Atmospheric Water Cycle: Evaporation, Condensation, and Precipitation, Exams of Geography

Download Atmospheric Water Cycle: Evaporation, Condensation, and Precipitation and more Exams Geography in PDF only on Docsity! GEOG 1900 STUDY GUIDE WINTER SEM 2024 ● What is air pressure? ○ Atmospheric pressure→ the weight of the column of air above a given unit area of the Earth’s surface ○ Exerted equally in all directions ● How do we measure it? What units do we use? Describe different instruments. ○ SI unit→ pascal (Pa), in US→ millibar (mb) = 100 Pa, in Canada→ kilopascal (kPa) = 1000 Pa= 10 mb ○ Mercury barometer→ avg. height of column at sea level = 76 cm ○ Aneroid barometer→ spring in flexible evacuated chamber ● How does pressure vary in the atmosphere vertically? Draw a profile through the atmosphere. ○ Pressure always decreases with altitude ○ Decrease NONlinearly because air is compressible→ rate of decrease is larger at lower altitudes and smaller at higher altitudes ○ Horizontal pressure changes are very minimal ● What is Dalton’s law of partial pressures? ○ Total pressure exerted = sum of partial pressures ● What is the ideal gas law? Write it down and explain the variables. ○ Equation of state: ○ P = ρ * R * T ■ Where P = pressure in pascals ■ Ρ (rho) = density ■ T = temperature ■ R=287 J/kg K (← constant) ● Density is inversely proportional to temperature ● How is pressure related to temperature? ○ Increase in temperature leads to increase in pressure; they are proportional ● Re-write the equation of state to define density. Now, describe how density changes with an air parcel’s temperature, if pressure is held constant. ○ ρ = P / (R * T ) ○ As temperature increases, density decreases→ they are inversely proportional ● What are lines of equal pressure on a weather chart called? → ISOBARS ● What is the pressure gradient? How is this seen on a weather chart? ○ Pressure gradient= rate of change in pressure ○ Spacing of isobars indicates strength of the pressure gradient→ dense clustering of isobars indicates a steep pressure gradient ( a rapid change in pressure with distance) ● What drives all wind? Explain how. ○ THE PRESSURE GRADIENT→ gives rise to pressure gradient force that sets air in motion ○ If the air over one region exerts a greater pressure than the air over the adjacent region, the higher- pressure air will spread out toward the zone of lower pressure as wind (always moves from high to low pressure) ○ Greater the PGF, stronger the wind ● Explain the hydrostatic equilibrium. ○ Vertical pressure gradient force and the force of gravity are normally of nearly equal value and operate in opposite directions (so the atmosphere doesn’t explode away or get sucked down to the very surface of the Earth) ● How does the vertical change in pressure differ between columns of warmer and colder air? ○ Cool air has a greater vertical pressure gradient because the warm column of air has the same mass of air, but expands upwards, thus, to reach the same pressure in each column, you have to go up higher in the warmer column of air ● Why do we care about the 500 mb heights, and how do we map these? ○ Look similar to isobars but they are telling us lines of equal height and at what height the pressure is 500 mb, this allows us to determine the pressure gradient force ● Explain how 500 mb heights reflect the relative density of air. ○ The lower the 500 mb height, the denser the air, and vice versa. ● What forces affect the speed and direction of wind? ○ Unequal distribution of air across the globe establishes the horizontal pressure gradient that initiate movement of air as wind ○ Planetary rotation alters direction of wind (Coriolis Force) ○ Friction slows the speed of the wind ● Describe the Coriolis force and how it changes with respect to latitude, and hemisphere. ○ Apparent deflection of path of objects moving on Earth due to Earth’s rotation; the Earth is spinning fastest at the equator ○ The intensity of the deflection is proportional to the speed of movement ○ At the equator, the CF = 0, and it increases with higher latitudes ○ In the NH, wind will be deflected to the right ○ In the SH, wind will be deflected to the left ● How does friction impact wind speed and direction? ○ Friction reduces wind speed and the lower wind speeds reduce the Coriolis force and thereby prevent the the flow from becoming gradient or geostrophic ○ Winds cross the isobars at an angle as they blow from high to low pressure ● Explain geostrophic winds. Where do they occur? How do they differ between hemispheres? ○ If pressure gradient and Coriolis force are equal, then a geostrophic wind occurs. ○ Non-Accelerating flow→ geostrophic flow ■ Flows to right in NH and left in SH ● What is gradient flow? ○ Close to geostrophic. ○ Small accelerations because isobars are not straight ○ Continual mismatch between pressure gradient and Coriolis force ○ Water will evaporate into the atmosphere as water vapor until the air becomes saturated, meaning it is holding the maximum amount of water vapor that it can, at which point the water vapor will condense into clouds. ○ At higher temperatures, more moisture is required for saturation. ● How does water change phase? What is each change called, and how much energy is required? ○ Water changes phases based on temperature and pressure ○ Evaporation: liquid water→ water vapor; process by which molecules break free of liquid volume ○ Condensation: water vapor→ liquid water ○ Sublimation: ice→ water vapor (without passing through liquid phase) ○ Deposition: water vapor→ ice (without passing through liquid phase) ● What is that energy called, and how does it influence temperature? ○ Latent heat? ○ Kinetic energy? ● What is humidity? Describe different ways to measure it. ○ Humidity refers to the amount of water vapor in the air ● Define vapor pressure→ the part of the total atmospheric pressure due to water vapor ○ Millibars, pascals, kilopascals ● How does saturation vapor pressure vary with temperature? ○ Higher temperatures→ higher saturation vapor pressures ○ Nonlinear relationship ○ SVP increases more rapidly with higher temperatures ● What is absolute humidity? Specific humidity? What units do we use for each? ○ Absolute humidity→ the density of water vapor, expressed as number of grams of water vapor contained in a cubic meter of air; value changes when air expands or contracts even though moisture content doesn’t change→ not widely used for this reason ○ Specific Humidity→ expresses the mass of water vapor existing in a given mass of air; proportion of atmospheric mass accounted for by water vapor; number of grams of water vapor per kilogram of air; affected a small amount by atmospheric pressure; doesn’t change as air expands or contracts; not temperature dependent. ● What is the mixing ratio? Saturation mixing ratio? ○ Mixing ratio: measure of the mass of water vapor relative to the mass of all of the other gases of the atmosphere; mass of water vapor/ mass of dry air; mixing ratio and specific humidity will always have nearly equal values ○ Saturation mixing ratio: maximum possible mixing ratio. ● Explain relative humidity, and define different ways we can calculate it. ○ Relative humidity: relates amount of water vapor in the air to the maximum possible at the current temperature ○ RH = (mixing ratio/ saturation mixing ratio) x 100% (could also probably use specific humidity and saturation specific humidity) ● What is the dew point? What happens when air temperature equals the dew point? ○ Dew point= the temperature at which saturation occurs ○ Dependent entirely on amount of water vapor present ○ Air is saturated/RH=100% when the air temperature equals the dew point ○ Dew point can never exceed temperature of air ○ High dew point → higher vapor content ○ Low dew point → lower vapor content ● Explain how dew point temperature is a good way to estimate overnight minimum temperature. ○ If no major wind shifts or other weather changes are anticipated, the minimum temp will often approximate the dew point. If the air temp drops to the dew point and there is little to no wind, a radiation fog has a good chance of forming. The fog would then inhibit further cooling → overnight low = dew point temp ● What is fog? Explain different types, and how/where they occur? ○ Fog is essentially a cloud whose base is at or near ground level ○ Precipitation fog: results from evaporation of falling raindrops ○ Steam fog: occur when cold, dry air mixes with warm, moist air above a water surface ○ Fogs resulting from cooling the air to the dew point: ■ Radiation (ground) fog: develop when nighttime loss of longwave radiation causes cooling to the dew point ● Most likely to form on cloudless night with light wind ● Most begin to dissipate within a few hours of sunrise; cloud droplets radiate away as air temp rises (“burned off” not lifted) ● Forme dby diabatic cooling; associated with cold air ■ Advection fog: form when relatively warm, moist air moves horizontally over a cooler surface; as air passes over cooler surface, it transfers heat downward, causes it to cool diabatically ● Ex: Bay Area ● Can also form over ocean when warm and cold currents are in proximity to each other ● Associated with greater winds than radiation fogs ■ Upslope fog: formed by adiabatic cooling; when air flows along a gently sloping surface, it expands and cools as it moves upward ● Where is most fog found in the US? Why are dew point temperatures lower in January than July? ○ Lower Jan. temps preclude existence of high water vapor contents ● What processes can lead to saturation? Explain. ○ Adding water vapor to air: ex: steam in bathroom when taking hot shower ○ Mixing cold air with warm, moist air: causes contrails to form behind planes flying at high altitudes ○ Lowering air temp to dew point: most common process for cloud formation ● What factors affect condensation? ○ Temperature, moisture vapor content ○ Considerable curvature of water droplets increases amount of moisture needed for them to be maintained relative to larger masses of water with flat surfaces ○ The curvature effect is largely offset by the fact that droplets do not occur as pure water but instead exist as solutions ● Explain the curvature effect, and how does it impact the amount of water required for saturation? ○ Larger spheres have smaller curvature effect than smaller ones (ex: Earth compares to tennis ball) ○ A highly curved droplet of pure water at any given temp has a higher saturation vapor pressure ○ Highly curved droplets of water require relative humidities in excess of 100% to keep them from evaporating away ○ Supersaturated: air with RH exceeding 100%; can exist because because the highly curved nature of suspended water droplets would cause them to rapidly evaporate in an atmosphere at 100% RH with respect to flat water surfaces ● What are condensation nuclei and ice nuclei? Which is more abundant? ○ Condensation nuclei: small particles typically 0.2 µm, or 1/100th the size of a cloud droplet on which water vapour condenses. Water requires a non-gaseous surface to make the transition from a vapour to a liquid (condensation) ○ The atmosphere contains more condensation nuclei than ice-forming nuclei ○ Most condensation/deposition of water vapor occurs on particles, or nuclei, which make the droplets/ice crystals more stable ○ Ice-forming nuclei activity is temperature dependent; formation of ice crystals at temps near ) degrees C requires ice nuclei ■ Ice-nuclei are rare in the atmosphere ● What is supercooled water? → water having a temp below the melting point of ice but still existing in a liquid state ○ Statically UNstable air: becomes buoyant when lifted and continues to rise if given an initial upward push ○ Statically Stable air: resists upward displacement and sinks back to its original level when the lifting mechanism ceases ○ Statically neutral: neither rises on its own following an initial lift nor sinks back to its original level ○ The buoyancy of a rising air parcel depends on its rate of cooling relative to the surrounding air ● What stops unstable air from rising? What is the most stable atmospheric condition? ○ They ascend into a layer of stable air ○ Entrainment (see below) ○ Most stable atmospheric condition: temperature inversion ● Explain absolutely unstable and absolutely stable conditions. Use diagrams. Now, describe the situation of conditionally unstable air. ○ ○ Absolutely unstable: the ELR is greater than the DALR; the rising air parcel is cooling more slowly than its surroundings; air parcel rises at an increasing speed ○ Absolutely stable: ELR is less than the SALR ○ Conditionally unstable: the ELR is between the DALR and SALR; tendency for a lifted air parcel to sink or continue rising depends on whether or not it becomes saturated and how far it is lifted ○ ● What factors influence the environmental lapse rate? ○ Heating or cooling of lower atmosphere→ heating increases ELR, cooling decreases ELR ○ Advection of Cold and Warm Air at different levels ○ Advection of an Air mass with a different ELR ○ In general terms, larger ELRs are associated with greater atmospheric instability and vice versa. This is usually associated with warmer surface temperatures. ● What is entrainment? Explain. How does it impact the growth of clouds? ○ As air rises, considerable turbulence causes ambient air to be drawn into the parcel ○ Especially active along edges of growing clouds ○ Entrainment suppresses the growth of clouds because it introduces unsaturated air into the margins and thus causes some of the liquid droplets to evaporate. The evaporation consumes latent heat and thereby cools the margin of the cloud, making it less buoyant. ● What is the lifting condensation level? ○ The height at which the relative humidity of an air parcel with reach 100% when it is cooled by dry adiabatic lifting ● What is the level of free convection? Explain how this is NOT the LCL. ○ The altitude in the atmosphere where the temperature of the environment decreases faster than the SALR of a saturated air parcel at the same level. ● What is an inversion, and describe some different ways inversions can form. ○ When air temperature increases with height; extremely stable condition of air ○ Radiation inversion→ results from diabatic cooling of the surface ■ Surface cools at night, cools lower atmosphere and decrease the ELR; with sufficient cooling, the air near the surface can become colder than the air above and create an inversion ○ Frontal inversion→ when a cold or warm front is present, a transition zone separates warm and cold air masses→ boundary forms a wedge of cold air under warm air; horizontal extent can be up to several hundred kilometers ● Describe the general classification scheme for clouds, based on height and form. List major subtypes of high, middle, and low clouds. ○ High Clouds: Cirrus, cirrostratus, cirrocumulus, contrails ○ Midlevel clouds: Altostratus, Altocumulus ○ Low clouds: Stratocumulus, Stratus, Nimbostratus ○ Noctilucent→ location in mesosphere allows them to be illuminated after sunset when the surface and the lower atmosphere are in Earth’s shadow ● Look over some of the more unusual clouds explained in the chapter. ○ Lenticular→ form downwind of mountain barriers and have curved shapes like eyeglass lenses ○ Banner → similar to lenticular but are individual clouds located immediately above isolated peaks ● Skies are mostly covered by clouds during broken and overcast conditions. The presence of some cloud cover that does not cover more than half of the sky is categorized as few or scattered conditions Chapter 7: Precipitation Processes ● Describe the relative size difference between cloud droplets and precipitation. ○ For precipitation to occur, the size of droplets/ice crystals inside a cloud must increase considerably ■ Rain drops are about 20 to 100 times larger than cloud droplets ○ Most cloud droplets do not become rain because, being too small, they fall too slowly and evaporate before reaching the ground ■ Terminal velocity increases with size ■ Typical cloud droplet ~ 10 µm (micrometers): rain droplet is 1000 µm ● How do cloud droplets grow? Explain the processes. ○ Condensation→ starting point for precip., increase cloud droplet growth to about 20 micrometers; rapid growth for very small droplets ○ Growth in warm clouds ■ Collision-coalescence process: depends on differing fall speeds of different-sized droplets ● Collector drop falls through warm cloud→ overtakes some of the smaller droplets in its path because of its greater terminal velocity ● Collision: likelihood of collision depends on both the absolute size of the collector and its size relative to the droplets below ○ If the collector drop is much larger than those below, the collision efficiency is low (because smaller droplets are pushed out of way of collector drop) ● Coalescence: when colliding droplets stick together; the percentage of colliding droplets that join together is the coalescence efficiency (most often near 100% ● What is terminal velocity? Under what cloud temperatures would you expect to see collision-coalescence causing precipitation? Why? ○ Terminal velocity: force = gravity (acceleration ceases) → object falls at a constant speed ○ Warm clouds (temps above 0 degrees C throughout) have collision-coalescence ■ Because they have liquid water ● Where do cold clouds occur, and how does this impact the process of precipitation formation? ○ At least a portion of midlatitude clouds have temps below 0 degrees C throughout and consist entirely of ice crystals, supercooled droplets, or mixture of both ○ Bergeron process necessary for precip outside of the tropics ● Explain how saturation vapor pressure for ice crystals and supercooled water droplets affect the development of precipitation. ○ The saturation vapor pressure over ice (the amount of water vapor needed to keep it in equilibrium) is less than that over supercooled water at the same temperature ■ This is because molecules in an ice crystal bond to each other more tightly than molecules of liquid water ● Remember: saturation exists when the vapor pressure of the air is at the point where evaporation from a water droplet would be exactly offset by condensation back onto it, or if sublimation from an ice crystal would be offset by deposition ○ Ice crystals do not sublimate ice to vapor as rapidly as water droplets evaporate liquid to water ○ Ice crystals continually grow at the expense of the supercooled droplets→ growth of ice crystals by the deposition of water vapor initiates precipitation→ as it grows, its increasing mass enables them to fall through the cloud and collide with droplets and other ice crystals → riming and aggregation ● Explain the Bergeron process. Why can’t it take place in warm clouds? (see above) ○ After formation cold clouds tend to have more supercooled droplets than ice crystals because there are more water condensation nuclei than ice deposition nuclei in the atmosphere. As the vapor saturation for ice is smaller than for liquid water, the supercooled water droplets lose water to the ice crystals (Clausius-Clapeyron Equation). Crystals grow, their terminal velocity increase, they begin to grow by collision with supercooled droplets (which freeze on impact) or other ice crystals. Some become large enough to fall below the cloud base. Falling ice crystals can also increase in size by accretion as they fall. ● What is riming? What is happening? Where does it occur? What is aggregation? ○ Riming→ (or accretion) when ice crystals fall through cloud and collide with supercooled droplets, the liquid water freezes onto them→ causes rapid growth of ice crystals→ further increases their fall speeds→ promotes further riming ○ Aggregation→ joining of 2 ice crystals to form a single, larger one; occurs most easily when ice crystals have a thin coating of liquid water to make them more “adhesive” ● Why do they get more rain in Mississippi than in Michigan? ○ Mississippi has warmer temperatures along with more moisture from the Gulf of Mexico; Michigan tends to have more snow because of colder temperatures ● Differentiate between different precipitation types and conditions they occur, like drizzle, rain, freezing rain, sleet, hail, graupel, snow. ○ snow→ initiated by bergeron process; occurs if falling crystals do not melt prior to reaching the ground; cloud temps must be less than -4 degrees C and temps from surface to cloud base not much more than 0 degrees C ■ Very cold snow forms small snowflakes that accumulate with lower density (“powder”) ■ Can never be too cold to snow at all (Can be too cold to snow A LOT) ○ rain→ C-C in warm clouds, B-P in cool/cold clouds (results as melting of falling snow) ■ Large water drops from nimbostratus and cumulonimbus clouds. In cold clouds bulk originates as snow or hail which melts on descent. ■ Virga: raindrops evaporate before hitting ground ○ Drizzle→ Small water drops that usually originate in warm stratus clouds Low, “thin” clouds, not much chance to grow through collision-coalescence. ○ Graupel→ ice crystal takes on additional mass by riming, its six-sided structure becomes obscured and edges are smoothed out. New ice contains very small air bubbles that give it a spongy texture and milky-white appearance ■ Attain diameters of up to 5 mm, can fall to ground or remain in cloud and provide nuclei upon which hailstorms form ○ hail→ multiple layers of ice usually no more than a few mm in thickness (can be much larger) ; large amounts of water must be available in the cloud; very strong updrafts ■ Form in cumulonimbus clouds ○ Freezing rain→ begins when a light rain or drizzle of supercooled drops falls through air with temp at or slightly below 0 degrees C ■ When raindrop hits surface, form thin film of water then freezes to form slick, continuous coating of ice ■ Occurs when rain falls from mild air layer aloft into a shallow layer of cold (below freezing) air ○ Rime: Ice crystals deposited by supercooled fog/clouds. Can generate significant accumulation on mountainous areas. ● Explain why sleet requires an inversion to form. ○ Sleet→ forms when raindrops freeze in air while falling; begins as falling ice crystals or snowflakes→ melts in warmer layer of air→ freezes in cooler layer of air ■ Semi-transparent pellets smaller than 5 mm ■ Requires an inversion, usually one associated with a warm front ● Layer of cold air beneath inversion must be fairly deep ● What are some methods and sources of uncertainty in precipitation measurement? ○ Rain gauges ■ Standard rain gauge: reduction in area between collecting funnel and measuring tube increases precision ■ Tipping bucket rain gauge: each tipping gets recorded. Bucket has known volume. Number of tipping events provides total rainfall during a determined period ○ Measurement accuracy is a concern due to such problems as evaporation from the gauge and winds than can prevent rain from entering gauge ○ Gauges only measure precip level at a single point/location ○ Weather Radar: emits microwave radiation with wavelengths of several cm ■ The more intense the backscattered radiation, the more intense the precip ○ Dual-polarization radar ○ Snow courses, snow pillows ○ Community Collaborative Rain, Hail, and Snow Network