You zip up your coat, throw a scarf around your neck, reach for a hat, and pull on a pair of warm gloves. Then you step out the front door and head for the car. The thermometer reads 10 degrees F. but a knife-edged wind is blowing from the north.
Suddenly, you feel cold, even though you thought you were dressed for the temperature at the time.
What you didn't count on was that 20-mile-an-hour wind; in other words, wind chill.
''When the temperature is 10 degrees F. and the wind is blowing at 20 miles an hour, the wind-chill factor is 24 degrees below zero,'' asserts C. Olof Lundwall, president of Thermius Inc., a small instrument plant located in Williamstown, Mass.
''It is really 34 degrees colder than the temperature will show,'' he reports.
Wind chill is no stranger to anyone who lives in the North where the radio and TV meteorologists often alert their listeners and viewers to the effect of that blast of cold air outside. Up to now, says Lundwall, they've had to arrive at the figure by using a chart.
Now his company is marketing a device which does all the figuring for you.
Instead of registering 10 degrees F., the Thermius Electronic Wind Chill Meter shows -34. That way, reasons Lundwall, a person who has to venture outside in such Arctic-like conditions can dress for it before he sticks his nose out the door.
''We feel cold when we lose heat,'' Lundwall explains. A stiff-blowing wind makes it feel much colder than the air temperature might indicate.
Not only how cold it actually is, but how cold it feels - that's what the wind-chill meter is all about.
The company is making a big push among the TV and radio stations, weather buffs, ski resorts, hotels, schools, companies that do outdoor work, such as construction firms; health-care departments, and the police.
Developed in Sweden by Bruno Nilsson while working on his doctorate at the University of Goteborg, the wind-chill meter is based on the heat-loss principle and simulates electronically the human body's reaction to cold, explains Lundwall.
Easy to install, a small electronic sensor is attached outside the house - on a TV antenna, for example, or maybe a gable peak - where it is exposed to the full impact of the weather. A 75-foot flexible cord links the sensor to a digital readout inside the house. The device runs on standard household current and an adapter reduces the current to the readout.
In computing the figure, the meter also adds a third ingredient, humidity, to the comfort equation.
''Up to now,'' says Lundwall, ''the reading has been based on temperature and wind speed alone.''
Biggest roadblock is the meter's cost. Price of the base unit, set inside a red plastic box, is $150; while a unit inside a walnut block costs $170. The electronics are made in Sweden and a small crew assembles the unit in Williamstown. If the sales numbers go up, the price should drop.
The company also has a battery-run unit in Sweden which can be used by outdoor workers, the military, etc., to measure how long a worker can safely be exposed to the cold.
Down the road is yet another unit which will ultimately be used to gauge the indoor ''comfort climate'' of buildings.