Chapter 7: Water and Atmospheric Moisture Notes
Chapter 7: Water and Atmospheric Moisture Notes Geog 1112
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Date Created: 02/25/16
Chapter 7: Water and Atmospheric Moisture Water’s Unique Properties Three states of water occur naturally (due to Earth’s distance from the sun) Three states of water = ice, liquid, vapor Water = H2O Hydrogen = positive charge Oxygen = negative charge Polarity result = water molecules attract each other Hydrogen Bonding = positive side attracts negative side of molecule Surface Tension Capillarity = hydrogen bonding causes this; example - when you dry something with a paper towel Meniscus = inwardly curved surface of water Capillary Action = component of soil-moisture processes Phase Changes and Heat Exchange Heat energy added or released = change from ice to liquid to vapor Phase Change = change between states Melting, Freezing = change between solid and liquid Condensation = water vapor in the air becomes a liquid Evaporation = liquid water becomes water vapor Vaporization = water is at boiling temperature Deposition = water vapor attaches directly to an ice crystal (leads to frost) Sublimation = ice changes to water vapor directly Ice, the Solid Phase Water Cools = goes up in density Water cooled to greatest density = still in liquid phase Hexagonal (6-sided) crystalline structure formed from continued cooling Temperature goes below freezing = ice expands in volume; decreases in density Water, the Liquid Phase Water as liquid = noncompressible fluid Ice to water = heat energy has to increase the motion of water molecules (to tear some of the hydrogen bonds) Ice and water = measures zero degrees; 32 degrees Fahrenheit Latent Heat = heat energy of a phase change (hidden) Phase change reversed = latent heat released Latent Heat of Melting and Freezing = 80 cal/g Water Vapor, the Gas Phase Water vapor = compressible and invisible Molecules = move independently Latent Heat of Vaporization = 540 calories are absorbed (liquid to vapor induced by boiling) Latent Heat of Condensation = 540 calories are released or given up (water vapor to liquid) Latent Heat of Sublimation = absorbs 680 cal (ice to vapor) Latent Heat Transfer Under Natural Conditions Latent Heat of Evaporation = most important cooling process in Earth’s energy budget Satellites = monitor water vapor in lower atmosphere Water Vapor = absorbs infrared wavelengths (long) Water Vapor = greenhouse gas; concentrate tied to temperature Global temperatures go up = evaporation increases Humidity Humidity = the amount of water vapor in the air Relative Humidity Most common measure of humidity in weather reports Relative Humidity = a ratio of the amount of water vapor that is actually in the air compared to the maximum water vapor possible in the air at a given temperature; varies due to changes from water vapor and temperature Formula for calculating Relative Humidity : (Actual Water Vapor in the Air)/ (Maximum water vapor possible in the air at that temperature) x100 Warm air = increases evaporation rate from water surfaces Cool air = increases condensation rate of water vapor onto water surfaces Saturation Equilibrium = rates of evaporation and condensation can reach equilibrium at some point Saturation and Dew Point Saturation = 100% relative humidity Dew-point Temperature = the temperature at which a given sample of vapor- condensing air becomes saturated and net condensation starts to form water droplets Daily and Seasonal Relative Humidity Patterns Temperature increases = relative humidity goes down Relative Humidity highest = at dawn Relative Humidity lowest = late afternoon Rate of evaporation = varies from morning to afternoon Specialized Expressions of Humidity Vapor Pressure and Specific Humidity = used to express humidity and relative humidity Vapor Pressure Vapor Pressure = the share of air pressure that is made up of water-vapor molecules Saturation vapor pressure = air that has as much water vapor as possible at a given temperature Specific Humidity Specific Humidity = the mass of water vapor per mass of air at any specified temperature Maximum Specific Humidity = the maximum mass of water vapor possible in a kilogram of air at any specified temperature Instruments for Measuring Humidity Hair Hygrometer = uses the principle that human hair changes as much as 4% in length; connects a standardized bundle of human hair through a mechanism to a gauge; as hair absorbs or loses water in the air, it changes length, indicating relative humidity Sling Psychrometer = contains two thermometers mounted side by side Dry-Bulb Thermometer = records the surrounding air temperature Wet-Bulb Thermometer = set lower in the holder, bulb is covered by a moistened cloth wick Atmospheric Stability Parcel = a body of air that has specific temperature and humidity characteristics Two opposing forces decide the vertical position of parcel 1. Buoyant Force 2. Gravitational Force Stability = the tendency of an air parcel either to remain in place or to change vertical position by ascending (going up) or descending (going down) Air parcel stable = resists displacement upward (also when disturbed tends to go back to its starting place) Air parcel unstable = continues to go up until it reaches an altitude where the surrounding air has a density and temperature similar to its own Adiabatic Processes Stability or instability of air parcel = depends on two temperatures 1. temperature inside 2. temperature outside The difference between inside and outside determine stability Ascending Parcel of Air = cool by expansion Descending Parcel of Air = heat by compression The cooling and heating mechanisms = adiabatic Diabatic = occurring with an exchange of heat Adiabatic = occurring without a loss or gain of heat; measured with one of two rates depending on moisture (DAR-Dry Adiabatic Rate and MAR-Moist Adiabatic Rate) Dry Adiabatic Rate DAR = the rate at which dry air cools by expansion as it rises or heats by compression as it falls Dry = air less than saturated Average DAR = 10 degrees Celsius / 1000 m Moist Adiabatic Rate MAR = the rate at which an ascending air parcel that is moist, or saturated, cools with expansion Average MAR = 6 degrees Celsius / 1000 m MAR is lower than DAR because of the latent heat of condensation MAR is lower than DAR = in warm air MAR and DAR are similar = in cold air Stable and Unstable Atmospheric Conditions Temperature relations (in the atmosphere) make three conditions: 1. Unstable 2. Conditionally Stable 3. Stable Examples of Lifting Mechanisms: mountain range, surface heating, weather fronts The Environmental Place Rate is between DAR and MAR = conditions are not stable or unstable Clouds and Fog Clouds can tell : moisture content, stability, and weather Clouds form = air becomes saturated with water Cloud Formation Processes Cloud = an aggregation of tiny moisture droplets and ice crystals that are suspended in the air; they are big enough in volume and concentration to be seen Fog = a cloud in contact with the ground Moisture Droplet = approximately 20 micrometers in diameter Air parcel goes up = may cool to 100 percent relative humidity and dew point temperature Cloud-Condensation Nuclei = microscopic particles that are present in the atmosphere all the time Nuclei come form = soot, ash, dust, particles from burned fuel Collision-Coalescence Process = involves falling coalescing droplets and warmer clouds Bergeron Ice-Crystal Process = super cooled water droplets evaporate and are absorbed by ice crystals that grow in mass and fall Cloud Types and Identification Altitude and Shape = important to cloud classification Stratiform = flat and layered clouds with horizontal development Cumuliform = puffy and globular clouds with vertical development Cirroform = wispy clouds (generally quite high in altitude; made of ice crystals) Low Clouds = Stratus, Stratocumulus, Nimbostratus Middle Clouds = Altostratus, Altocumulus High Clouds = Cirrus, Cirrostratus, Cirrocumulus Vertically developed = Cumulus, Cumulonimbus Stratus Clouds = gray, dull, featureless; when start to rain, they are called: nimbostratus Cumulus Clouds = puffy, bright (cotton balls) Stratocumulus Clouds = lumpy, low-level, grayish Altocumulus Clouds = a lot of styles such as: wave patterns, patchy rows, lens-shaped Cirrus Clouds = can indicate on oncoming storm Cumulonimbus Clouds = thundercloud Processes That Form Fog Fog = cloud layer on the ground (sight restricted to less than 1 km) Presence of fog = the air temperature and the dew point temperature are almost identical at the ground level (saturated conditions) Fog = warm (almost all fog) Moisture droplets of fog = above freezing Supercooled fog = moisture droplets below freezing; can be dispersed by means of artificial seeding with ice crystals Radiation Radiation Fog = when radioactive cooling of a surface chills the air layer directly above that surface to the dew point temperature (saturated conditions) Radiation Fog = does not occur over water but over moist ground Advection Advection fog = when air in one place moves to another place where conditions are right for saturation Upslope Fog = forms when moist air flows to higher elevations along a mountain or hill Valley Fog = associated with topography; cool air is denser than warm air and it settles in low-lying areas; chilled and near the ground Evaporation Evaporation Fog = steam fog; forms when cold air lies over the warm water of an ocean, or lake; can form as water molecules evaporate from the water surface into the cold overlying air (humidifies the air to saturation, then to condensation to make fog)