Earth Science Lecture Summary Notes Chapter 7 - Water and Atmospheric Moisture (based on Christopherson, Geosystems, 6th Ed., 2006) Prof. V.J. DiVenere - Dept. Earth & Environmental Science - LIU Post Properties of water dipole molecule: the H2O molecule has an electric charge imbalance so that one end is slightly negative and the other end is slightly positive hydrogen bonding: the opposite charges attract water molecules to each other surface tension: because of hydrogen bonding water drops bead up, not spread out into an extremely thin layer universal solvent: because of its dipole nature ionic solids like sodium chloride readily dissolve in water phases: water can exist at Earth surface conditions as solid (ice), liquid (water), and gas (water vapor) phases (solid-liquid-gas) interesting H2O facts: H2O molecules in ice are bonded to each other in an open hexagonal (6-sided) ring structure - the open structure makes solid H2O (ice) less dense than liquid H2O (water) (in most substances, the solid form is denser than its liquid form) - because of the hexagonal crystal structure of ice, snowflakes are 6-sided latent heat of freezing-latent heat of melting water molecules absorb 80 calories per gram to change from the solid (ice) to the liquid (water) state at 0 C water molecules release 80 calories per gram to change from the liquid (water) state to the solid (ice) at 0 C specific heat - much higher than most substances, including land (soil & rock) water molecules absorb 1 calorie per gram to raise the temperature 1 C (it takes a lot, so it warms slowly) water molecules release 1 calorie per gram to cool by 1 C (it takes a lot, so water temperature falls slowly) latent heat of evaporation-latent heat of condensation water molecules absorb 540 calories per gram to change from liquid (water) to gas (water vapor) water molecules release 540 calories per gram to change from gas (water vapor) to liquid (water) latent heat of sublimation water molecules absorb 680 calories per gram to change from solid (ice) directly to gas (water vapor) water molecules release 680 calories per gram to change from gas (water vapor) to solid (ice) huge amounts of heat are stored in water and is slowly released as water cools over night or in winter responsible for moderation of climate in coastal and oceanic settings huge amounts of energy are stored in water vapor and is released when it condenses or sublimates this is the power source that drives cyclonic storms
Humidity relative humidity = how much water vapor is in the air compared to how much it can hold at that temperature (actual water vapor content) divided by (maximum vapor capacity for that temperature) relative humidity = water vapor content water vapor capacity Warm air has the capacity to hold more water vapor than cool air so relative humidity changes with changing temperature even though actual moisture content doesn't saturation: the condition of air that is at 100% relative humidity - the air can't hold any more moisture its actual moisture content is equal to its moisture capacity for that temperature dew point: the temperature at which the air is saturated for a given actual moisture content daily humidity pattern: as temperature rises during the day, relative humidity goes down because the moisture capacity of the air increases as temperature falls overnight, relative humidity goes up because the moisture capacity of the air decreases this pattern holds for all seasons absolute humidity measures specific humidity: the weight of water vapor in air compared to the weight of that air specific humidity = weight of water vapor in air weight of the air (e.g., 5 grams of water vapor in every kilogram of air) vapor pressure: the pressure exerted by just the water vapor in the air remember, air pressure is a measure of the weight or pressure exerted by all the molecules in the air (e.g., 24 millibars of pressure exerted by water vapor compared to about 1013 millibars exerted by the atmosphere as a whole including the water vapor) tools for measuring humidity hair hygrometer - hair gets longer as humidity increases sling psychrometer - based on the amount of cooling of the wet bulb thermometer
Atmospheric Stability air is stable if it is at the same density as surrounding air, so it will not tend to rise or fall but will tend to remain in place Air is cooler at higher elevations because it is farther from the warmth radiated from the Earth's surface... normal lapse rate is a global average rate of temperature reduction with elevation (6.4 C/1000 m) environmental lapse rate is the actual lapse rate on a given day at a specific location Adiabatic Processes: An adiabatic process is one in which no heat is exchanged with the surrounding air. Rising air expands as the pressure of the surrounding air decreases (there is less air above it). The expansion results in cooling because energy is used to separate the molecules. Sinking air compresses as the pressure of the surrounding air increases (there is more air above it). The compression results in warming - the energy used to separate the molecules is "given back." The adiabatic cooling (or warming) rate of rising (or sinking) depends on whether or not condensation occurs. Dry Adiabatic Rate (DAR): if adiabatic cooling does not lower the temperature below the dew point for the moisture content of the rising parcel of air, then the rate of cooling with rise and expansion is 10 C/1000 m. Moist Adiabatic Rate (MAR): if rising air has reached the dew point, then condensation will continue as the air rises. Since condensation liberates latent heat, the cooling rate is not as fast, only 6 C/1000 m. stable and unstable atmospheric conditions: If a parcel of air is moved upward due to local convergence, convection, orographic lifting, or frontal lifting (see Chapter 8) we will want to know if that air is likely to continue upward and expand and cool until it reaches the vapor point resulting in condensation. Stable: If rising air cools faster (DAR or MAR) than the surrounding air (Environmental Lapse Rate), then it will be cooler and denser than the surrounding air and will therefore tend to sink back down to its original elevation. Unstable: If rising air cools more slowly (DAR or MAR) than the surrounding air (Environmental Lapse Rate), then is will be warmer and less dense than the surrounding air and will continue to rise, expand, and cool resulting in continued condensation.
Clouds cloud formation: cloud-condensation nuclei, cloud droplets For condensation to occur at the dew point water vapor needs a surface to condense on. In the atmosphere water vapor starts to condense on microscopic particles suspended in the air: dust, soot, volcanic ash, salt spray, etc. These are called cloud-condensation-nuclei. Once microscopic cloud droplets form, they grow larger as water vapor continues to condense on them and droplets merge by collisions. Eventually they grow large enough that they are visible and heavy enough that they may fall as rain (or snow if condensation occurred below freezing). cloud forms stratiform: flat or layered cumuliform: puffy or globular cirroform: wispy, made of ice crystals low level clouds stratus (unbroken layer of cloud), nimbostratus (dark, unbroken layer, rain cloud) cumulus (puffy, irregular clouds), stratocumulus (cumulus clouds arranged in a broad layer) clouds of vertical development cumulonimbus (thunderhead) forms by vertical growth (upward convection) of cumulus cloud strong upward convection carries growing cloud droplets to high elevations (where they may freeze to form hail) strong downdrafts produce strong winds associated with thunderstorms midlevel clouds altostratus (midlevel flat, layered cloud) a corona closely circles the sun or moon shining through altostratus altocumulus (midlevel puffy clouds) often form in a patchwork or in rolls high clouds cirrus (wispy, "mare's tails") cirrostratus (high altitude flat, layered cloud) a large halo closely circles the sun or moon shining through cirrostratus cirrocumulus (high altitude puffy clouds) often form in a patchwork or ripples, but the clouds are/look smaller than altocumulus
Cloud Types Fog advection fog - results from air movement advection fog - when moist air migrates over a cool land or water surface evaporation fog - warm water evaporates into cool air upslope fog - moist air cools as it rises up over hills or mountains valley fog - cool air sinks downslope into valley bottom radiation fog - results from radiative cooling of the land (and air over it) as a land surface cools (by giving off infrared radiation) eventually it gives off decreasing amounts of infrared; the overlying air cools causing saturation