Description Of Humidity

Factors Affecting Humidity

• As humidity is a function of the water vapor present in the air, the potential for a high or low humidity in a given area is determined largely by temperature and the influence of nearby bodies of water. Coastal areas typically have high humidity because of the vast amount of water available to evaporate into the atmosphere. Lakes and rivers also contribute to high humidity under the right conditions.
  
  In contrast, desert areas typically have low humidity except when a wet storm passes through the area. The change in humidity in a relatively dry area can predict whether a storm is approaching.
  
  

Absolute Humidity

• Humidity measured as the mass of water vapor present in a given volume of air refers to absolute humidity. This can tell an observer exactly how much water is present in the atmosphere and available to condense out under the right conditions. However this measurement of humidity is somewhat unreliable because changes in atmospheric pressure and temperature will change the volume occupied by a given mass of air. Absolute humidity is useful as a comparison point to describe two different geographic areas under similar atmospheric conditions.
  
  

Specific Humidity

• Specific humidity describes the mass of water vapor present in a given mass of air that includes the water vapor. This avoids the temperature-pressure effects on measuring humidity in an absolute sense and allows an observer to detect how concentrated water vapor is in the atmosphere as well as track changes in this concentration over time. Air always has a certain carrying capacity for water vapor and understanding the mass ratios between water vapor and air vapor allows for predictions about condensation.
  
  

Descriptions of Relative Humidity

• The most common description of humidity most people are familiar with is that of relative humidity. This is expressed as a percentage and indicates how close a given air sample is to being fully saturated with water vapor --- how close it is to its carrying capacity. Relative humidity cannot exceed 100 percent because any water vapor past that point would condense out of the atmosphere. This percentage is valuable because the higher the relative humidity, the wetter the air will feel and the better it will hold heat. Areas of high humidity typically have highly moderated atmospheric temperatures but also increased air pressure. This contributes to the "heavy" feeling of the air in many warm coastal regions.