Specific humidity measures the amount of water vapor in a given mass of air. It remains constant regardless of pressure or temperature and is useful in predicting weather changes. Absolute humidity measures the mass of water to the total volume of air and changes with air pressure. Weather forecasters use specific humidity to predict precipitation.
Specific humidity is a way of measuring the amount of moisture, or water vapor, suspended in the air. In this case, specific humidity refers to the direct relationship between the amount of water vapor-laden air versus dry air in a predetermined mass of air. It is measured by taking the amount of water vapor and dividing it by the total mass of air in a given quantity to get a percentage ratio or result, usually expressed as grams of water vapor per kilogram of air.
The specific humidity remains constant regardless of pressure or temperature, as long as no moisture is added or reduced from a given mass. This differs from relative humidity in that it changes with fluctuations in the environment. Because of this rather stable method of measuring humidity, specific humidity is considered to be a very useful reading in the process of predicting weather changes in meteorology. Chemical engineering processes also calculate specific humidity to determine how it affects the outcome of chemical reactions. It is also used in mechanical engineering to test the stress level of building materials such as precast concrete.
Absolute humidity is a closely related concept. In absolute humidity one compares a ratio of the mass of water to a total volume of air, or grams per cubic meter, whereas with specific humidity it is a mass of water to a total mass of air, grams per kilogram . The main difference between them is that absolute humidity readings change as the volume and, therefore, the air pressure of a quantity of air changes. The term absolute humidity can be misleading, as it is a value that fluctuates with the air pressure, so in British standards it is referred to as volumetric humidity.
Weather forecasters often refer to specific humidity in the context of precipitation forecasting, as water vapor has a saturation point in the air depending on temperature and atmospheric pressure. If the specific humidity of the air increases due to evaporation of water entering the air from the ground or other sources and the temperature does not change, the saturation point of the air is also approaching, which could lead to precipitation . Relative humidity, dew point, and other factors also factor into weather calculations. These are all affected by temperature variations and air pressure changes based on the height of a given region above or below sea level.
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