9/12 & 9/14 Geography Notes
9/12 & 9/14 Geography Notes GEOG 1112
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This 8 page Class Notes was uploaded by Kaley Notetaker on Thursday September 15, 2016. The Class Notes belongs to GEOG 1112 at University of Georgia taught by Dr. Elgene Box in Fall 2016. Since its upload, it has received 54 views. For similar materials see Intro to Weather and Climate in Geography at University of Georgia.
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Date Created: 09/15/16
Geography 9/12 (2 nd half of ch.5) Condensation – especially Adiabatic Cooling FORM PROCESS CHARACTERISTICS Dew Lowering of temp to the Coating of liquid on dew point near surface; surfaces often happens under clear skies and no wind. Diabatic Process Frost Lowering of air Large number of small, temperature to white crystals on saturation point (when surface saturation is below 0C). Diabatic Process Frozen Dew Formation of dew at Continuous layer of temperatures above solid ice on surface 0C, followed by cooling of temperatures below 0C; Diabatic Process Fog Usually happens by Large concentration of cooling of layer of air suspended droplets in with light winds; can later of air near ground. happen by mixing warm and cold winds; Adiabatic or Diabatic Process Radiation fog Cooling of air to dew Same as “fog” point by longwave radiation loss. Diabatic process. Advection fog Cooling of air to dew Same as “fog” point as it passes over cool surface. Diabatic Process Upslope fog Cooling of air as it flowsSame as “fog” upslope. Adiabatic process. Precipitation fog Increasing the water Same as “fog” vapor content of the air by evaporation from falling droplets. Adiabatic process Steam fog Mixing warm, moist air Same as “fog” with cold air. Adiabatic process. Clouds Usually by lifting of air Concentration of and adiabatic cooling. suspended droplets and/or ice crystals in air well above the surface ^^Chart from book, page 147. Adiabatic process– processes where the temperature changes but no heat is added or removed from a substance Diabatic process – a process where energy is added to or removed from a system Warmer air – greater maximum water vapor possible Cooler air – lesser maximum vapor possible Cooling and Saturation Saturation: resulting equilibrium state of air after evaporation and condensation (opposing processes) have offset gains and losses - As long as the rate of evaporation exceeds the rate of condensation, the amount of water vapor increases. - When condensation rate = evaporation rate, saturation occurs There are 3 ways by which air can become saturated with water vapor: 1. By adding water to the air 2. By mixing wet (usually warmer) and drier, usually cooler air 3. By cooling to the dew-point temperature How do we cool air to get to dew-point temp? - Cooling beyond dew-point temperature results in condensation - Water vapor usually condenses onto condensation nuclei, such as dust particles, ice crystals or particulate pollution (called condensation nuclei) Water vapor does not always condense immediately at 100% relative humidity; sometimes it overshoots slightly Droplet size and supersaturation Super-saturation takes place when the air > 100% relative humidity before water vapor actually condenses - Because molecules evaporate faster from small droplets, with greater curvature, humidity > 100% is needed to keep the air saturated - In short, Droplet size DOES make a difference - The bigger the droplet, the higher relative humidity is needed to saturate the droplet Reaching Saturation in Nature - Advection of wet air may add water to the air - Wet and dry air may mix along their interface as along coastlines - Cooling Cooling is the most common way of reaching saturation - Adiabatic cooling ** of rising air = the idea that air that is rising cools automatically Gas behavior in the atmosphere: The volume of a gas can vary greatly, depending on temp of gas and pressure acting on the gas by a surrounding medium; such as the atmosphere. Ideal gas law describes this: P V = n R T P = constraining (external) pressure (atmospheric pressure) V= volume of gas N = number of moles of the gas R= universal gas constant T = temperature of gas This law is used to describe the behavior of “parcels” of air rising or sinking in the atmosphere - Rising air encountering progressively less external pressure around it can expand in volume - Sinking air encountering progressively more external pressure around it can be compressed Thus: - Rising air cools adiabatically – AKA it cools without any net gain or loss of energy o Approx. 10C per 1000m - Sinking air warms adiabatically o Approx. 10C per 1000m **Adiabatic cooling of rising air is the MAIN mechanism to reach dew-point temp and cause cloud formation **Adiabatic warming of descending air results in drier air (lower r.h.), with eveaporation of clouds and less chance of precipitation Counterpart of expansional cooling = compressional warming Temperature = measure of internal heat energy - As heat in rising air expands into a larger volume it is “diluted” and temp goes down - As heat in descending air is compressed, it is concentrated and the temp of that air increases Rising air cools and sinking air is warmed adiabatically, at a constant rate – as long as air is not saturated Dry Adiabatic Lapse Rate (DALR) : Rate at which a rising parcel of unsaturated air cools and sinking parcel of unsaturated air warms - 9.8C per 1000m / approx. 1C per 100m - Remember* Adiabatic means there is no net loss or gain of energy! Example: If surface temp is 30 degrees C, what temp would rising air have … At 500 m? 25 At 1000m? 20 At 2000m? 10 Cooling and Condensation in the atmosphere Adiabatic cooling: is the main mechanism for reaching dew-point temperature in the atmosphere to result in condensation and cloud formation **clouds are formed of liquid water droplets, must have condensation before you have clouds Conversely, adiabatic warming of descending air reduces its relative humidity and results in “drier” air with less cloud formation (and evaporation of existing clouds) Cooling is done by expansion; heating is done by compression For any condensation to occur: - Dew point = air temperature o ^ This happens by: raising vapor content of air to saturation level mixing warm, moist air with cool, dry air lowering air temp to dew point temp (most important in cloud formation) Most clouds Form by adiabatic cooling (lowering of air temp without the removal of heat) Adiabatic cooling results from the expansion of air that occurs when st is lifted and it is a direct application of 1 law of thermodynamics (energy cannot be created or destroyed, just transformed) If air has a temp of 10C when it goes over a mountain range 2000m high, what temp would that air have - At 1000m? 20c - At 500m? 25C - At 100m? 29-30C What would rel. humidity of that air be if it was nearly 100% over the mountains? -25% What happens once saturation is reached and condensation occurs? - Condensation continues if air is still rising How are air parcels affected by condensation process? - Air gets warmer because latent heat (condensation is a change of state, so latent heat is released) released in the form of sensible heat - Air parcel would not continue to rise and cool because cooling has been offset (it’s not cooling as fast because the heat being given off), it would continue to rise though because of the heat So which rises better? Dry air or wet air? - Wet air! (aka saturated air) because you have condensation going on and heat being released to keep that air warmer longer, thus making the air rise faster Wet air/saturated cools more slowly: WALR vs DALR (due to release of latent heat during condensation) Wet adiabatic lapse rate is NOT a constant but it is typically less than 10 (let’s call it 5) because it is approx. half of the DALR (10C/1000m) After condensation level is reached, condensation and cloud formation begins Height of condensation level depends on how dry the air is at the bottom where the process begins; clouds usually form between 1000 and 2000 m - Physical Laws cont. – (full list in study guide!) 1. Pressure of a gas is inversely related to volume (the has expands with lower atmospheric pressure) a. – ideal gas law PV = n R T 2. Rising air cools automatically, sinking air warms automatically (adiabatically) 3. Saturated air is more buoyant than unsaturated air Condensation without adiabatic cooling - Dew: direct condensation as a result of cooling (often radiative cooling, at night) Hoar frost: direct deposition of water vapor as ice (if temperature below freezing), without passing through liquid phase; forms as ice crystals typically around the edges of leaves Fog: condensation as in clouds – but near the ground Fog situation and Types: Fog: essentially a cloud at ground level; forms by the lowering of air temperature to the dew point, increasing water vapor content, or mixing cold air with warm, moist air o Steam fog Occurs when cold, dry air mixes with warm, moist air above the a water surface must have water rapid evaporation o Precipitation fog – Results from evaporation of falling raindrops Advection fog – o result of either warm or cold advection (warm more common) Advection refers to horizontal movement of air o As air passes over the cooler surface, heat is transferred downward o Can also form over water when warm and cold ocean currents are in close proximity to one another o Common during summer months in San Francisco bay area Evaporation fog o water vapor near warmer (Wet) surface rises and condenses in cooler later (sometimes from cold advaection above) o referred to as an “overshoot phenomenon” because the water evaporates quickly but then re-condenses in cooler air above Radiation fog (ground fog) – o results from strong radiative cooling of the air very near the surface on a clear night (L>>>L) o most common type of fog, esp. in autumn o result of radiative cooling on a clear night; can be gently ‘stirred’ with light winds o burns off quickly after sun comes up o common in Central Valley of California Upslope fog o results from adiabatic cooling when air flows along a gently rising sloped surface, the air cools andd expands as it moves upward (ex: Western slope of Great Plains) Fog is often the result of more than just one thing 9/14 Lifting and atmospheric stability Already: adiabatic cooling, dry (DALR) and wet (WALR) lapse rates Air does not rise if it is cooler than the surrounding air, so a “lift” is needed to get enough cooling for cloud formation Processes that life air: Orographic lifting Convergent lifting Frontal lifting Free convection Atmospheric stability – will the air rise? (or will it require a ‘lift’?) Key: adiabatic cooling vs. temp profile (ELR) Determining atmospheric stability: 1. Project temperature of an air parcel (by ALR) to some other elevation 2. Comparing that with the temperature of the air already there Static stability: air’s susceptibility to uplift - Statically unstable air becomes buoyant when lifted and continues to rise if given an initial upward push; - Statically stable air neither rises on its own following an initial lift, not sinks back to it’s original level; it just rests at the height which it was displaced Four cases of stability: 1. Stable a. Happens if ELR is less than saturated adiabatic lapse rate b. Air will resist lifting, regardless of whether or not it is saturated c. Stability is “resistance to upward motion” 2. Unstable a. Happens if ELR is greater than the adiabatic cooling rate (DALR if dry, WALR if saturated) 3. Neutral atmosphere a. Temp lapse rate is the same as ELR 4. Conditional stability a. Happens when ELR is between the dry and saturated adiabatic lapse rates b. Tendency for a lifted parcel to sink or continue rising depends on whether or not it becomes saturated and how far it is lifted Know How to determine atmospheric stability (with numbers) Lift Mechanisms: Convergent lifting Low pressure coming from flat land Horizontal movement toward a common location ` Convectional lifting Lifting that results from heating the air near the surface Free Convection – ascent of warmer air, especially as a result of surface heating o Lifts large amount of air quickly, thus the air is cooled quickly and made available for precipitation See diagram on p.42 of workbook! Onographic lifting Case of forced convection by which air is forced to rise when blown against a mountain range Upward displacement of air leads to adiabatic cooling and possible cloud formation Copious orographic precipitation takes place on windward side On the other side of the mountain, air descends the slope and warms via compression to create a rain shadow (area of lower precipitation) example of this is Death Valley! Frontal lifting Case of forced convection by which moving air (pushed by a pressure gradient) is forced to “override” colder air; therefore it is seemingly lifted by a cold-air wedge Fronts- Transition zones that exist where great temperature differences occur in relatively short distanes Results in the widespread development of cloud in 1 of 2 ways: o When cold air adcances towards warmer air (aka a cold front), dense cold air displaces light warm air o When warm air flows towards a wedge of cold air (aka a warm front), warm air is forced upward Free vs. Forced convection: Free Convection – warm air rising (less dense, lighter) naturally (p.42) Forced convection – air is PUSHED upwards - By a mountain range (orographic lifting) - Another air mass (frontal lifting) - Or convergence of different air masses Free Vertical Air Movement - A parcel of air moves vertically until they are comfortable – when they reach a level where surrounding air has same density o Stablility: if parcel is cooler (heavier/ more dense) that surrounding air, parcel will sink (or stay at surface) o Instability: if parcel is warmer (lighter/ less dense) than surrounding air, parcel will rise o Both the parcel’s temp and that of surrounding air may be changing vertically Density and Buoyancy – Air is buoyant (unstable) if less dense than the air around it Buoyancy: tendency of an air parcel to rise because it is less dense than surrounding air o Saturated air is more buoyant Density: mas of a substance per unit of volume Buoyancy force air parcel gravitational force Determining atmospheric stability : to compare if atmosphere is stable, compare ad cooling rate of rising air with vertical temp structure of air around them (given by ELR) ELR = dif, in temp and dif. Of height above surface Determining atmospheric stability: - As air parcel rises or sinks, it is subject to adiabatic cooling or warming - determining atmospheric stability involves comparing projected temperature of air parcel vs. the temperature of the surrounding air at some height where temperatures are available SO, IF: - Air parcel warmer than surrounding air -> unstable, air would rise o ELR > DALR DALR = 10C/1000m - Air parcel cooler than surrounding air -> stable, air would not rise o ELR < DALR DALR = 10C/1000m Stable atmosphere - air cools slowly upward and rising air cools faster - If air resists vertical motion, it is stable (although it can be forced to rise) - Stability occurs when cold air is beneath warm air - Atmosphere is stable if ELR < DALR o – all temperature inversions are stable - ex: in latter part of the night and near dawn, when the ground is cold from nighttime radiative cooling - normally, stable air does not produce clouds or precipitation 15 degrees / 1000 m = high ELR (more than 10) = unstable atmosphere because Unstable atmosphere - Air cools rapidly upward and rising air cools more slowly - If air is lighter (less dense) and rises without being forced, it is said to be unstable - Instability occurs when warmed air is beneath cold air o – ex: on a sunny afternoon, when ground is hot from absorbing solar radiation - Atmosphere is unstable if ELR > DALR - Cloud form if unstable air rises far enough to cool t dew-point temp and cause condensation (aka precipitation) Conditional stability - If “dry” (unstaturated) air resists upward motion but rises on its own after being saturated (relative humidity = 100%) - Clouds can form if air is lifted to its condensation level - DALR > ELR > WALR - “atmosphere may be stable but…” Neutral Atmosphere: - lifted parcel of air does not return to its original position nor continues to rise - ELR = ALR (wet or dry) - Temperature inside parcels matches temperature of surroundings ELR = .7 degrees/100 m; air is slightly colder than surrounding air, so you have to kick the air up a little bit, it will reach the level of free convection where that air is no longer heavier than the air around it, the air becomes unstable and it rises and can condensate and become a saturated air mass Factors that can influence ELR: 1. Surface heating --- surface air warmed and may rise 2. Surface cooling --- surface air cooled, will not rise 3. Advection of warmer air --- new air may be warmed than air above and thus could rise 4. Advection of cooler air --- new air may destroy existing instability 5. Radiative cooling of cloud tops --- creates larger ELR and can cause instability indirectly Ground surface changes ELR the most because the temp of it fluctuates so greatly (warms during the day, surface cools under a clear sky ((may even get a temp inversion)))