Humidity

Humidity is the word used to denote the amount of dampness or concentration of water vapour present in the atmosphere and depends on the quantity of water vapour present in the air. In other words, temperature increases with the increase of the amount of water vapour needed to achieve saturation.

Everybody is aware that water is the most important of all the vital components that constitute the human body and the functioning of all organs depends on the amount of fluid consumed. Hence, too much or too little humidity can be dangerous for life. High temperaturesassociated with high humidity can be the cause of a major health hazard, especially for the very young and the very old. On the other hand, air without water vapour is unimaginable, which might be like the weather of Mars, without clouds, rain, snow, thunder, or lightning. Humidity is nature’s indicator of possibilities for precipitation, dew or fog.

Although humidity depends on the water vapours dissolved in the air, the amount of humidityis also dependent on various factors like the distance from the sea or a forest. While humidity and temperature are closely related, the water holding capacity of air increases with the increase in temperature. Alternatively, as the temperature of air decreases, it loses the capacity to hold water vapour and a haze appears when we breathe, or dews appear during the night. However, the same amount of water vapour results in higher humidity in cool air than warm air.Although the water holding capacity of air increases with the increase of temperature, the rate of increase is slow at low temperature and increases very fast at thetemperature.

Human comfort level depends considerably on the humidity of the atmosphere. While the human body dispels heat through perspiration and its evaporation, low humidity causes the skin to become rough and cracked.

However, although humidity is a general term, which indicates the amount of water vapours in the air, there are different types of humidity known as absolute, relative and specific humidity.

Absolute Humidity refers to the actual amount of water vapour that is saturating the air. When a volume of air at a given temperature holds the maximum amount of water vapour, the air is said to be saturated. Absolute humidity is the total mass of moisture or water vapour present in a given volume or mass of air without considering the temperature. However, it changes with the change of air temperature or changes of pressure, provided the volume is not fixed. To calculate absolute humidity, the mass of water vapour in an area is to be divided by the mass of air in the same area.

Relative Humidityis the ratio of how much water vapour is in the air and how much water vapour the air could potentially contain at a given temperature. It only considers the invisible water vapour and is normally expressed as a percentage. A higher percentage means that the air and water mixture is more humid. When humidity is high, the air is clogged with water vapour and a rise in relative humidity in hot summer days increases the apparent temperature by hindering the evaporation of perspiration from the skin.

Under conditions of high humidity, the rate of evaporation of sweat from the skin decreases, since it is harder for the air with more moisture to absorb the sweat from our skin, and instead of feeling any relief, we simply feel hot and sticky. Thus, high humidity makes hot weather more unbearable, which can lead to heat exhaustion, even dehydration and death. The Relative humidity is used in weather forecasts and reports, as it is an important indicator of the likelihood of precipitation, dew, fog, even hurricanes.

Specific Humidity is the ratio of the amount of water vapour in the air to the amount of dry air in the area. It does not vary as the temperature or pressure of a body of air changes, provided the amount of water vapour or moisture remains the same. As the rate of evaporation of water from any surface is directly proportional to the specific humidity difference between the surface and the adjoining air, specific humidity is an extremely useful quantity in meteorology.