Why it’s not the humidity ... it’s the dewpoint

One of our least-favorite weather clichés is,  "It's not the heat, it's the humidity."

Actually, what is commonly called "humidity" is the "relative humidity,"  one of the great red herrings of weather, distracting attention from what really matters – the dewpoint.

Relative humidity is deceptive because it represents the amount of vapor in the air relative to what the atmosphere could hold. So the higher the temperature, the more vapor the air can hold. Humidities often hit 100 in dead of winter.

By contrast, the dewpoint is an absolute measure of moisture and a quick reference point for knowing the level of discomfort that awaits outside. So just what is it?

Water vapor is a mystical, magical phenomenon. It is ever present invisibly in the atmosphere.

But if the air is sufficiently cooled, it is forced out of hiding, condensing into rain, snow, fog --  or dew.

The temperature at which the atmosphere can't hold any more vapor is the dewpoint. When temperatures slip beneath the dewpoint, vapor turns to liquid.

For example, dew forms in the morning when air is cooled by coming in contact with a cool surface, such as a car roof; dew will form on a cold soda can (however heavily taxed) if taken outside on a steamy day like this.

The higher the water-vapor content of the atmosphere, the higher the dewpoint.

At 2 p.m. Thursday, the relative humidity at Philadelphia International Airport was an unimpressive 57 percent.

But the dewpoint was an oppressive 72; anything above 70 is in the realm of heat-wave level. And with a temperature of 91 at 2 p.m., the heat index hit 100.

The dewpoint is a critical measure for gauging human comfort.

Sweat is a life-saver. The whole process is a poor man's evaporative-cooling system. When sweat evaporates it gives off a cooling effect to the skin.

If the air is full of moisture, sweat can't evaporate. You probably know the feeling. If not, you can take a handy refresher course by taking a brisk walk this afternoon.