# Atmospheric Moisture and Stability

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Atmospheric Moisture and Stability
Lecture 8

Water is responsible for many of Earth’s natural processes

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

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 dewpoint Mixing Ratio is the ratio of the mass of water to the mass of dry air

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 air 2. Cool the air until its temperature is closer to the dew point temperature Remember 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.

Moisture An 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.

Two parcels of air: PARCEL 1: Temperature = 31 oF, Dew point = 28 oF PARCEL 2: Temperature = 89 oF, Dew point = 43 oF Parcel 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.

Types of Heat Sensible Heat is the sort of heat you can measure with a thermometer It’s also the type of heat you feel when you step on a hot surface with bare feet Latent Heat is the heat required to change a substance from one phase to another This is most commonly important with water, which is the only substance that exists on the Earth is three different phases Gases 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

Latent Heat

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 cycle A city with high water vapor concentration (like Key West, FL) will have a small diurnal cycle Late July averages: Sacramento: ~94/60 Key West: ~90/79

The other key component to the hydrologic cycle- Stability
What is stability? Stability refers to a condition of equilibrium If we apply some perturbation to a system, how will that system be affected? Stable: System returns to original state Unstable: System continues to move away from original state Neutral: System remains steady after perturbed

Stable: Marble returns to its original position
Stability Example Stable: Marble returns to its original position Unstable: Marble rapidly moves away from initial position

How does a bowl and marble relate to the atmosphere?
Stability How 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:

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:

What do we mean by an air parcel?
Imaginary small body of air a few meters wide Can expand and contract freely Does not break apart Only considered with adiabatic processes - External air and heat cannot mix with the air inside the parcel Space occupied by air molecules inside parcel defines the air density Average speed of molecules directly related to air temperature Molecules colliding against parcel walls define the air pressure inside

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 buoyant In 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)

Atmospheric Stability
This is all well and good but what about day to day applications?

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.” Temperature Dew point temperature Wind - Direction and Speed Pressure

Adiabatic Lapse Rate Mixing Ratio Moist Adiabatic Lapse Rate Temperature Dewpoint Temperature

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.

Lapse Rates Lapse 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 parcel Dry Adiabatic Lapse Rate (DALR): Dry air parcel

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 elevation Parcel temperature decreases by about 10° if parcel is raised by 1km, and increases about 10° if it is lowered by 1km

MALR As rising air cools, its RH increases because the temperature approaches the dew point temperature, Td If 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

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 cooling Remember no heat exchanged with environment

DALR vs. MALR

Absolute Stability The atmosphere is absolutely stable when the environmental lapse rate (ELR) is less than the MALR ELR < MALR <DALR A saturated OR unsaturated parcel will be cooler than the surrounding environment and will sink, if raised

Absolute Stability Inversion layers are always absolutely stable
Temperature increases with height Warm air above cold air = very stable

Absolute Instability The atmosphere is absolutely unstable when the ELR is greater than the DALR ELR > DALR > MALR An unsaturated OR saturated parcel will always be warmer than the surrounding environment and will continue to ascend, if raised

Conditional Instability
The atmosphere is conditionally unstable when the ELR is greater than the MALR but less than the DALR MALR < ELR < DALR An unsaturated parcel will be cooler and will sink, if raised A saturated parcel will be warmer and will continue to ascend, if raised

Conditional Instability
Example: parcel at surface T(p) = 30°C, Td(p) = 14°C (unsaturated) ELR = 8°C/km for first 8km Parcel is forced upward, following DALR Parcel saturated at 2km, begins to rise at MALR At 4km, T(p) = T(e)…this is the level of free convection (LFC)

Conditional Instability
Example continued… Now, parcel will rise on its own because T(p) > T(e) after 4km The parcel will freely rise until T(p) = T(e), again This is the equilibrium level (EL) In this case, this point is reached at 9km Thus, parcel is stable from 0 – 4km and unstable from 4 – 9km EL LCL

Rising Air Consider an air parcel rising through the atmosphere
The parcel expands as it rises The expansion, or work done on the parcel causes the temperature to decrease As the parcel rises, humidity increases and reaches 100%, leading to the formation of cloud droplets by condensation

Rising Air If the cloud is sufficiently deep or long lived, precipitation develops. The upward motions generating clouds and precipitation can be produced by: Convection in unstable air Convergence of air near a cloud base Lifting of air by fronts Lifting over elevated topography

Lifting by Convection As the earth is heated by the sun, thermals (bubbles of hot air) rise upward from the surface The 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

Lifting by Convection A deep stable layer restricts continued vertical growth A deep unstable layer will likely lead to development of rain-producing clouds These clouds are more vertically developed than clouds developed by convergence lifting

Lifting by Convergence
Convergence exists when there is a horizontal net inflow into a region When air converges along the surface, it is forced to rise

Lifting by Convergence
Large scale convergence can lift air hundreds of kilometers across Vertical motions associated with convergence are generally much weaker than ones due to convection Generally, clouds developed by convergence are less vertically developed

Lifting due to Topography
This type of lifting occurs when air is confronted by a sudden increase in the vertical topography of the Earth When air comes across a mountain, it is lifted up and over, cooling as it is rising The type of cloud formed is dependent upon the moisture content and stability of the air

Lifting due to Topography

Lifting Along Frontal Boundaries
Front – The transition zone between two air masses of different densities Lifting occurs along two different types of fronts Cold Front Warm Front

Lifting Along Cold Fronts
A colder,denser air mass lifts the warm, moist air ahead of it As the air rises, it cools and condenses, producing clouds and precipitation The steep slope of the cold front leads to more vigorous rising motion Hence, cold fronts are often associated with thunderstorms

Lifting Along Cold Fronts

Lifting Along Warm Fronts
A warmer, less dense air mass rises up and over the cold air ahead of the warm front Air rises, cools and condenses Warm fronts have gentler slopes and move slower than cold fronts Generally, precipitation is more steady and widespread

Lifting Along Warm Fronts

Lifting Along Frontal Boundaries
Will discuss origin more in detail later in the semester as we begin to discuss cyclones and fronts NEXT WEEK: Severe weather!