Arc'teryx Women's Thorium Vest with Hood Size: MThis Thorium Vest is an excellent midweight down layering piece from Arc'teryx. This is also somewhat of a hybrid piece that combines 750 fill power down with Coreloft synthetic insulation strategically placed in moisture prone areas. Pefect to layer over a next to skin first layer or as a midlayer underneath a waterproof hardshell jacket in colder or wet conditions.