Humidity transmitters provide electrical outputs that are proportional to humidity inputs. They measure variables such as relative humidity, absolute humidity, specific humidity, moisture content, dew point, temperature, and pressure. Relative humidity is a measure of the amount of water in the air compared with the amount of water that the air can hold at the measured temperature. Absolute humidity is the mass of water vapor in a given volume of air. Specific humidity is absolute humidity divided by the total mass of the given volume of air. Moisture content or water content is the ratio of water mass to total mass. Dew point is the temperature to which air must be cooled in order for saturation to occur. Some humidity transmitters measure pressure. Others measure temperature. Typically, devices that measure pressure or temperature include additional probes.