Atmospheric Moisture and Stability

Lecture 8

Water is responsible for many of Earth’s natural processes

http://www.srh.weather.gov/jetstream/atmos/hydro.htm

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

There are only TWO ways There are only TWO ways to saturate the airto saturate the air

(or increase the relative humidity) (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 CycleMoisture 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

Stability ExampleStability Example

Stable: Marble returns to its original position

Unstable: Marble rapidly moves away from initial position

StabilityStability

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:

http://www.chitambo.com/clouds/cloudshtml/humilis.html

Stability Cont.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:

http://www.physicalgeography.net/fundamentals/images/cumulonimbus.jpg

– Imaginary small body of air a few meters wide• Can expand and contract freely• Does not break apart• Only considered with adiabatic processesadiabatic 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

What do we mean by an air parcel?

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 buoyantParcel 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 SoundingsReview: 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.”

http://www2.ljworld.com/photos/2006/may/24/98598/

• Temperature

• Dew point temperature

• Wind

- Direction and Speed

• Pressure

Mixing RatioAdiabatic Lapse Rate

Temperature

Moist Adiabatic Lapse Rate

TemperatureDewpoint

Vertical Profile of Atmospheric Temperature allows us to assess

stability of the atmosphere

Lapse RatesLapse 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

LCL

EL

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

• Will discuss origin more in detail later in the semester as we begin to discuss cyclones and fronts

NEXT WEEK: Severe weather!