The chemical compound that we call water is a unique substance in that it can exist in three phases within the ranges of temperatures and pressures found in the atmosphere. At very cold temperatures, water is a solid, ice, that covers our lakes and puddles. Warm it some and it melts to liquid that covers almost two-thirds of the Earth and falls from clouds above. At all temperatures some water exists in the invisible vapor phase. The transition between these forms is crucial to the conditions we associate with "weather". A volume, even without air in it, has a maximum capacity for vapor that depends on the temperature. As the temperature rises, more ice can sublimate or water can evaporate to provide additional vapor for the volume. As the temperature falls, some vapor must condense back to the liquid or solid form. The maximum amount of vapor in the volume at its associated temperature produces the condition called saturation.
Normally a volume of air is not saturated, meaning that less than the maximum possible vapor is present. Saturation can occur in one of two ways. More vapor can be added to the volume to reach the maximum. This addition is seen just above the spout of a boiling tea kettle, or with so-called steam fog, a version of your breath on a cold day. Generally the atmosphere creates saturation a second way, by cooling the air volume to reach saturation with the amount of vapor originally present. Cooling to saturation can be by one of two processes. Rising air cools as it expands into lower surrounding pressures at higher altitudes. Saturation and clouds can result. Overnight radiational cooling near the ground may create fog.
For more information describing the global hydrological cycle, you may consult the optional material in Tuesday's electronic Supplemental Summary Information .
To be submitted on the lines for Tuesday on the Study Guide, Part B, Applications, Week 6 Chapter Progress Response Form, under section B. Daily Summary.