While nitrogen and oxygen constitute the bulk of the atmosphere, their concentrations are stable worldwide. In contrast, water vapor is highly localized and dynamic. Its presence depends heavily on the air’s temperature; warmer air can hold significantly more moisture than cold air. This variability is central to meteorology, as the phase changes of water—from vapor to liquid or ice—release or absorb latent heat. This energy exchange fuels storms, regulates global temperatures and determines regional climates. Because of this constant fluctuation in space and time, it is classified as a variable constituent rather than a permanent one.