Water in the Atmosphere

Transcription

1 Chapter 5 Lecture Understanding Weather and Climate Seventh Edition Water in the Atmosphere Frode Stordal, University of Oslo Redina L. Herman Western Illinois University

2 The Hydrologic Cycle Earth has been called the water planet as over ¾ of the Earth s surface is covered by water. Water is unique in that it can simultaneously exist in all three states (solid, liquid, gas) at the same temperature and shift between states easily. The hydrologic cycle refers to the cycle of water through Earth and atmosphere.

3 The Hydrologic Cycle

4 Water Vapor and Liquid Water Evaporation and Condensation Molecules escape into the overlying volume as water vapor during evaporation. Energy must be available at the water surface. Water vapor increases in air as surface water evaporates. Water vapor molecules randomly collide with the water surface and bond with adjacent molecules during condensation. There is an equilibrium between evaporation and condensation during saturation. Upon saturation, condensation begins and water returns to the surface. The air/atmosphere does not hold water vapor. Water vapor exists in gas form just like any other atmospheric gas: oxygen, nitrogen, etc.

5 Water Vapor and Liquid Water Evaporation and Condensation

6 Indices of Water Vapor Content Introduction Humidity is the amount of water vapor in air. Humidity is expressed in several ways. Each has advantages and disadvantages, depending upon use.

7 Indices of Water Vapor Content Vapor Pressure Vapor pressure is the amount of pressure exerted on the atmosphere by water vapor. Vapor pressure is dependent upon both density and temperature of the vapor.

8 Indices of Water Vapor Content Vapor Pressure Saturation vapor pressure is the vapor pressure of the atmosphere when it is saturated. The movement of water vapor molecules exerts vapor pressure on surfaces.

9 Indices of Water Vapor Content Vapor Pressure Saturation vapor pressure is temperature dependent. At low temperatures the saturation vapor pressure increases slowly, but it increases rapidly at higher temperatures. It is not a linear increase. Nonlinear increase in saturation vapor pressure with increase in temperature.

10 Indices of Water Vapor Content Absolute Humidity Absolute humidity is the density of water vapor, expressed as the number of grams of water vapor per cubic meter of air (g/m 3 ). The absolute humidity value changes as air volume expands and contracts.

11 Indices of Water Vapor Content Specific Humidity Specific humidity represents a given mass of water vapor in a given mass of air (dry + water vapour) (g/kg). Specific humidity does not change as air expands and contracts. Saturation specific humidity is the specific humidity of the atmosphere when it is saturated.

12 Indices of Water Vapor Content Mixing Ratio Mixing ratio is a measure of atmospheric moisture; it is the mass of water vapor per unit mass of dry air, usually expressed in grams per kilograms (g/kg). Mixing ratio is very similar to specific humidity in that it expresses the mass of water vapor relative to air mass. Maximum mixing ratio is the saturation mixing ratio.

13 Indices of Water Vapor Content Relative Humidity Relative humidity (RH) is the amount of water vapor relative to the maximum that can exist at a particular temperature. RH = (specific humidity/saturation specific humidity) x 100% Relative humidity describes the amount of water vapor relative to a saturation point. The saturation point, or the relative humidity term, is relative to air temperature and total water vapor.

14 Indices of Water Vapor Content Relative Humidity The highest RH occurs in the morning, during the coolest time of the day. The lowest RH occurs in late afternoon, during the warmest time of the day. Because of temperature dependency, the term cannot be used to compare moisture content at different locations having different temperatures.

15 Indices of Water Vapor Content Relative Humidity The relationship between RH and temperature.

16 Indices of Water Vapor Content Dew Point The dew point temperature is the temperature at which saturation occurs in the air and is dependent upon the amount of water vapor present. High dew points indicate abundant atmospheric moisture. Dew points can be only equal or less than air temperatures. If saturation is reached and air temperatures cool further, water vapor is removed from the air through condensation. When air reaches saturation at temperatures below freezing, the term frost point is used.

17 Processes That Cause Saturation Air can become saturated in three ways: The addition of water vapor Mixing cold air with warm air Moist air by cooling the air to dew point

18 Factors Affecting Saturation and Condensation Effect of Curvature Small drops exhibit greater curvature than larger ones. Curvature influences saturation vapor pressure with highly curved drops. For very small drops, requires supersaturation of 200 percent. Hygroscopic (water attracting) aerosols act as condensation nuclei (particles onto which water droplets form). Condensation onto condensation nuclei, called heterogeneous nucleation, causes dissolution of the aerosol.

19 Factors Affecting Saturation and Condensation Effect of Curvature Larger drops have less curvature than smaller ones.

20 Factors Affecting Saturation and Condensation The Role of Condensation Nuclei Evaporation from solutions is lower than for pure water. This opposes curvature influences in a way that condensation typically occurs at RHs near 100 percent. Hygroscopic nuclei is abundant in the atmosphere from many natural sources (salt, dust, ash, etc.) and anthropogenic sources (combustion derivative). Small condensation nuclei lead to very tiny water drops, or haze.

21 Factors Affecting Saturation and Condensation Effect of Curvature Small droplets require higher RHs to remain liquid.

22 Factors Affecting Saturation Ice Nuclei Atmospheric water does not normally freeze at 0 C. Supercooled water refers to water having a temperature below the melting point of ice but nonetheless existing in a liquid state. Ice crystal formation requires ice nuclei, a rare temperaturedependent substance similar in shape to ice (six-sided). Examples: clay, ice fragments, bacteria, etc. Ice nuclei become active at temperatures below -4 C Between -10 and -30 C, saturation may lead to ice crystals, supercooled drops, or both. Below -30 C, clouds are composed solely of ice crystals. At or below -40 C spontaneous nucleation, the direct deposition of ice with no nuclei present, occurs.

23 Cooling the Air to the Dew or Frost Point Adiabatic Processes Cloud formation typically involves temperature changes with no exchange of energy (adiabatic process), according to the first law of thermodynamics. Rising air expands through an increasingly less dense atmosphere, causing a decrease in internal energy and a corresponding temperature decrease. Parcels expand and cool at the dry adiabatic lapse rate (DALR), 1 C/100 m. Parcels may eventually reach the lifting condensation level, the height at which saturation occurs. Parcels then cool at the saturated adiabatic lapse rate (SALR), ~0.6 C/100.

24 Cooling the Air to the Dew or Frost Point Adiabatic Processes Dry adiabatic cooling.

25 Cooling the Air to the Dew or Frost Point The environmental (ambient) lapse rate (ELR) refers to an overall decrease in air temperature with height. This rate, which changes from place to place, stems from the fact that air located farther from surface heating is typically cooler than that nearer the surface. A comparison of adiabatic and environmental cooling rates.

26 Forms of Condensation Dew Dew is liquid condensation on surface often occurring during the early morning hours. Loss of longwave radiation at night can cause the surface to cool diabatically. Surface air becomes saturated and condensation forms on objects acting as condensation nuclei.

27 Atmospheric Moisture and Climate Change The IPCC studies around the world have found that increases in specific humidity near the surface have been associated with increasing temperatures since Over most oceanic areas, relative humidities have remained fairly constant, as increasing water vapor contents have been offset by increases in the saturation specific humidity. Over some land areas, increases in specific humidity have been more than offset by increases in the saturation levels, leading to locally unchanged or slightly reduced relative humidities.