers will be a must in your kit to pull on over your main walking trousers as soon as you see or feel that the rain is on the way. They can add another layer against the wind of course too.
As mentioned before, personal preference comes in to play once you know what you need. I get hot very easily when I walk especially on the hills so I may go for lighter fleeces as a mid layer as opposed to someone who feels the cold. You will get to know this and do listen to your body and trust, let it be your guide.