Water-saturated air

Water-saturated air
Diagram showing that the cold air behind a cold front forces the warm, moist air ahead of it to rise, causing the development of clouds in advance of the cold front.
How clouds form in response to a cold front

Air contains water vapor and the amount of water in a given mass of dry air, known as the Mixing Ratio, is measured in grams of water per kilogram of dry air (g/kg).[1][2] The amount of moisture in air is also commonly reported as relative humidity; which is the percentage of the total water vapor air can hold at a particular air temperature.[3] How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible and tiny water and ice particles suspended above the Earth's surface)[4] depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The dew point is the temperature to which a parcel must be cooled in order to become saturated.[5]

There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands.[6] The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface,[7] usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath.[8] Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.[9]

The main ways water vapor is added to the air are: wind convergence into areas of upward motion,[10] precipitation or virga falling from above,[11] daytime heating evaporating water from the surface of oceans, water bodies or wet land,[12] transpiration from plants,[13] cool or dry air moving over warmer water,[14] and lifting air over mountains.[15] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature)[16] force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus.[17] Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions.[18]
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