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Warm Air Over a Cold Surface
When air moves over a cold surface and lowers the air temperature, it can reach saturation. This occurs, for instance, when a warm stream of air flows over an ocean or lake. Because the body of water does not hold as much heat as land does, the air cools naturally, as the air and water begin to reach equilibrium in temperature. If the air cools enough, it will reach saturation.
Cold Air and Warm Air
When cold air mixes with warm air, the temperature is lowered. This often occurs when a breeze from a sea or large lake goes over the mainland and mingles with a stream of air from the mainland. The cooler sea breeze will lower the temperature of the warmer mainland breeze and result in saturation of the air.
Air Cooling
Air can be cooled by nighttime contact with a cool ground. As the sun goes down, the earth begins to lose some of the heat it had been retaining from exposure to the sun. As this happens, the lower temperatures of the ground cause the air to lose temperature as well. If the air loses enough temperature, it may result in saturation of the air.
Air Forced Upwards
Air can be forced upwards in a few ways, such as when it flows over a mountain or when the sun heats up the ground, causing convection currents. Convection currents occur all the time within bodies that have different temperatures through their systems. For instance, when you heat water, the water at the bottom, closest to the fire, will eventually rise, forcing colder water from the top to come down. As the molecules of the water flow upwards, they lose heat and expend energy. Air works in the same way, and as air temperature decreases, saturation is easier to achieve.
Increase in Humidity
When humidity increases, the gap between dew point and temperature is decreased. Humidity is the measure of how much water is in the air, compared to how much it could hold. If the humidity is 50 percent, that means that currently the air has half of the condensation it could hold before becoming saturated. If you increase the humidity by adding water content to the air, you will reduce the gap between the dew point and the temperature, and if the humidity increases to 100 percent, the dew point will equal the air temperature. The humidity index also increases as the temperature of the air decreases.
Water Vapor
When water vapor is added to the air, saturation is increaed. Water vapor can be added to the air by transpiration and by evaporation. Transpiration occurs when plants absorb water and release it into the atmosphere from their leaves, and evaporation occurs when water is heated up enough to evaporate into the air. Evaporation accounts for the large majority of water in the air. As you add water to the air, the humidity increases, and the dew point gets closer to the temperature. If you add enough water to the air, the air will become saturated.