"DOWN" is a pure, natural, thermal-insulation product. It is grown by birds and in cold climates it enables them to survive severe weather conditions. The best down, therefore, is to be found on birds which spend winters in arctic or sub-arctic conditions. Down is unlike normal feathers in that it grows without a hard, central "stalk" or quill. In mature birds it forms a soft, fluffy underlayer close to the body beneath the bird's main feathers. The most common sources of down are ducks and geese. Down is used as an insulating material in various man-made products such as sleeping bags, insulated jackets, and bed coverings. In such applications the "down" insulation that is used is never 100% pure down but is a "down mixture" containing a small percentage of fine feathers also. This mixture is represented numerically as a ratio - for example 85:15, which means 85% pure down and 15% feathers. The greater the pure down % the better the insulation value, in theory, as long as you compare like-for-like qualities. In practice, above a 90:10 ratio it is quality differences which are of more significance in determining how good the insulation is. Quality is indicated by "FILL POWER" which is a measure of the down's ability to LOFT - please see below. Cumulus sleeping bags contain a 92:8 ratio of pure down to feathers (small, fine feathers only) in our Cumulus Ultralight and Mysterious Traveller models. Our up-market Cumulus Prime, Cumulus Expedition and Cumulus Quantum models are insulated with 96:4 'diamond grade' Polish goose down. Goose down is better than duck down, and POLISH GOOSE DOWN is recognised world-wide as being of THE highest quality. In particular it is beloved of the Japanese who are well known for demanding top quality and who buy large quantities of Polish down each year. The excellent quality of Polish down is partly due to climatic conditions, but is also the result of extensive academic research into breeding techniques, which are unsurpassed anywhere. Cumulus sleeping bags contain only Polish goose down, whose source is the non-polluted region of forests, lakes and beautiful, natural landscapes of North-Eastern Poland. This is a particularly cold part of Poland, suffering long winter weeks of arctic air with temperatures reaching as low as -36°C in January 2003. It is to combat such severe conditions that the geese, whose natural habitat has been this region for thousands of years, have evolved their protective down to a quality which is unmatched anywhere, both for heat retention and for durability. Polish goose down is available in several qualities, as measured by its ability to LOFT (see below). Cumulus sleeping bags incorporate the two HIGHEST qualities of Polish goose down. The higher the quality, the LESS WEIGHT of down is needed to achieve the required temperature rating for the sleeping bag. Hence high quality down means a lighter weight sleeping bag.