Overview
Moisture refers to the presence of water in any of its forms—vapour, liquid droplets or ice—occurring in the atmosphere, soils, manufactured materials and biological tissue. In everyday language it often denotes small or partly wet amounts, but in science and engineering it is quantified and characterised because it affects physical, chemical and biological processes.
Forms and how it is expressed
Atmospheric moisture appears primarily as water vapour; clouds and fog are collections of tiny liquid droplets and precipitation is larger liquid or solid particles. Within materials, moisture can be free (liquid that moves under gravity) or bound (water held by chemical or physical adsorption). Common metrics include relative humidity (percentage of saturation), absolute humidity (mass of vapour per volume), dew point, and moisture content (percentage of water by mass in a material).
Measurement and instruments
- Hygrometer and electronic humidity sensors measure air moisture and relative humidity.
- Psychrometers use wet- and dry-bulb temperatures to infer humidity.
- Moisture meters (electrical resistance or capacitance types) estimate water content in wood, drywall and soil.
- Specialised devices such as tensiometers and neutron probes assess soil moisture for agriculture.
Effects, importance and examples
Moisture influences weather, plant growth, corrosion, biological decay and comfort. High indoor moisture can produce condensation, mould, structural damage and poor air quality; low moisture can cause drying and cracking of wood and increased static. In manufacturing and food storage, controlling moisture prevents spoilage, chemical degradation and product failure. In agriculture, soil moisture is a key variable determining crop yields and irrigation needs.
Control and mitigation
Common control measures include ventilation, dehumidification, heating, use of desiccants and vapour barriers in buildings, waterproofing treatments, and drainage systems for soils. Understanding the form and location of moisture is essential to choose the right remedy and to prevent recurring problems.
Notable distinctions and historical notes
Although often used interchangeably, "humidity" typically refers to moisture in the air, while "moisture content" refers to water in materials. Instruments and methods to observe and quantify moisture have evolved from simple qualitative devices to precise electronic sensors, allowing modern meteorology, building science and industrial processes to manage moisture more effectively.