It depends upon the season.  In Spring, Autumn, and Summer light clothes such as half pants and light shirts are worn during the days.  Early morning and evenings can be chilly, so warmer layers are added. In the winter season, cold weather apparel for at least -10 degree C is necessary for treks.  There are now garments made especially for trekking, using fabrics which wick bodily moisture while maintaining dryness, and which give protection from wind and sun and are light in weight.