INSULATION
Trap still air
Fibres and fill work by holding air still. Several loose layers trap more of it than one tight one.
How warmth is measuredHOW WARMTH WORKS
Warm is still air.
Dry is everything.
A jacket does not make heat. It keeps yours.
LAYERS keep air still
feels like −19°F 0°F air with a 15 mph wind
- Trap still air
- Block the wind
- Stay dry
- Layers beat bulk
Your body warms a thin layer of air next to the skin. A good jacket holds that air still, keeps the wind from stripping it away and keeps water from soaking through. Here is how that works, in plain English.
Plain English, with the sources linked.
THE SHORT ANSWER
Hold the air, shut out the weather.
Three jobs, and every jacket
does them in its own way.
WIND
Block the wind
Wind strips away the warm air next to your skin. A tightly woven outer layer keeps it in place.
See wind chill in numbers
WATER
Stay dry
Water carries heat away far faster than air, so a soaked layer stops insulating well.
Check the weatherA QUICK WEATHER CHECK
What is your jacket
up against today?
Pick the weather outside. The bar shows how much the outer shell matters compared with the warm layers underneath, based on how forecasters describe heat loss.
A rough guide, not a safety rating. In severe cold, follow your national weather service's advice.
DRESS FOR IT
Four habits
for cold, wet days.
Layer loosely
Several light, loose layers trap more still air than one tight, heavy one, and you can take one off before you overheat.
Finish with a shell
Make the outer layer tightly woven, water-repellent and hooded, as forecasters advise, so wind and rain stay outside.
Cover the ends
Wear a hat, and choose mittens snug at the wrist: the National Weather Service rates them better than gloves in the cold.
Read the "feels like" figure
Dress for the feels-like temperature in the forecast, not just the air temperature. On a windy day the two can be several degrees apart.
TWO NUMBERS WORTH KNOWING
Wind and water,
in numbers.
The air temperature is only
part of the story.
feels like −19°F
0°F air with a 15 mph wind
In the National Weather Service's own example (about −18°C feeling like −28°C), exposed skin can freeze in 30 minutes. Its wind chill index has been in use since 1 November 2001.
WIND CHILL · US NATIONAL WEATHER SERVICE
up to 26×
Heat loss to water, compared with air
Water conducts body heat away up to 26 times faster than air of the same temperature. It is the main reason a soaked layer feels so much colder than a dry one.
WET VERSUS DRY · US NATIONAL WEATHER SERVICE
Neither number needs a special jacket to understand. They explain why forecasters keep giving the same advice: stay dry and out of the wind.
JACKET FACTS
Five things worth
knowing.
The air does the insulating
The US National Weather Service advises loose-fitting, lightweight layers because the air trapped between them insulates you. The fabric mostly holds that air in place.
"Feels like" has a definition
The Met Office works out UK wind chill from the heat lost by a bare face at a walking speed of 3 mph, using the wind at about 5 feet, roughly human height.
Warmth has a unit
Clothing insulation is measured in clo. ASHRAE puts 1 clo at about 0.155 m²·°C/W, roughly what typical winter indoor clothing provides.
Synthetic layers shed in the wash
A 2016 University of Plymouth study estimated that an average 6 kg wash released 728,789 fibres from acrylic fabric, 496,030 from polyester and 137,951 from a polyester-cotton blend.
Rainwear chemistry is changing
In September 2024 the European Commission restricted PFHxA, one group of PFAS chemicals, in consumer textiles such as rain jackets, with transition periods of 18 months to 5 years depending on the use.
QUICK QUESTIONS
Jackets, answered.
Short answers to the questions people ask most.
Why does a jacket feel thinner on a windy day?
On a calm day your body keeps a thin layer of warmed air next to the skin. The Met Office explains that wind takes that boundary layer away, so skin temperature drops and you feel colder than the thermometer says.
Is one thick coat better than several layers?
The US National Weather Service advises several loose-fitting, lightweight, warm layers, because the air trapped between them insulates you, with a tightly woven, water-repellent, hooded layer on the outside.
Why does getting wet make you feel so cold?
Water conducts body heat away much faster than air. The National Weather Service puts it at up to 26 times faster than air of the same temperature, which is why staying dry matters as much as adding layers.
What is a clo?
The clo is a unit for the insulation that clothing provides. ASHRAE describes 1 clo as roughly typical winter indoor clothing, equal to about 0.155 square metre degrees Celsius per watt.
Do fleece jackets shed microplastics in the wash?
Synthetic fabrics do shed fibres. A 2016 University of Plymouth study estimated that an average 6 kg wash could release over 700,000 fibres from acrylic fabric and about 496,000 from polyester.
Where the facts come from
Updated
- Wind chill temperature index (brochure)US National Weather Service
- Wind chill factor and the "feels like" temperatureMet Office
- The hazards of cold waterUS National Weather Service
- Thermal comfort vocabulary: the cloASHRAE
- Washing clothes releases thousands of microplastic particles (2016)University of Plymouth
- Commission restricts a sub-group of PFAS chemicals (2024)European Commission