Water in the Atmosphere Chapter 18.1 Water in the Atmosphere

Precipitation is any form of water that falls from a cloud. Water vapor is the source of all condensation and precipitation. Clouds, fog, rain, snow, sleet, and hail are all examples of weather.

Water’s Changes of State Water can change from solid to liquid to gas on Earth. This makes the water cycle possible. All water at one point, in the water cycle, must enter the atmosphere as water vapor.

Solid to Liquid When a state change takes place energy is transferred in the form of heat. When heat is transferred to a glass of ice water, the temperature of the ice water will remain at 0 ˚C until all of the ice has melted. When heat is added to ice water it is used to break apart the molecules in the ice cubes. This cause the ice to melt. Latent heat is the heat used to melt ice but does not produce a temperature change. (Latent means hidden) Latent heat can be important in the atmosphere. It can for thunderstorms, tornadoes, and hurricanes.

Liquid to Gas This process, of going from a liquid to a gas is called evaporation. Again, heat will be transferred to break apart the molecules in liquid to become gas. This energy needed is called latent heat of vaporization. The opposite process – water vapor to liquid – is called condensation. In the atmosphere condensation makes fog and clouds. In condensation water vapor releases energy. This release of energy can produce a lot of different weather.

Solid to Gas Sublimation is the direct conversion of a solid to a gas. Dry ice is a great example of sublimation. Deposition is when a vapor goes directly to a solid. This happens when water vapor turns to frost on cold objects like grass or windows.

Humidity Humidity is the amount of water vapor in the air.

Saturation As water evaporates it increases pressure in the air above it. At this point it forces more and more water molecules to liquid. At a certain point the number of water molecules leaving the water’s surface equals the amount of water returning. This is the point when the air is saturated. Warm air will contain more water vapor when saturated than saturated cold air.

Relative Humidity Relative humidity is the ratio of the air’s actual water-vapor content (how much water-vapor it has) compared to the amount of water vapor the air can hold at that temperature and pressure. Relative humidity shows how close the air is to saturation.

Relative humidity can be changed in 2 ways: Adding or removing water vapor – water vapor can be added to the atmosphere by evaporation from the oceans and smaller bodies of water. The second way is the amount of moisture needed for saturation depends on temperature, so relative temperature will change with temperature. Lowering the temperature increases the relative humidity and raising the air temperature decreases relative humidity.

Dew Point Dew-point temperature or dew point is the temperature the air would need to be cooled to reach saturation. If the air was cooled any more condensation would occur, we would have dew, fog, or clouds. We have dew on plants because in evening hours the air cools below the dew-point.

Measuring Humidity A hygrometer is used to measure relative humidity. Read pgs. 508 and 509 for how a hygrometer works.