The Hydrological Cycle

Transcription

1 Introduction to Climatology GEOGRAPHY 300 The Hydrological Cycle Tom Giambelluca University of Hawai i at Mānoa Atmospheric Moisture Changes of Phase of Water Changes of Phase of Water 1

2 Changes of Phase of Water Phase changes involve energy Processes that absorb (require) energy Evaporation Melting Sublimation (solid to gas) Processes that release energy Condensation Freezing Sublimation (gas to solid) Kathy Hadley; Humidity = the water vapor content of the air Water vapor content can be expressed in various ways: Specific Humidity = mass of water vapor per mass of air (g/kg) Absolute Humidity = mass of water vapor per volume of air (g/m 3 ) Mixing Ratio = mass of water vapor per mass of "dry air" (g/kg) Vapor Pressure = partial pressure of water vapor (mb) Relative Humidity = actual water vapor content of the air maximum water vapor capacity of the air Relative Humidity = vapor pressure saturation vapor pressure x 100% Water vapor content can be expressed in various ways: Specific Humidity = mass of water vapor per mass of air (g/kg) Absolute Humidity = mass of water vapor per volume of air (g/m 3 ) Mixing Ratio = mass of water vapor per mass of "dry air" (g/kg) Vapor Pressure = partial pressure of water vapor (mb) Relative Humidity = e e sat e = vapor pressure (mb) e sat = saturation vapor pressure (mb) 2

3 Diurnal cycles of air temperature and relative humidity Saturation: condition of equilibrium between liquid water and water vapor or between ice and water vapor At the molecular level: Liquid to Gas: Occurs when liquid water molecules hit the air-water interface with sufficient kinetic energy to overcome surface tension. Velocity (kinetic energy) of molecules can be observed as the temperature of the water. The warmer the water, the more frequently molecules will "escape" the liquid to become gas molecules. Gas to Liquid: Gas molecules are also moving around, and those striking the liquid water surface will become liquid. The higher the the concentration of water vapor molecules (the higher the humidity), the more often water vapor molecules will strike the water surface and become liquid. The transition from liquid to gas depends mostly on the water temperature and the transition from gas to liquid depends mostly on the humidity. What would happen if you suddenly increase the temperature of the whole box? In the figure above, if the box is sealed and initially has no water vapor, the humidity will start to increase as liquid water molecules escape the surface tension. As the humidity increases, gas molecules hit the water surface more and more frequently. Eventually the humidity increases to a point where the rate of gas to liquid transitions is equal to the rate of liquid to gas transitions. This is an equilibrium. At this point, the humidity no longer increases and the air is said to be saturated. The relative humidity is 100%. What would happen if you suddenly increase the temperature of the whole box? Answer: That would increase the kinetic energy of the water causing the humidity to start increasing. We would observe evaporation taking place. Eventually, the air will reach a new equilibrium at a higher humidity. What would happen if you suddenly decreased the temperature of the box? Answer: The rate of gas-liquid transitions would continue at about the same rate, while the rate of liquid-gas transitions would decrease. We would observe condensation taking place. This would result in a reduction in the humidity. Eventually a new equilibrium would be reached at a lower humidity. 3

4 Saturation Vapor Pressure Saturation Vapor Pressure e sat is a function of temperature T (ºC) 10 e sat (mb) 12.3 T (ºC) 23 e sat (mb) Humidity Sample Problems Humidity Sample Problems If the vapor pressure is 10 mb and the temperature is 20ºC, what is the relative humidity? If the relative humidity is 50% and the temperature is 29ºC, what is the vapor pressure? RH=100 x e/e sat RH=100 x e/e sat First, you have to get the saturation vapor pressure, which is a function of the temperature. From the table, we get e sat = 23.4 mb. RH = 100 x 10/23.4 =42.7% e = (RH/100) x e T= 29ºC, e sat = 40.0 mb e = (50/100) x 40.0 = 20.0 mb 4

5 Dew Point Temperature Unsaturated air, if cooled sufficiently, will eventually reach saturation. The temperature to which you would have to cool a parcel of air in order to saturate it is called the Dew Point Temperature. Example: Dew Point Temperature Air with a vapor pressure of 20 mb, and a temperature of 25ºC is unsaturated (because e T = 25ºC is 31.7 mb). What is the dew point temperature? Dew point temperature is strictly a function of the vapor pressure. Look at the table to find out the temperature at which 20 mb would be the saturation vapor pressure. From the table, we see: e T = 17ºC is 19.4 mb e T = 18ºC is 20.6 mb By interpolation, we can get T = 17.5ºC is 20 mb Therefore, the dew point temperature is 17.5ºC. In other words, if this air were cooled from 25ºC down to 17.5ºC, the air would become saturated. Types of Condensation Importance of Fog in Hawai i 5

6 Importance of Fog in Hawai i Cloud Water Interception: Direct Contribution of Fog to the Hydrological Cycle Cloud Water Interception Adds Significant Amount of Water in Hawaii's Cloud Zone How Air Becomes Saturated Diabatic process: direct addition or removal of heat energy Adiabatic processes: No net exchange of energy Expansion of rising air causes the air to cool Dry (unsaturated) Adiabatic Lapse Rate: -1 deg C per 100 m (-5.5 deg F per 1000 ft) Sinking air experiences compression and warms by the same rate Saturated adiabatic lapse rate: ~-0.5 deg C per 100 m (-3.3 deg F per 1000 ft) 6

7 Dry Adiabatic Cooling Dry Adiabatic Lapse Rate In the atmosphere, vertical motion of air always induces cooling or heating. Rising unsaturated air cools at a specific, known rate, called: The DRY ADIABATIC (LAPSE) RATE: DAR DAR = 10ºC per 1000 m = 1ºC per 100 m=0.01ºc per m For example, air at 20ºC that is lifted 1500 m, will cool by 15ºC, decreasing to a temperature of 5ºC. Dry Adiabatic Lapse Rate DAR = 10ºC per 1000 m = 1ºC per 100 m=0.01ºc per m For example, air at 27ºC that is lifted 2500 m, will cool by 25ºC, decreasing to a temperature of 2ºC. Dry Adiabatic Lapse Rate Conversely, when unsaturated air descends, it warms at the DAR. Air starting at T = -20ºC at 3000 m, will warm by 30ºC by descending to the surface, increasing its temperature to 10ºC. 7

8 Moist Adiabatic Lapse Rate (MAR) If the air rises and cools to it's DEW POINT TEMPERATURE, it becomes saturated and water begins to condense out into droplets forming a cloud. Because condensation releases energy, the rising air no longer cools at the DAR. The addition of heat from condensation together with the continued cooling effect of expansion gives a net cooling rate lower than the DAR. This is called: The MOIST ADIABATIC (LAPSE) RATE: MAR MAR varies depending on the mositure content and temperature of the air. Average MAR = around 5 or 6ºC per 1000 m. Changing Buoyancy of Rising Air Environmental Lapse Rate Atmospheric Stability The ENVIRONMENTAL LAPSE RATE is highly variable. Its value in relation to the DAR and MAR determines the STABILITY of the air. STABLE AIR is not easily moved vertically. UNSTABLE AIR will move vertically on its own due to bouyancy effects. CONDITIONALLY UNSTABLE AIR will become unstable if condensation occurs. The stability is determined by the ENVIRONMENTAL LAPSE RATE. 8

9 Atmospheric Stability Atmospheric Stability Atmospheric Stability Environmental Lapse Rate in Hawaii Example of changing ELR: Diurnal Surface Heating The Trade Wind Inversion 9

10 Effect of Inversion on Rising Air 10