As anyone knows who has been outdoors on a windy winter day, moving air can seem to go right through layer after layer of clothing, chilling a person to the bone.

And that wind chill is more than just a nasty sensation, or a number providedby the local weathercaster. It can be a serious hazard at this time of year.Those especially vulnerable to the dangers of cold are the very young and the very old, medical experts point out. But virtually anyone can become a victim of hypothermia, or extreme cooling of the body.

The wind chill exists all the time, of course. Consider the pleasant summer day when it suddenly seems cold after emerging from a swimming pool.

That's wind chill at work, too, but it's not much of a hazard then.

Come this time of year the wind can whisk away body heat faster than you can say "Jack Frost," leaving you shivering or worse.

The reason is the body's own cooling mechanism depends on evaporation of moisture, a process that is very effective in removing heat.

Moving air carries away molecules of evaporated water and the air that the body has warmed. More water then evaporates into the dry air, which the body has to heat, the result is that the process makes the body even cooler.

And that, in reality, is the problem.

The wind doesn't make it colder, it carries away heat to make it cold faster.

If the air is at 20 degrees Fahrenheit and the wind is 20 miles per hour the windchill is minus 10 degrees.

Folks unfortunately exposed to these conditions won't actually cool down to minus 10. At the worst they will stop cooling at 20 degrees, when they reach the air temperature.

But they will get there faster, cooling at the same rate as if the temperature were minus 10.

So what this all means is listen to the latest weather reports before leavingthe house and dress for the weather. Layers of clothing are more effective than one or two thick garments.

Since more than half of the heat produced by the body can be lost through thehead and neck, a cap or hat is vital in winter, the Center for Environmental Physiology points out.

Trapped air is the key to insulation, and there are several ways to do this. Animals use fur. Birds opt for feathers.

People can manage the same effect with layers of warm clothing, especially wool and down garments which trap air in layers.

Cold puts extreme stress on the body, which can prove deadly to people with heart or lung disease and to those whose bodies have poor heat regulation systems- the very young and very old especially.

The ill elderly can suffer at temperatures as high as 60 degrees, according to physicians, who urge that friends and family keep track of the old and ill at this time of year.

Signs of hypothermia include changes in a person's appearance or behavior, uncontrollable shivering or total lack of shivering when it would be expected; stiff muscles, slow or irregular heartbeat, slurred speech, weak pulse, confusion, drowsiness and a drop in body temperature.

Victims of hypothermia need rewarming and medical attention, the Department of Health and Human Services stresses.

While waiting for emergency medical help, HHS experts say, prevent further heat loss by wrapping the victim in a warm blanket. Hot water bottles or a warm heating pad - not hot - can help if placed on the victim's abdomen.

If the victim is alert, small quantities of food and drink can help, but never give alcoholic beverages.

Warming can also be accomplished by lying close to the victim and using one'sbody heat to warm them. But do not handle the victim too roughly. Rubbing, for example, can make things worse, the HHS experts warn.