Water in the Atmosphere

Transcription

1 Water in the Atmosphere Chapter 24 Solid to Liquid The process of changing state, such as melting ice, requires that energy be transferred in the form of heat. Latent heat is the energy absorbed or released during a change in state. Liquid to Gas Evaporation is the process of changing a liquid to a gas. Condensation is the process where a gas, like water vapor, changes to a liquid, like water. Latent Heat Precipitation is any form of water that falls from a cloud. When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere. Precipitation Solid to Gas Sublimation is the conversion of a solid directly to a gas without passing through the liquid state. Deposition is the conversion of a vapor directly to a solid. 1

2 Changes of State Dew Point Dew point is the temperature to which a parcel of air would need to be cooled to reach saturation. Specific Specific is the number of grams of water vapor in 1 kg of air. Very moist air in tropical regions will contain more water vapor than the cold dry air in the polar regions. Dew on a Spider Web Warm air holds more moisture than cold air. is a general term for the amount of water vapor in air. Saturation Air is saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure. When saturated, warm air contains more water vapor than cold saturated air. Relative Relative humidity is a ratio of the air s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure. ACTUAL:MAXIMUM (%) LET s EXAMINE: When the water-vapor content of air remains constant, lowering air temperature causes an increase in relative humidity, and raising air temperature causes a decrease in relative humidity. Measuring Psychrometer A psychrometer is a hygrometer with dry- and wet-bulb thermometers. Evaporation of water from the wet bulb makes air temperature appear lower than the dry bulb s measurement. The two temperatures are compared to determine the relative humidity. 2

3 Hydrometer/Hygrometer Two Conditions for Cloud Formation Saturation In addition, for clouds to form, the air must be saturated with water vapor. When air temperature reaches the dew point, the air is saturated and water vapor begins to condense. Temperature plays a major role in cloud formation. As warm, moist air is cooled, the dew point decreases which can lead to cloud formation. Two Conditions for Cloud Formation Solid Surface A solid surface must be available on which condensation can take place. Suspended particles in the atmosphere, including ice, salt, dust, and other solid matter, are called condensation nuclei. These particles provide the necessary surfaces for cloud-forming condensation. #1 Convective Cooling Process When air in the atmosphere rises, the temperature of the air decreases. This occurs due to the following process: As air rises, it moves from a region of high air pressure to a region of low air pressure. As a result, the molecules in the air move further apart. The work of expanding the distance between molecules of air uses potential energy that is stored in compressed air. As this energy is used, the temperature falls. And just as expansion causes air to cool, compression causes air to become warmer. Most Clouds are formed by convective cooling. Adiabatic Temperature Changes These are temperature changes that results solely from the expansion or compression of air. If the rising air reaches a level where its temperature is lower than its dew point, then the moisture in the air condenses to form a cloud. 3

4 2/3/2014 Cloud Formation by Adiabatic Cooling CONDENSATION LEVEL Dew Point = 2oC Dew Point = 5oC Dew Point = 10oC Dew Point = 15oC Condensation Level is the level at which the air temperature is equal to its dew point. Lower temps will result in condensation = CLOUDS. Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting. Orographic Lifting Orographic lifting occurs when mountains act as barriers to the flow of air, forcing the air to ascend. The air cools adiabatically; clouds and precipitation may result. Orographic Lifting Frontal Wedging A front is the boundary between two adjoining air masses having contrasting characteristics. Convergence Convergence is when air flows together and rises. Localized Convective Lifting Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy. Convergence and Localized Convective Lifting 4

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