Therefore, atmospheric pressure always decreases with increasing height. A column of air in cross section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 pounds per square inch. The amount of water vapor present in the air is known as humidity. The higher the temperature, the greater the number of water molecules the air can hold. Humidity is also commonly used to describe the relative humidity.
Absolute humidity is the mass of water vapor present per unit volume of air.
Thermometer is an instrument for measuring temperature consisting typically of a glass bulb attached to a fine tube of glass with a numbered scale and containing a liquid that is sealed in and rises and falls with changes of temperature. A barometer on the other hand is used to measure atmospheric pressure. A typical barometer employs a mercury-filled glass tube, sealed at one end with a mercury reservoir at the other end. The mercury then adjusts between the tube and the reservoir to reflect atmospheric pressure. Low pressure will cause the mercury to drop in the tube.