Humidity Confuses

Humidity is a confusing element.  People often talk about 90 degrees and 90 per cent humidity but this combination cannot happen naturally on Earth.  The percentage people casually refer to as humidity is actually relative humidity, which is the percentage of saturation.  This percentage varies either by a change in the moisture in the air or by a change in the temperature of the air.  When the temperature is near 90 degrees, the relative humidity cannot be 90 per cent because warm air can contain so much more water vapor.  But consider that even when the relative humidity is around 90 per cent, this does not mean the air is 90 per cent water vapor.  Relative humidity is a percentage of saturation, not a percentage by mass or volume.  The actual percentage of water vapor in the air by volume is varies from near zero per cent to as about four per cent. The lowest actual water vapor content is found in cold, winter air.  Even on a dry summer day, there is more water vapor in the air than there is most of the winter.    Meteorologist John Wheeler

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