Thermal Comfort

Almost by definition, astronomical observing takes place during the coldest part of the day. Each of us not only has a different sensitivity to and tolerance of cold, but this individual variation itself varies from day to day and even during the course of a night's observing. Therefore, our personal insulation needs to be both adequate and adjustable; this implies layering. Furthermore, viewing through the eyepieces of the binocular is not a physically active task and, for this reason alone, we need to dress as though it were about five to ten degrees colder than it really is in order to compensate for the lack of physical activity. Those who are used to being in cold conditions will undoubtedly have their own "clothing solutions." Those who are not may find some useful ideas in the following advice, whose principles have kept me comfortable for over a decade.

• Undergarments: If you want to keep warm, avoid cotton; it is cold because it absorbs moisture (which is why we use it for towels) and uses our body heat to evaporate the water. If you wish to stick to natural fibres, change to wool (itchy) or silk (expensive), otherwise you need to use a synthetic "thermal" hydropho-bic fabric that wicks water away from the skin without absorbing it.

• Insulting layer: The middle layer of clothing is the insulating layer, which needs to trap as much air as it can, because air is an excellent insulator. By far the most efficient insulating layer per unit weight, or per unit volume, is dry goose down, but the damper it gets the less effective it is. It takes ages to dry out, and it is extremely expensive. The moisture that has been wicked through the inner layer comes to the middle layer, which it will dampen unless it passes through. If you wish to stick to natural fibres, wool has the reputation for being a good insulator when it is wet, although it can be a bit heavy. Modern synthetics such as Hollofil, Thinsulate, and Polartec are excellent insulators and will wick moisture away from the body without absorbing it. It makes sense to have a zipped garment as this middle layer, so you can adjust your insulation to different conditions by opening and closing the zip. Also remember that this layer should not be too tight or be compressed by the outer layer, or its insulating properties (related to the amount of trapped air) are reduced.

• Outer layer: While we tend not to observe in strong winds, even a 8 km/h (5 mph) breeze can make a great deal of difference to our thermal comfort if it can get into the insulating layer. The outer layer therefore needs to be windproof, but it must also pass the water vapor that has been wicked away from our bodies by the inner layers. A windproof Polartec fleece is ideal as a combination insulator/outer.

• Hats: It is said that we can lose up to 40 percent of our body's heat through our heads. Even if this is as low as 25 percent, it indicates that we can regulate our body temperature by changing our headware, thereby reducing the need to deal with the insulating middle layer of clothing. "Extreme conditions" headware would follow the same pattern as our other clothing (i.e., silk or polypropylene balaclava, covered by a wool or polartec layer, covered by a windproof layer) but, for most of us, a simple fleecy hat, preferably with ear flaps, is adequate.

• Gloves: There is no particularly elegant solution to the need to keep the fingers warm and also have them free and sufficiently sensitive to make fine adjustments. The best solution I have found is an insulated "hunter's" glove that has a fold-back mitten over a fingerless glove as well as split thumbs. This keeps the fingers warm but enables the forefinger and thumb to easily slip out when necessary.

• Footwear: On clear nights the ground cools faster than the air and we lose heat rapidly to the cold ground if we wear thin soles. Ordinary thick-soled shoes worn with two layers of socks (inner "wicking" sock, outer insulating "cushion loop" sock) are sufficiently warm for most temperate zone conditions, and for the more cold-footed among us,there are alternatives such as snow boots,which have thick soles and a removable inner sock of thick felt, often combined with a sandwiched reflective layer.

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