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The Science Behind the Down Jacket

The most critical garment for outdoor enthusiasts is the Down Jacket. The garment has a hugely impressive warmth-to-weight ratio making it possible to keep warm in cold and dry conditions. Due to its enormous benefits adventure seekers have ventured higher in extreme cold winter conditions. In this photo blog Maninder Kohli shares the science behind the Down Jacket and offers guidance on getting the right one for the serious outdoor enthusiast.

Seema Jindal wearing a Canada Goose Women's PBI Camp Down Hoody

750 Fill Duck Down as she poses on the trail to Goecha La

Winter in Ladakh: In recent years I have been heading out to Ladakh in peak winter with day temperatures hitting -15 deg C, using my trustworthy Down Jacket which keeps me warm and comfortable. It is interesting to observe what our local guide Scalden shows up in a jacket made of sheep wool. When temperatures drop further he switches to a similar jacket made of Yak Wool! The local jackets have an interesting look to them but weight a lot more and are not as compressible as a Down.

Maninder Kohli wearing an Arcteryx Cerium SV Hoody in January in Leh. The Down Jacket has a 850 Fill and the weight of the jacket is 400 grams will down weight of 160 grams.

The Science of the Down Jacket: Down is a soft, heat-trapping fluff from duck or geese that keeps them warm in the wild. Down Jackets work on the same principal and use this natural material as insulation because it’s great at locking in warmth. Each ounce of down has approximately two million fluffy filaments that interlock and overlap to create insulating pockets of air, which is what keeps you warm. Goose down is generally superior to duck down due to its larger down clusters. From 2014, many manufactures have adopted a regulation titled ‘Ethically Sourced Down’ which signifies that no live plugging took place to develop the Down Jacket

The down from the belly of the bird makes its way into a Down Jacket.

Understanding Fill Power: Two key aspects to check when buying a Down Jacket is the Fill Weight which measures the quantity of Down and the Fill Power which is based on the loft the clusters achieve-more air is trapped and more insulation is achieved. The higher the Fill Power number the warmer the jacket, keeping the quantity of Down fill constant. With high Fill Power numbers, such as 850, less Down is required for a given level of insulation. A Fill Power number below 700 in my opinion does not offer adequate warmth to weight ratio for high altitude or winter trips.

Fill Power

Description & Use

Less than 600

Used in cities for day to day use

similar to a Uniqlo jacket

600 - 700

Light & Warm Jacket for

everyday mountain use

700 - 800

Superior warmth and performance.

Active mountain use, cold camp evenings

800 - 900

Exceptional warmth-to-weight ratio,

high altitude treks & winter expeditions

On the Trail to Annapurna: I recollect it was 2015 and we were heading on a trek to Annapurna Base Camp. Down Jackets had at that time become mainstream and trekkers were getting access to them. Several on our team acquired Down Jackets and when we reached higher camps they came our like a riot of colour. There was a fair amount of excitement as the jackets were put to work. As we went higher complaints started to come in as team members were still feeling cold. In our haste to get a Down Jacket we had got light jackets. In these jackets the Fill Power was high but the absolute quantity of Down Fluff was low. While the girls looked great they felt cold.

Team members sporting a variety of jackets on the trail to

Annapurna Base Camp

Problem with the Parka: Learning from the Annapurna experience, when it was time to get my wife a Down Jacket my desire was that she should not feel cold. I went for a warmer option, a Parkha which extends to the hip, to ensure that she is comfortable in the cold. We were heading to Nanda East Devi Base Camp and as temperatures dropped she whipped out the Down Jacket. She was pleased as she was super warm. But issues surfaced when she returned home and started to review the images. She felt the jacket bloated up and was making her look very fat! My experience is that pleasing women is tougher than climbing Mt. Everest!

Shamila Kohli wearing a Patagonia Fitz Roy Down Hoody with 800-fill Goose Down

with a total weight of 600 grams.

Quantity of Down: When you look at the warmth level of a jacket the overwhelming variance comes in from the actual amount of Down fluff in the jacket. As you increase the quantity of Down feather the jacket becomes heavier but warmer. This table does a review of 4 men’s Rab jackets to show that while the Fill Power remains constant as you add the down fluff both the warmth level and the weight increase.


​Low Altitude Treks

High Altitude Treks

Peaks < 6000m

Peaks > 6000m







Men's Microlight Alpine Down Jacket

​Men's Neutrino Pro Down Jacket

Men's Positron Pro Down Jacket

Men's Batura

Down Jacket

Fill Power





Fill Weight

148 grams

212 grams

300 grams

450 grams

Jacket Weight

465 grams

585 grams

760 grams

1190 grams

Recent Innovation’s in Down Jackets : One of the issues with the Down Jacket is that if it is subject to moisture the Down starts to clump up and the insulation gets badly compromised. To manage this issue some manufacturers are working on two areas, treating the Down before infusing it into the jacket and looking at the fabric to play a role in preventing moisture to gain contact with the Down.

Sumedha Sarogi standing on a frozen river in Ladakh in winter. She is wearing a Rab Women's Infinity Alpine Down Jacket with 800 fill Goose Down with a filling of 162 grams and jacket weight being 490 grams. The Nikwax Fluorocarbon-Free Hydrophobic Finish of the down. External fabric is GORE-TEX INFINIUM™ WINDSTOPPER® fabrics.

Selecting your Down Jacket: The 5 key aspects to keep in mind when selecting your Down Jacket if you are going to high altitudes or winter excursions :

1. Go for a Down Jacket with Fill Power between 700 - 850

2. Quantity of Down fluff should be more than 200 grams for a mens jacket

3. Give preferences to Goose Down as compared to Duck Down.

4. Use ethically sourced Down and Down feather treated for moisture protection

5. Ensure your Down Jacket has a hood

Rajesh Pratap Singh on the descend from Green Lake, base camp of Kanchenjunga wearing a Rab Neutrino Pro Down Jacket with 800 fill Goose Down with a filling of 212 grams and jacket weight being 600 grams.

The 1000 Fill Jacket comes to New Delhi: Paul and Shark have announced "7X1000" Ripstop nylon Puffer Jacket with 1000 Fill Power. The jacket uses Goose Down and I would describe it as a super light weight, water repellent, jacket to own!

Shamila posing with the new The "7X1000" Ripstop nylon Puffer Jacket with 1000 Fill Power developed by Paul & Shark at our home in New Delhi

Early Morning and Annapurna : It was April 2015, 3 days before the worst earthquake in Nepal history would strike. I was keen on an early morning shot of the first light hitting the top of Annapurna. We were staying the night at the Annapurna Base Camp at 4000m. From 5 am, I waited in pitch darkness, with my Down for warmth. In my view temperatures would have been about -10 Deg C. All my efforts were rewarded as I got this fantastic shot.

First light striking the peak of Annapurna 8091m, the 10th highest mountain in the world

as viewed from the southern basecamp

The Bali Pass Crossing : On the Pass crossing day on Bali Pass we woke up at 3 am, the plan was to set off at 3.30 am. The outside temperature was biting cold at around - 5 deg C. We set out with our Down Jackets. As the sun emerged we had to took off the Down Jacket as it was getting warm. We finally reached the cold and windy Pass at noon. Our Downs went back on for the most tricky and technical descend.

Smita Aggarwal on the final approach to Bali Pass with Swargarohini

dominating the background

Tired trekker Rohit Aggarwal rests on Bali Pass 4950m after an exhausting climb, Kalanag 6387m and Bandarpunch 6313m can be seen in the background.

The Story of Kuntal Joisher: On May 19, 2016 Kuntal Joisher from Mumbai became the first Vegan to climb Mount Everest. When he reached the summit he was elated but carried a regret. Through the climb he had maintained a vegan diet but he had worn a Down Suit which contained bird fluff as that was the only option to manage the cold. This thought troubled him and he reached out to an organisation ‘Save the Duck’ which uses micro fibres to mimic the look and feel of natural down. ‘Save the Duck’ developed for the first time ever a Synthetic Suit for mountaineering use. On May 23, 2019 Kuntal again reached the summit of Everest wearing a Synthetic Suit completing a personal goal of climbing Mt Everest without any animal or bird influence.

Kuntal wearing a regular Down Suit on Mt. Everest in 2016

On Mt. Everest in 2019 Kuntal wearing a Synthetic Suit by ‘Save the Down’.

On the trail to Green Lake, the Eastern basecamp of Kanchenjunga 8596m,

the 3rd highest mountain in the world

Summary: For critical outdoor products like a Down Jacket my view is to go for established brands from both a safety and value perspective. The coldest I have ever felt on a trek was at Green Lake when we were going to the Base Camp of Kanchenjunga. The altitude coupled with a higher humidity level in the Eastern Himalaya made even day temperates difficult to deal with. It's in these kind of conditions where your gear gets well and truly tested and one realises the value of quality gear.

Maninder Kohli


Juniper Outdoor

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