The Atmosphere and the Weather

Water in the Atmosphere

The significance of water as an atmospheric variable is a result of its unique physical properties.

Water is the only substance that exists as a gas, liquid, and solid at temperatures found at the earth's temperatures.

The water molecule, H₂O, is made up of two atoms of hydrogen and one of oxygen.

Little energy is available at low temperatures, and the bonds binding the water molecules are firm. The water molecules pack tightly in a fixed geometric in the solid phase.

As temperature increases, the available energy causes the bonds of the ice phase to weaken. Because they are not firmly set, bonds form, break, and form again. This permits flow to occur and represents the liquid phase of water. In this liquid stage, there is still bonding, but it is much less compact than in the ice phase.

At higher temperatures and with more energy, the bonding of the water molecules breaks down and the molecules move in a disorganized manner, which is the gas phase. If the temperature decreases, the molecules will revert to a less energetic phase and revert the process. Gas will change to liquid and liquid to solid.

Schematic diagram of the arrangement and motion of water molecules in different phases

Phase	Temperature	Atomic Arrangement of water molecules	Water molecule motion
Ice (solid)	Cold		Vibration about a fixed point
Water (liquid)	Warmer		Molecules slide over one another freely
Water Vapor (gas)	Hot		Widely spaced molecules move about rapidly

Deposition 680 cal/g Releases heat to the atmosphere
	Freezing 80 cal/g Melting		Condensation 590 cal/g Evaporation
Environment heat needed Sublimation 680 cal/g

The processes of melting, evaporation, and sublimation absorb energy. This added energy causes the molecules to change their bonding pattern.

The energy absorbed is latent heat and goes back to the environment when the phase changes reverse.

Water vapor is also extremely important in the atmosphere because:

• It transforms into both liquid and solid cloud particles that grow and fall to Earth as precipitation.

• Releases large amounts of heat - called latent heat when it changes from vapor into water or ice.

• Latent heat is an important source of energy, especially for storms.

• Water vapor strongly absorbs infrared radiation, making it an important gas in the Earth's heat-energy balance.

Now, how do we measure the amount of water vapor in the atmosphere?