Mid layers are focused on heat insulation and breathability for function and is most commonly known as a soft shell or fleece (although not limited to these types of clothes). In a way, you can consider the mid layer to be similar to the base layer, however it does not need to be as comfortable as it doesn’t touch the skin as much. The requirements of the base layer on the other hand are very similar as the mid layer is focused on retaining the body’s warmth and helping move moisture away from the body.
Once again, synthetic fabrics reign supreme in this section as cotton once again has the potential to retain unwanted moisture and is not recommended for any activity that involves exercise. There are many different thicknesses and effective insulation that is available in the form of jumpers, fleeces and soft shell jackets. Normally it is worth choosing a good mid layer based on how cold the environment will be. If you choose a fabric that is too warm, you will overheat in warmer climates and/or when the body gets too warm. It is worth keeping the mid layer relatively light and allowing the outer layer and/or the base layer to accumulate the necessary insulation needed to keep the body at optimal temperatures.
One feature that many seem to neglect is that the mid layer must be quick at drying but not essentially required to be waterproof. Try to avoid using branded hoodies which are normally merchandised and made of cotton as it’s cheaper and choose a good level of insulation that fits the climate temperatures. Once again, wind resistance and water proofing is not essential for a mid layer, especially if an outer layer is going to be a part of your equipment.