2 Water is responsible for many of Earth’s natural processes
3 Water can exist in all three phases in our atmosphere What atmospheric variable do we use to quantify the amount of water in any given volume of air at one time?Answer: Moisture
4 Moisture (Variables)Relative Humidity (RH) is defined as the ratio of the amount of water vapor in the air to the amount of water vapor the air can hold (given as a percentage)Dewpoint is defined as the temperature the air would have to be cooled to reach saturation (RH=100%)Warmer air can hold more water vapor, so warmer air will, by definition, have a higher dewpointMixing Ratio is the ratio of the mass of water to the mass of dry air
5 1. Add more water vapor to the air There are only TWO ways to saturate the air (or increase the relative humidity)1. Add more water vapor to the air2. Cool the air until its temperature is closer to the dew point temperatureRemember the water vapor molecules are moving faster in warm air and less likely to stick together and condense. If air cools to the dew point temperature, there is saturation.
6 MoistureAn air parcel with a large moisture content has the potential for that parcel to produce a great amount of precipitation.- Air with a mixing ratio of 13 g/kg will likely rain a greater amount of water than air with a mixing ratio of 6 g/kg.
7 Two parcels of air:PARCEL 1: Temperature = 31 oF, Dew point = 28 oFPARCEL 2: Temperature = 89 oF, Dew point = 43 oFParcel 2 contains more water vapor than Parcel 1, because its dew point is higher.Parcel 1 has a higher relative humidity, because it wouldn’t take much cooling for the temperature to equal the dew point! Thus, Parcel 1 is more likely to become saturated. But if it happened that both parcels became saturated then Parcel 2 would have the potential for more precipitation.RH is not simply equal to the dew point divided by the temperature but is a good representation.
8 Types of HeatSensible Heat is the sort of heat you can measure with a thermometerIt’s also the type of heat you feel when you step on a hot surface with bare feetLatent Heat is the heat required to change a substance from one phase to anotherThis is most commonly important with water, which is the only substance that exists on the Earth is three different phasesGases are more energetic than liquids, which are more energetic than solids, so to move up in energetic states, energy is taken from the environment, and vice versa
10 Moisture and the Diurnal Temperature Cycle Review: Water has a high heat capacity (it takes lots of energy to change its temperature)As a result, a city with a dry climate (like Sacramento, CA) will have a very large diurnal (daily) temperature cycleA city with high water vapor concentration (like Key West, FL) will have a small diurnal cycleLate July averages:Sacramento: ~94/60Key West: ~90/79
11 The other key component to the hydrologic cycle- Stability What is stability?Stability refers to a condition of equilibriumIf we apply some perturbation to a system, how will that system be affected?Stable: System returns to original stateUnstable: System continues to move away from original stateNeutral: System remains steady after perturbed
12 Stable: Marble returns to its original position Stability ExampleStable: Marble returns to its original positionUnstable: Marble rapidly moves away from initial position
13 How does a bowl and marble relate to the atmosphere? StabilityHow does a bowl and marble relate to the atmosphere?When the atmosphere is stable, a parcel of air that is lifted will want to return back to its original position:
14 Stability Cont.When the atmosphere is unstable (with respect to a lifted parcel of air), a parcel will want to continue to rise if lifted:
15 What do we mean by an air parcel? Imaginary small body of air a few meters wideCan expand and contract freelyDoes not break apartOnly considered with adiabatic processes - External air and heat cannot mix with the air inside the parcelSpace occupied by air molecules inside parcel defines the air densityAverage speed of molecules directly related to air temperatureMolecules colliding against parcel walls define the air pressure inside
16 Buoyancy and Stability Imagine a parcel at some pressure level that is held constant, density remains the same so the only other variable that is changing is temperature. (REMEMBER: the Ideal Gas Law)So if ρparcel < ρenv. Parcel is positively buoyantIn terms of temperature that would mean:T of parcel > T of environment – buoyant! (unstable)T of parcel < T of environment – sink! (stable)T of parcel = T of environment – stays put (neutral)
17 Atmospheric Stability This is all well and good but what about day to day applications?
18 Review: Atmospheric Soundings Vertical “profiles” of the atmosphere are taken at 0000 UTC and 1200 UTC at ~95 stations across the country and many more around the world. Sometimes also launched at other times when there is weather of interest in the area.Weather balloons rise to over 50,000 feet and take measurements of several meteorological variables using a “radiosonde.”TemperatureDew point temperatureWind- Direction and SpeedPressure
20 Adiabatic Lapse RateMixing RatioMoist Adiabatic Lapse RateTemperatureDewpointTemperature
21 Vertical Profile of Atmospheric Temperature allows us to assess stability of the atmosphere We must compare the parcel's temperature Tp with the temperature of the surrounding environment Te.
22 Lapse RatesLapse Rate: The rate at which temperature decreases with height (Remember the inherent negative wording to it)Environmental Lapse Rate: Lapse rates associated with an observed atmospheric sounding (negative for an inversion layer)Parcel Lapse Rate: Lapse rate of a parcel of air as it rises or falls (either saturated or not)Moist Adiabatic Lapse Rate (MALR): Saturated air parcelDry Adiabatic Lapse Rate (DALR): Dry air parcel
23 DALR Air in parcel must be unsaturated (Relative Humidity < 100%) Rate of adiabatic heating or cooling = ~10°C for every 1000 meter (1 kilometer) change in elevationParcel temperature decreases by about 10° if parcel is raised by 1km, and increases about 10° if it is lowered by 1km
24 MALRAs rising air cools, its RH increases because the temperature approaches the dew point temperature, TdIf T = Td at some elevation, the air in the parcel will be saturated (RH = 100%)If parcel is raised further, condensation will occur and the temperature of the parcel will cool at the rate of ~6.5°C per 1km in the mid-latitudes
25 DALR vs. MALR The MALR is less than the DALR because of latent heating As water vapor condenses into liquid water for a saturated parcel, LH is released, lessening the adiabatic coolingRemember no heat exchanged with environment
27 Absolute StabilityThe atmosphere is absolutely stable when the environmental lapse rate (ELR) is less than the MALRELR < MALR <DALRA saturated OR unsaturated parcel will be cooler than the surrounding environment and will sink, if raised
28 Absolute Stability Inversion layers are always absolutely stable Temperature increases with heightWarm air above cold air = very stable
29 Absolute InstabilityThe atmosphere is absolutely unstable when the ELR is greater than the DALRELR > DALR > MALRAn unsaturated OR saturated parcel will always be warmer than the surrounding environment and will continue to ascend, if raised
30 Conditional Instability The atmosphere is conditionally unstable when the ELR is greater than the MALR but less than the DALRMALR < ELR < DALRAn unsaturated parcel will be cooler and will sink, if raisedA saturated parcel will be warmer and will continue to ascend, if raised
31 Conditional Instability Example: parcel at surfaceT(p) = 30°C, Td(p) = 14°C (unsaturated)ELR = 8°C/km for first 8kmParcel is forced upward, following DALRParcel saturated at 2km, begins to rise at MALRAt 4km, T(p) = T(e)…this is the level of free convection (LFC)
32 Conditional Instability Example continued…Now, parcel will rise on its own because T(p) > T(e) after 4kmThe parcel will freely rise until T(p) = T(e), againThis is the equilibrium level (EL)In this case, this point is reached at 9kmThus, parcel is stable from 0 – 4km and unstable from 4 – 9kmELLCL
33 Rising Air Consider an air parcel rising through the atmosphere The parcel expands as it risesThe expansion, or work done on the parcel causes the temperature to decreaseAs the parcel rises, humidity increases and reaches 100%, leading to the formation of cloud droplets by condensation
34 Rising AirIf the cloud is sufficiently deep or long lived, precipitation develops.The upward motions generating clouds and precipitation can be produced by:Convection in unstable airConvergence of air near a cloud baseLifting of air by frontsLifting over elevated topography
35 Lifting by ConvectionAs the earth is heated by the sun, thermals (bubbles of hot air) rise upward from the surfaceThe thermal cools as it rises, losing some of its buoyancy (its ability to rise)The vertical extent of the cloud is largely determined by the stability of the environment
36 Lifting by ConvectionA deep stable layer restricts continued vertical growthA deep unstable layer will likely lead to development of rain-producing cloudsThese clouds are more vertically developed than clouds developed by convergence lifting
37 Lifting by Convergence Convergence exists when there is a horizontal net inflow into a regionWhen air converges along the surface, it is forced to rise
38 Lifting by Convergence Large scale convergence can lift air hundreds of kilometers acrossVertical motions associated with convergence are generally much weaker than ones due to convectionGenerally, clouds developed by convergence are less vertically developed
39 Lifting due to Topography This type of lifting occurs when air is confronted by a sudden increase in the vertical topography of the EarthWhen air comes across a mountain, it is lifted up and over, cooling as it is risingThe type of cloud formed is dependent upon the moisture content and stability of the air
41 Lifting Along Frontal Boundaries Front – The transition zone between two air masses of different densitiesLifting occurs along two different types of frontsCold FrontWarm Front
42 Lifting Along Cold Fronts A colder,denser air mass lifts the warm, moist air ahead of itAs the air rises, it cools and condenses, producing clouds and precipitationThe steep slope of the cold front leads to more vigorous rising motionHence, cold fronts are often associated with thunderstorms
44 Lifting Along Warm Fronts A warmer, less dense air mass rises up and over the cold air ahead of the warm frontAir rises, cools and condensesWarm fronts have gentler slopes and move slower than cold frontsGenerally, precipitation is more steady and widespread