Chapter 6: Atmospheric Moisture

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1 Chapter 6: Atmospheric Moisture I. The Impact of Atmospheric Moisture on the Landscape A. Atmospheric moisture influences landscape both in short term and long term. 1. Short term, with puddles, flooding, snow and ice; 2. Long term, with precipitation integral to weathering and erosion, critical to vegetation. II. Water Vapor and the Hydrological Cycle A. Water vapor the gaseous state of water; atmospheric moisture. 1. Changes easily from one state to another with temperature and pressure changes. a) This ease of changing results in erratic distribution around the world. 2. Can be virtually absent in some parts of world, constitutes as much as 4% of atmospheric volume in other parts. 3. Essentially restricted to lower troposphere. B. Hydrologic cycle ceaseless interchange of moisture in terms of its geographical location and its physical state: 1. Water evaporates, becomes water vapor; a) Goes into atmosphere; (1) Vapor condenses, becomes liquid or solid state; (a) Returns to Earth. 2. Hydrologic cycle intricately related to many atmospheric phenomena. a) Important determinant of climate: (1) Rainfall distribution (2) Temperature modification III. Evaporation A. Evaporation process by which liquid water is converted to gaseous water vapor. 1. Molecules of water escape the liquid surface into the surrounding air. B. Temperature 1. Temperature is a key factor in evaporation, both in water and in the air around it. a) Molecules become more agitated the higher the temperature, and this agitation leads to evaporation. 2. Temperature works in conjunction with pressure. a) Vapor pressure the pressure exerted by water vapor in the air. (1) At any given temperature, there is a maximum vapor pressure that water vapor molecules can exert. (a) Saturated air the point at which some water vapor molecules must become liquid because maximum vapor pressure is exceeded. (i) The warmer the air, the more water vapor it can hold before becoming saturated. C. Still Versus Moving Air 1. Movement in air through windiness and/or turbulence helps promote evaporation by removing saturated air. a) Disperses vapor molecules and thus makes air above water surface less saturated, so rate of evaporation can increase. D. Evapotranspiration 1. Evapotranspiration the process of water vapor entering the air from land sources. a) Evapotranspiration occurs through two ways: (1) Transpiration the process by which plant leaves give up their moisture to the atmosphere; (2) Evaporation from soil and plants. b) Most evapotranspiration occurs through plants. 2. Potential evapotranspiration the maximum amount of moisture that could be lost from soil and vegetation if the ground were sopping wet all the time. 3. Potential evapotranspiration rate and actual rate of precipitation play a key role in determining a region s groundwater supply (or lack of it). IV. Measures of Humidity A. Humidity the amount of water vapor in the air. B. Absolute Humidity C. Absolute humidity a direct measure of the water vapor content of air. 1. Expressed as the weight of water vapor in a given volume of air, usually as grams of water per cubic meter of air. a) Amount is a function of how much volume is being considered. (1) If the volume of air doubles, the absolute humidity halves. b) Absolute humidity is limited according to temperature. (1) The colder the air, the less vapor it can hold. D. Specific Humidity

2 1. Specific humidity a direct measure of water-vapor content expressed as the mass of water vapor in a given mass of air (grams of vapor/kilograms of air). E. Relative Humidity 1. Relative humidity an expression of the amount of water vapor in the air in comparison with the total amount that could be there if the air were saturated. This is a ratio that is expressed as a percentage. a) Relative Humidity = Actual Water Vapor in Air/Capacity x 100 b) Relative humidity changes if either the water vapor content or the water vapor capacity of the air changes. c) Also changes if temperature changes. (1) Relationship between temperature and relative humidity is one of most important in all meteorology. (a) Inverse relationship as one increases, the other decreases. (2) Relative humidity can be determined through the use of a psychrometer (see Appendix IV for a description of humidity measurement via this instrument). F. Related Humidity Concepts 1. Dew point the critical air temperature at which saturation is reached. 2. Cooling is the most common way that air is brought to the point of saturation and condensation. 3. Sensible temperature a concept of the relative temperature that is sensed by a person s body. V. Condensation A. Condensation process by which water vapor is converted to liquid water; opposite of evaporation. 1. For condensation to take place, air must be saturated. a) Condensation cannot occur, however, even if the air is saturated, if there is not a surface on which it can take place. (1) Air becomes supersaturated if surface is not available. b) In upper atmosphere, surfaces are available through hygroscopic particles or condensation nuclei tiny atmospheric particles of dust, smoke, and salt that serve as collection centers for water molecules. (1) Most common are bacteria blown off plants or thrown into air by ocean waves. B. Adiabatic Processes 1. Key physical geographic fact: a) Large masses of air can be cooled to the dew point ONLY by expanding as they rise. (1) Because of this limitation, adiabatic cooling is the only prominent mechanism for development of clouds and production of rain. 2. Dry adiabatic lapse rate the rate at which a parcel of unsaturated air cools as it rises; this rate is relatively steady (5.5 F per 1000 feet) (10 C/km). a) Air is not necessarily dry, just not saturated. b) Descending air warms, and it does so at the dry adiabatic lapse rate. 3. Lifting condensation level (LCL) the altitude at which rising air cools. sufficiently to reach 100% relative humidity at the dew point temperature, and condensation begins. 4. Saturated adiabatic lapse rate the diminished rate of cooling, which occurs when air rises above the lifting condensation level. It depends on temperature and pressure, but averages about 3.3 F per 1000 feet (6 C/km). C. Clouds 1. Not all clouds precipitate, but all precipitation comes from clouds. 2. At any given time, about 50% of Earth is covered by clouds. 3. Clouds play an important role in the global energy budget. a) Receive insolation from above and terrestrial radiation from below. (1) They absorb, reflect, scatter, or reradiate this energy, and so influence radiant energy. 4. Clouds are classified on the basis of two factors: a) Form b) Altitude 5. Three forms of clouds: a) Cirriform clouds a cloud that is thin, wispy, and composed of ice crystals rather than water particles; it is found at high elevations. b) Cumuliform clouds a cloud that is massive and rounded, usually with a flat base and limited horizontal extent, but often billowing upward to great heights. c) Stratiform clouds a cloud form characterized by clouds that appear as grayish sheets or layers that cover most or all of the sky, rarely being broken into individual cloud units. (1) These 3 cloud forms are subclassified into 10 types based on shape. (a) One type may evolve into another. (2) Three of these 10 are purely one form, while the other 7 are combinations of these three. (a) Three pure forms: (i) Cirrus cloud high cirriform clouds of feathery appearance. (ii) Cumulus cloud puffy white cloud that forms from rising columns of air. (iii) Stratus cloud low clouds, usually below 6500 feet (2 km), which sometimes occur as individual clouds but more often appear as a general overcast.

3 6. Precipitation comes only from clouds that have nimb in their name; specifically, nimbostratus or cumulonimbus. a) Cumulonimbus cloud cumuliform cloud of great vertical development often associated with a thunderstorm. b) Nimbostratus cloud a low, dark cloud, often occurring as widespread overcast and normally producing precipitation. 7. Other cloud types include a) Altocumulus clouds middle-level clouds, between about 6500 and 20,000 feet (2 and 6 km), which are puffy in form and are composed of liquid water. b) Altostratus clouds middle-level clouds, 6500 and 20,000 feet (2 and 6 km), which are layered and are composed of liquid water. c) Cirrostratus clouds high cirriform cloud that appears as whitish, translucent veils. d) Stratocumulus clouds low clouds, usually below 6500 feet (2 km), which sometimes occur as individual clouds but more often appear as a general overcast. D. Fog 1. Fog a cloud whose base is at or very near ground level. a) No physical differences between cloud and fog. (1) Important differences in how fog and clouds form. (a) Most clouds develop as a result of adiabatic cooling in rising air. (b) Most fogs are formed either when Earth s surface cools to below its dew point or when enough water vapor is added to the air to saturate. (i) Only rarely is uplift involved in creating fog (occurs in upslope fog [orographic fog] when humid air climbs a topographic slope and cools by adiabatic cooling). E. Dew 1. Dew the condensation of beads of water on relatively cold surfaces; if temperature is below freezing, ice crystals (white frost) forms. VI. The Buoyancy of Air A. Buoyancy the tendency of an object to rise in a fluid. 1. A parcel of air moves vertically until it reaches a level at which the surrounding air is of equal density (equilibrium level). B. Stability 1. Stable air resists vertical movement; nonbuoyant, so will not move unless force is applied. 2. Unstable air buoyant, will rise without external force or will continue to rise after force is removed. 3. Conditional instability intermediate condition between absolute stability and absolute instability. Occurs when an air parcel s adiabatic lapse rate is somewhere between the dry and wet adiabatic rates. Acts like stable air until an external force is applied; when forced to rise, it may become unstable if condensation occurs (release of latent heat provides buoyancy). C. Determining Air Stability 1. Air stability is related to adiabatic temperature changes, as discussed in Review Question Accurate determination of stability of any mass of air depends on temperature measurements, but one can get a rough indication from looking at cloud patterns. a) Unstable air is associated with distinct updrafts, which are likely to produce vertical clouds. b) Cumulous clouds suggest instability. c) Towering cumulonimbus clouds suggest pronounced instability. d) Horizontally developed clouds, most notably stratiform, characterize stable air forced to rise. e) Cloudless sky indicative of stable, immobile air. VII. Precipitation A. Most clouds do not yield precipitation. B. Condensation alone is insufficient to produce raindrops. C. The Processes 1. Still not well understood why most clouds do not produce precipitation. 2. Two mechanisms are believed to be principally responsible for producing precipitation: a) Ice-crystal formation b) Collision and coalescence of water droplets (1) Bergeron process process by which ice crystal formation occurs; is believed to account for the majority of precipitation outside of tropical regions. (a) Ice crystals and supercooled water droplets in cloud are in direct competition for water vapor not yet condensed. (b) Ice crystals will attract most of the vapor if liquid droplets are in state of equilibrium. (i) If ice crystals grow at expense of water droplets, the crystals will grow large enough to fall. (ii) As they descend, they grow warmer and pick up more moisture, growing still larger. (iii) They then either precipitate as snowflakes or melt and precipitate as raindrops.

4 (2) Collision/Coalescence most responsible for precipitation in the tropics and produces much precipitation in the middle latitudes. (a) Rain is produced by the collision and coalescing (merging) of water droplets (i) No ice crystals because cloud temperatures are too high. (b) Must coalesce enough that the droplets become large enough to fall. (c) Coalescence is assured only if atmospheric electricity is favorable, so that positively charged droplets collide with negatively charged ones. D. Forms of Precipitation 1. Rain the most common and widespread form of precipitation, consisting of drops of liquid water. Most rain is the result of condensation and precipitation in ascending air that has a temperature above freezing, but some results from thawing of ice crystals. 2. Snow solid precipitation in the form of ice crystals, small pellets, or flakes, which is formed by the direct conversion of water vapor to ice. 3. Sleet small raindrops that freeze during decent, reaching ground as small pellets of ice. 4. Glaze rain that turns to ice the instant it collides with a solid object. 5. Hail rounded or irregular pellets or lumps of ice produced in cumulonimbus clouds as a result of active turbulence and vertical air currents. Small ice particles grow by collecting moisture from supercooled cloud droplets. E. Atmospheric Lifting and Precipitation 1. Significant amounts of precipitation can originate only by rising air and adiabatic cooling. 2. There are four principal types of atmospheric lifting: a) Convective lifting b) Orographic lifting c) Frontal lifting d) Convergent lifting 3. More often than not, the various types operate in conjunction. 4. Convective precipitation showery precipitation with large raindrops falling fast and hard; caused by convective lifting, which occurs when unequal heating of different air surface areas warms one parcel of air and not the air around it. a) This is the only spontaneous of the four lifting types; the other three require an external force. 5. Orographic precipitation occurs with orographic lifting, caused when topographic barriers force air to ascend upslope; only occurs if the ascending air is cooled to the dew point. a) Rain shadow area of low rainfall on the leeward side of a topographic barrier; can also apply to the area beyond the leeward side, for as long as the drying influence continues. 6. Frontal precipitation occurs when air is cooled to the dew point after unlike air masses meet, creating a zone of discontinuity (front) that forces the warmer air to rise over the cooler air (frontal lifting). 7. Convergent precipitation showery precipitation caused by convergent lifting, the least common form of lifting, which occurs when air parcels converge and the crowding forces uplift, which enhances instability. This precipitation is particularly characteristic of low latitudes. VIII. Global Distribution of Precipitation A. The spatial distribution of precipitation is the most important geographic aspect of atmospheric moisture. B. Broad scale pattern is based on latitude, but many other factors are involved in this complex pattern. C. Isohyet a line joining points of equal quantities of precipitation. D. Average Annual Precipitation E. Nature of the air mass and the degree to which that air is uplifted determine the amount of precipitation in an area. 1. Humidity, temperature, and stability are mostly dependent on where air originated and on the trajectory it has followed. 2. Uplifting (and its amount) determined largely by zonal pressure patterns, topographic barriers, storms, and other atmospheric disturbances. F. Some generalizations possible: 1. Coastal regions usually receive more precipitation than interior regions because they are closer to moisture sources. 2. Because they have warm trade winds that are forced to rise, tropical latitudes contain most of the wettest areas in the world. 3. The remaining wettest areas are narrow zones along the western coasts of North and South America. a) Caused by a combination of onshore westerly airflow, frequent storminess, and mountain barriers that run perpendicular to the westerly winds. G. Seasonal Precipitation Patterns 1. Summer/winter variation in precipitation occurs over most of Earth. a) Strongest over continental interior, because strong summer heating of surface causes instability. b) Coastal areas often are more balanced in their seasonal precipitation regime (always close to moisture sources).

5 2. The displacement of wet and dry zones mirrors the seasonal shifting of major pressure and wind systems, which follows the Sun northward in July and southward in January. a) Summer (in each hemisphere) is the time of maximum precipitation over most of the world. 3. Monsoon regions present the most conspicuous variation in seasonal precipitation, with very wet summers and generally dry winters. H. Precipitation Variability 1. In any given year or any given season, the amount of precipitation may or may not be similar to the long-term average. 2. Precipitation variability expected departure from average precipitation in any given year (expressed as a percentage; can go above or below average). a) Regions of normally heavy precipitation experience the least variability. b) Normally dry regions experience the most variability.

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