By STEPHEN REGENOLD

It weighs about 3 ounces and packs in a tiny stuff sack smaller than a fist. But the Cirrus Pull-On from Rab can add enough warmth to save the day on a hike or a remote climb.

I have kept this thin jacket, which is made of a synthetic fabric called Pertex Quantum, in my pack for the past few months. It rarely gets used, as I do not count it as part of my apparel or outerwear for the day. But more than once I’ve shivered and remembered the Cirrus top in my pack. It unfurls into a billowy wind shirt you pull on and then zip up to your neck. It blocks wind and can add a surprising amount of warmth with its thin, silky sheen.

Founded in 1981 by the company’s namesake, the climber Rab Carrington, the Rab label has roots in the world of rock climbing and mountaineering. Equip Outdoor Technologies Ltd. is now the parent of the Rab brand. The company, based in Derbyshire, England, notes the Cirrus wind shirt as “ideal for carrying in a bumbag or for clipping to your harness.”

Though I am not British enough to know what a “bumbag” is, I could see clipping the stuffed shirt on a harness gear loop in back during a fast and light climb. It is that small. This year, on climbing trips and for adventure races, I have kept my Cirrus in a small waterproof sack in my backpack. It sits next to a lighter, two zip-ties, and iodine pills to purify water.

The shirt, as noted, for me has mainly been used as an emergency or backup item, though unlike zip-ties or iodine pills it is one I employ just for comfort’s sake. That said, the pull-on has serious limitations. It blocks wind but not water. Rain drops seep through the Cirrus’ fabric — its single ply of fabric is made to cut wind but not precipitation.

The shirt, a simple product with elasticized cuffs and a chest zipper, cannot be used as a replacement for a jacket except in the summer. There’s a drawcord closure on the waist and neck to seal it up, but no hood.

At 3 ounces in weight — actually 2.8 ounces on my scale for the size XL top — the Cirrus is warm and generous for its miniscule bulk. It retails for $85, which is pricey for a simple top, but online it costs about $50 from various stores.

Pack a top like the Cirrus along for emergencies and in times where you might need a backup layer. It weighs literally about the same as a single Clif Bar. Its significant warmth and protection from wind are worth every extra ounce.

—Stephen Regenold is founder and editor of www.gearjunkie.com.