The shoulder seasons can be a confusing time. The morning starts out sunny, the first few miles of your hike warming up nicely, and you find yourself comfortable enough with the long sleeve performance shirt you're wearing. When the early afternoon wind start blowing in some cloud cover, however, you'll be happy you packed the Arc'teryx Women's Atom SL Vest. The lightweight insulation offers a nice addition of core warmth without overdoing it, and without restricting your free arms, and it packs down into its own pocket so you can easily clip it to your waist-pack if you're traveling light or incorporating trail running into your hike. A durable water resistant finish helps shed light rain and block the cool breeze, while the fleece panels provide ventilation so you can keep the pace up.