Chapter 4: Moisture and Atmospheric Stability The hydrologic cycle

Transcription

1 Chapter 4: Moisture and Atmospheric Stability The hydrologic cycle from: USGS Evaporation: enough water to cover the entire surface of Earth to 1 meter cycles through the atmosphere each year Most evaporation occurs from the oceans A much smaller amount from lakes, streams, and runoff Infiltration Less rain falls on oceans than evaporates from the surface of the oceans More rain falls on land surface than evaporates from the land Most water runs off the land surface but some infiltrates the ground Even water the infiltrates the land surface eventually makes its way back to the ocean Transpiration Plants use water for photosynthesis but they also put water into the atmosphere by transpiration

2 Water and change of state Under the conditions existing on Earth, water exists in three states: solid, liquid, and gas From Wikipedia. Heating water in the liquid state requires 1 calorie per degree per gram Latent heat of fusion: melting ice requires 80 calories per gram Note the temperature does not change during the melting process Note that when water freezes it releases 80 calories per gram The hidden or latent heat is breaking some of the bonds in the ice allowing the molecules to flow Latent heat of vaporization: evaporating water requires 540 to 600 calories per gram When water vapor condenses it releases the same amount of energy per gram The range occurs because it requires less heat to vaporize water at 100 C than at 0 C The latent heat is breaking all the remaining bonds: the molecules to become gaseous Sublimation ice can go directly to the gaseous phase The energy required for this process is equal to the sum of the energy involved in melting and vaporization combined Deposition is the opposite process and releases the same amount of energy required during sublimation Humidity water vapor in the air Humidity the general term for the amount of water vapor in the air Absolute humidity the mass of water vapor in a given volume of air Absolute humidity = Mixing ratio the amount of water vapor in a unit of air compared to the remaining mass of dry air Mixing ratio = Vapor pressure that part of the total atmospheric pressure attributable to its water-vapor content Saturation an equilibrium condition in which the number of water molecules leaving the surface of a Figure 1: The picture shows the particle liquid water reservoir is equal to the number of water transition, as a result of their vapor molecules returning to the liquid reservoir pressure, from the liquid phase to the gas Saturation vapor pressure the pressure exerted by the phase and converse. motion of water-vapor molecules in a sample of (from: saturated air

3 The saturation vapor pressure is an exponential function of the temperature of the water (or air assuming equilibrium temperature) See Figure 2 Relative humidity the ratio of the air s actual water-vapor content compared with the amount of water vapor required for saturation at that temperature (and pressure) Example: Using Table 4 1 on page 102, calculate the relative humidity for air that has a mixing ratio of 10 g of water vapor per kilogram at 25 C From the table, saturation occurs at 20 g per kg at a temperature of 25 C, therefore: / 100% 50% relative humidity / Figure 2: A graph showing the temperature dependence of water-vapor pressure. (from: Changes in relative humidity Add or subtract moisture Evaporation from oceans, lakes, bodies of water Transpiration from plants Change the temperature Heating air causes relative humidity to drop Cooling air causes relative humidity to rise A 10 C change in air temperature doubles the amount of water required to reach saturation Natural changes in relative humidity 1. Daily changes in temperature (day / night) 2. Temperature changes as air moves horizontally 3. Temperature changes as air moves vertically Dew point (or dew point temperature) the temperature to which a parcel of air would need to be cooled to reach saturation Advantages of dew point Unlike relative humidity (how near air is to being saturated), dew point is a measure of actual moisture content As long as moisture is not being added or subtracted from the air, the dew point is constant as the temperature changes (unlike relative humidity which changes with changes in the temperature) Humidity measurement Hygrometer an instrument used to measure humidity Psychrometer two identical thermometers mounted side-by-side, one with a thin muslin wick tied around the bulb is called the wet bulb Air is passed over the two bulbs either by fanning or by swinging the thermometers causing evaporation from muslin wick so that the temperature of the wet bulb thermometer is lower than the dry bulb thermometer The higher the humidity is in the air, the smaller the temperature difference between the thermometers

4 Tables (like those in Appendix C-1 and C-2) are consulted to determine the relative humidity or the dew point Example: A swing psychrometer is used to measure the humidity on a day where the dry bulb measures 24ºC and the wet bulb measures 20ºC. Find the relative humidity and the dew point Find the difference in wet and dry bulb temperatures 24ºC - 20ºC = 6ºC From the tables C-1: relative humidity = 55% C-2: dew point temperature = 14 C Hair hygrometer hair changes length in proportion to humidity The limitation is that this hygrometer is very slow Electric hygrometer has an electrical conductor coated with a moisture-absorbing chemical which causes the current to vary with humidity Many weather stations have converted to electrical hygrometers Adiabatic temperature changes changes in temperature of the air in which heat is neither added nor subtracted from the air Causes of adiabatic temperature changes Expansion causes air to cool Compression causes air to warm Adiabatic cooling and condensation Parcel an imaginary volume of air that acts independently of the surrounding air in which it is assumed no heat enters or leaves Entrainment when surrounding air does infiltrate a vertically moving column of air No entrainment occurs in a parcel Dry adiabatic rate the rate of heating or cooling of a vertically moving column of unsaturated air (air less than 100% relative humidity) Wet adiabatic rate the rate of heating or cooling of a vertically moving column of saturated air (air at 100% relative humidity) Note that cloud formation occurs during wet adiabatic cooling (rising air) Lifting condensation level the altitude at which a parcel reaches saturation and cloud formation begins Processes that lift air 1. Orographic lifting air is forced to rise over a mountainous barrier Rain shadow desert air that reaches the leeward side of a mountain warms as it descends causing the relative humidity to drop making condensation or precipitation very unlikely 2. Frontal wedging warmer, less dense air is forced over cooler, more dense air Front masses of warm and cold air collide producing a front 3. Convergence a pile up of horizontal air flow results in upward movement 4. Localized convective lifting unequal surface heating causes pockets of air to rise because of their buoyancy The critical weathermaker: atmospheric stability Stable air if a parcel that is forced to rise cools fast enough that it is cooler than the surrounding air it will be more dense than the surrounding air and if allowed to do so will tend to sink back to its original position Unstable air if a parcel that is forced to rise cools slowly enough that it is warmer than the surrounding air it will be less dense than the surrounding air and will rise until its temperature cools enough to equal the temperature of the surrounding air

5 Absolute stability occurs when the environmental lapse rate is less than the wet adiabatic rate Example: suppose the environmental lapse rate is 5 C per 1000 m, the dry adiabatic rate is 10 C per m, the wet adiabatic rate is 6 C per m, the lifting condensation level is 2000 m, and the air temperature at the surface is 25 C. Show that the air exhibits absolute stability. Height Air Temp Parcel Temp 5000 m 5 C 13 C 4000 m 5 C 7 C wet rate: 6 C per 1000 m 3000 m 10 C 1 C 2000 m 15 C 5 C clouds form 1000 m 20 C 15 C dry rate: 10 C per 1000 m Surface 25 C 25 C Note that the rising parcel is always cooler than the surrounding air (stable air) Absolute instability occurs when the environmental lapse rate is greater than the dry adiabatic rate Example: suppose the environmental lapse rate is 12 C per 1000 m, the dry adiabatic rate is 10 C per m, the wet adiabatic rate is 6 C per m, the lifting condensation level is 2000 m, and the air temperature at the surface is 25 C. Show that the air exhibits absolute stability. Height Air Temp Parcel Temp 5000 m 35 C 13 C 4000 m 23 C 7 C wet rate: 6 C per 1000 m 3000 m 11 C 1 C 2000 m 1 C 5 C clouds form 1000 m 13 C 15 C dry rate: 10 C per 1000 m Surface 25 C 25 C Note that the rising parcel is always warmer than the surrounding air (unstable air) Conditional instability occurs when moist air has an environmental lapse rate is between the dry and wet adiabatic rates Example: suppose the environmental lapse rate is 9 C per 1000 m, the dry adiabatic rate is 10 C per m, the wet adiabatic rate is 6 C per m, the lifting condensation level is 2000 m, and the air temperature at the surface is 40 C. Show that the air exhibits conditional instability. Height Air Temp Parcel Temp 5000 m 5 C 2 C 4000 m 4 C 8 C wet rate: 6 C per 1000 m 3000 m 13 C 14 C 2000 m 22 C 20 C clouds form 1000 m 31 C 30 C dry rate: 10 C per 1000 m Surface 40 C 40 C Note that the rising parcel is always cooler than the surrounding air below the condensation level (stable air) but is warmer than the surrounding air above the condensation level (unstable air)

6 How stability changes Factors that enhance instability 1. Intense solar heating that warms the lowermost layer of the atmosphere 2. Heating of an air mass from below as it passes over a warm surface 3. General upward movement of air (caused by any of the lifting processes) 4. Radiation cooling from cloud tops Factors that enhance stability 1. Radiative cooling of Earth s surface after sunset 2. Cooling of an air mass from below as it passes over a cold surface 3. General subsidence within an air column