Humidity is the amount of water vapor in air. Absolute humidity, relative humidity, and specific humidity are different ways to express the water content in a parcel of air. Relative humidity is the most frequently used of these expressions because of its importance in weather forecasting. Relative humidity indicates the likelihood of precipitation, dew, or fog. High relative humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by preventing the evaporation of perspiration from the skin. This effect is calculated in a heat index table. Warm water vapor has more thermal energy than cool water vapor and therefore more of it evaporates into warm air than into cold air.

A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries

In meteorology, precipitation (also known as hydrometeor) is any product of the condensation of atmospheric water vapor that is deposited on the earth's surface. It occurs when the atmosphere (being a large gaseous solution) becomes saturated with water vapour and the water condenses and falls out of solution (i.e., precipitates). Air becomes saturated via two processes, cooling and adding moisture.

Dew Point is the temperature at which air becomes saturated when cooled without addition of moisture or change of pressure. Any further cooling causes condensation; fog and dew are formed in this way.

Heat Index is a measure that combines air temperature and relative humidity measurements to indicate how hot it actually feels. Warm, dry air does not feel as hot as warm, humid air.