Subsection 10.1.2 Atmospheric Moisture
Atmospheric moisture refers to the water vapor present in the Earth’s atmosphere. Water vapor is one of the most important greenhouse gases, and it plays a critical role in regulating the Earth’s temperature and climate. The amount of water vapor in the atmosphere can vary depending on factors such as temperature, humidity, and atmospheric pressure. Water vapor is constantly moving in and out of the atmosphere through processes such as evaporation, transpiration, and condensation. As warm air rises, water vapor in air expands, cools, and condenses into clouds, leading to precipitation or rain Water Cycles. When air cools to its dew point, water droplets may form, which can lead to fog or dew. The amount of moisture (water vapor) contained in the air is known as humidity. Humidity is typically measured in relative humidity. Relative humidity (R.H.) is the ratio of the amount of water vapor present in the air to the maximum amount of water vapor needed to saturate the air at a given temperature. Relative humidity expressed in percentage.
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gpm.nasa.gov/education/sites/default/files/videos/WaterCycleMovie-NoText.mp4
\begin{equation*}
R.H. = \frac{\text{water vapor in air}}{\text{maximum water vapor the air can hold at that temperature}} \times 100\%
\end{equation*}
Warmer air can hold more water vapor than cooler air, so relative humidity can change depending on temperature changes. When air contains the maximum amount of moisture, it is said to be saturated.
One cubic meter volume of air van hold a maximum of about 30 grams of water vapor at temperature \(80 ^oF \text{.}\) Dry air has zero relative humidity but saturated air has 100% relative humidity. Humid (or saturated) air is very uncomfortable, since the rate of evaporation is slowed by the high levels of water contents already in the air. At that time, human skin cannot sweat (perspirate) properly and we feel uncomfirtable. Dew point: is another way of expressing the saturation. The dew point is the temperature at which the air becomes saturated with the available water vapor causing condensation to occur. At this point, the air cannot hold any more moisture, and the excess water vapor begins to condense into liquid droplets. The dew point is an important measurement in weather forecasting, as it helps determine the likelihood of dew, fog, or frost formation.
