Thermodynamics Study Set Flashcards
What does density do with height in the molecular boundary layer?
Density increases with height
What does Rho stand for?
Density
What does Theta represent?
Real Heat
What does Theta E represent?
Total Heat
What is important to note regarding total heat?
The amount of total heat never changes dry or wet
What weather process is responsible for making real heat?
Condensation
What is another abbreviation for the Planetary Boundary Layer?
CBL
What does the word eccentric represent?
Dry adiabatic
What is important to note regarding the Planetary Boundary Layer?
The Planetary Boundary Layer is where eddies are located
What are the two different types of eddies?
Mechanically Induced and Thermally Induced
What is responsible for mechanically induced eddies in the atmosphere?
The wind
When do thermally induced eddies occur?
During the day
What is important to note regarding Mechanically Induced eddies?
You can still get mechanically induced eddies even on a cloudy day
What are the important of eddies in the atmosphere?
They’re responsible for mixing the water vapor and the heat within the Planetary Boundary Layer
In the Planetary Boundary Layer, what two things remain constant with height?
The temperature and the mixing ratio
What is the Dewpoint Lapse Rate regarding the Dry Adiabatic?
2 degrees Celsius per kilometer
The measure of the energy required by the atmosphere, at a given time and place, to inhibit the ability of an air parcel to rise is what meteorological quantity?
Lifted condensation level
When convective inhibition values are high, the risk of thunderstorm development is what?
Low
In which of the following ways can convective inhibition values be determined?
Skew-T plotting and mathematical calculations
What is the wet-bulb temperature?
The lowest temperature that can be reached by evaporating water into the air
What is the importance of the skew-T/log-P thermodynamic diagram?
The skew-T/log-P thermodynamic diagram is used to plot vertical profiles of atmospheric temperature, moisture, and wind
Data for the vertical atmospheric profiles, also known as sounding plots, come from numerous sources such as what?
Radiosondes, dropsondes, pibals, aircraft, NWP model output, and satellite sounders
Most weather soundings are generated in what way?
Electronically
How does pressure decrease regarding the atmosphere?
Pressure decreases logarithmically with increasing height in the atmosphere
What are the horizontal lines on the skew-T/log-P thermodynamic diagram referred to as?
Isobars
What are Isobars?
Lines of equal pressure
How are the Isobars spaced on a skew-T/log-P thermodynamic diagram?
Logarithmically from about 1050 hPa at the bottom to 100 hPa at the top
Isobar lines are plotted every what on a skew-T/log-P thermodynamic diagram?
Every 50 hPa
What are Isotherms?
Lines of constant temperature
When talking about Isotherms on a skew-T/log-P thermodynamic diagram, how are they plotted?
On the skew-T diagram, isotherms are straight, solid lines, sloping from the lower left to upper right
What do Dry Adiabats represent?
Lines of constant potential temperature
When talking about Dry Adiabats on a skew-T/log-P thermodynamic diagram, how are they plotted?
On the skew-T diagram, dry adiabats are the slightly-curved, solid lines sloping from the lower right to upper left
What is the importance of the Dry Adiabat lines on a skew-T/log-P thermodynamic diagram?
They indicate the rate of temperature change in a parcel of dry air rising or descending adiabatically, i.e., with no loss or gain of heat by the parcel
The dry adiabat for each multiple of 10°C shares a label with the what on a skew-T/log-P thermodynamic diagram?
Isotherms
What do Saturation Adiabats lines represent?
Lines of constant equivalent potential temperature
What are other names for Saturation Adiabats?
Moist adiabats or saturation pseudo-adiabats
When talking about Saturation Adiabats on a skew-T/log-P thermodynamic diagram, how are they plotted?
On the skew-T diagram, saturation adiabats are the slightly curved lines
What should someone note regarding Saturation Adiabats on a skew-T/log-P thermodynamic diagram?
The slope and spacing of the lines varies significantly with height and temperature, particularly at lower levels
What is the importance of Saturation Adiabats?
Saturation adiabats represent the rate of temperature change in a rising parcel of saturated air (assuming that all the condensed water vapor is liquid and falls out immediately as the parcel rises—the pseudo-adiabatic assumption)
When do the saturation adiabats become parallel to the dry adiabats when talking about a skew-T/log-P thermodynamic diagram?
At low values of moisture, temperature, and pressure
What are Saturation Mixing Ratio lines also known as?
Humidity mixing ratio lines
What do Saturation Mixing Ratio lines represent?
Constant values of water vapor capacity—specifically, the number of grams of water required to saturate one kilogram of dry air at a particular temperature and pressure
When talking about Saturation Mixing Ratio lines on a skew-T/log-P thermodynamic diagram, how are they plotted?
On the skew-T diagram, the saturation mixing ratio (ws lines are the slightly-curved, dashed lines sloping from the lower left to upper right
What should someone note regarding Saturation Mixing Ratio lines?
Since the vapor capacity of air varies non-linearly with temperature, the labeling interval for Ws lines is not uniform
In order for someone to create temperature and dewpoint curves on a skew-T diagram, what must they do first?
Plot the temperature and dewpoint at each individual pressure level in the sounding. All mandatory and significant level sounding data should be plotted
In order for someone to create temperature and dewpoint curves on a skew-T diagram, what is the second step they must do?
Connect the temperature points with straight lines between successive pressure levels, often using red. Similarly, connect the dewpoints with straight lines, often using green or blue
Define Dewpoint
The temperature to which a parcel of air at a constant pressure becomes saturated without any addition of water vapor
What is the Saturation Mixing Ratio?
The saturation mixing ratio (ws) is the ratio of the mass of water vapor (Mv) to the mass of dry air (Md) in a parcel of air at saturation. In other words, Ws is the maximum amount of water vapor that a parcel can hold without condensation
The Saturation Mixing Ratio is expressed in parts per what?
Parts per thousand. Usually grams of water vapor per kilogram of dry air
How can someone find the Saturation Mixing Ratio when using a skew-T diagram?
To find the saturation mixing ratio for a given temperature and pressure on a plotted sounding, read the value, either directly or by interpolation, of the saturation mixing-ratio line that crosses the T curve at that pressure
In a sample of moist air, what is the mixing ratio?
The ratio of the mass of water vapor to the mass of dry air
What is the mixing ratio expressed in?
Parts per thousand, usually grams of water vapor per kilogram of dry air
How is the mixing ratio different from the saturation mixing ratio?
It measures the actual amount of water vapor present while the saturation mixing ratio measures the amount of water vapor that would be present at saturation
How can someone find the mixing ratio on a Skew-T diagram?
Read the value, either directly or by interpolation, of the saturation mixing ratio line that crosses the dewpoint temperature curve at that certain pressure
What should someone always remember regarding the mixing ratio?
The mixing ratio at a particular pressure is the saturation mixing ratio of the dewpoint. Not the temperature
What is Relative Humidity?
The ratio (expressed as a percent) of the amount of water vapor in a given volume of air to the amount that volume would hold if the air were saturated
What can the Relative Humidity be computed by?
The mixing ratio and the saturation mixing ratio
What is a Dewpoint Depression?
The difference between the temperature and the dewpoint temperature at a particular pressure level
The moisture output from rawinsonde observations is typically reported in terms of what?
The dewpoint depression
Saturated conditions have a dewpoint depression of what?
Zero
Dry conditions have a dewpoint depression of what?
More than 30 degrees Celsius
Rapid drying conditions above the fog support what?
Radiative and evaporative cooling at the fog top. This cooling helps to maintain the fog
What is Saturation Vapor Pressure?
The part of the total atmospheric pressure attributable to water vapor if the air were saturated
How can someone find the Saturation Vapor Pressure while using a Skew-T diagram?
From the temperature curve at the given pressure on the sounding, always follow the isotherm to the 622 hPa isobar
What is Vapor Pressure?
The part of the total atmospheric pressure attributable to water vapor
What is the difference between finding the Vapor Pressure and the Saturation Vapor Pressure on the Skew-T diagram?
One starts with the dewpoint to find the vapor pressure while one starts with the temperature to find the saturation vapor pressure
How can someone find the Vapor Pressure while using a Skew-T diagram?
From the dewpoint temperature curve at the given pressure on the sounding, always follow the isotherm to the 622 hPa isobar
Define Virtual Temperature
The temperature at which dry air would have the same density as the moist air at a given pressure
Two air samples with the same virtual temperature have the same what?
Density, regardless of their actual temperature or relative humidity
How does water vapor compare to dry air?
Water vapor is less dense than dry air
How does warm air compare to cool air?
Warm air is less dense than cool air
Since water vapor is less dense than dry air and warm air is less dense than cool air, the virtual temperature is always what to the actual temperature?
Always greater than or equal to
Since the saturation mixing ratio increases exponentially with temperature, the virtual temperature correction becomes what?
Increasingly important for higher dewpoints
How can someone figure out the Virtual Temperature while using a Skew-T diagram?
- Determine the mixing ratio, which is the value of the saturation mixing ratio line passing through the dewpoint at a given pressure
- The virtual temperature is then computed as follows:
Tv minus T plus (w/6)
What is Potential Temperature (Theta)?
The temperature that a sample of air would have if it were brought dry-adiabatically to a pressure of 1000 hPa
Potential Temperature is commonly expressed in what unit?
Kelvin
How can someone figure out the Potential Temperature while using a Skew-T diagram?
From the temperature curve at the given pressure, follow the dry adiabat to the 1000 hPa isobar
How does a dry adiabat relate to potential temperature?
The dry adiabat is an isotherm of constant potential temperature
What is the Lifting Condensation Level?
The height at which a parcel of air becomes saturated when it is lifted dry-adiabatically
How can someone figure out the Lifting Condensation Level while using a Skew-T diagram?
The LCL is located on a sounding at the intersection of the saturation mixing ratio line that passes through the surface dewpoint temperature with the dry adiabat that passes through the surface temperature
When the moisture content in the near-surface layers varies significantly, an average moisture value of the lower layer may be used in place of the surface-parcel moisture value to compute the what?
Lifting Condensation Level
What is Equivalent Temperature?
The temperature a sample of air at a pressure level would have if all its moisture were condensed out by a pseudo-adiabatic process
What is another name for Equivalent Temperature?
Adiabatic Equivalent Temperature
How can someone figure out the Equivalent Temperature while using a Skew-T diagram?
- From the dewpoint at the given pressure, draw a line upward parallel to the saturation mixing-ratio lines. Also, from the temperature curve at the given pressure, draw a line upward along a dry adiabat until it intersects the line drawn from the dewpoint. Recall that this level is the LCL
- From the LCL, follow a saturation adiabat upward to a pressure where the saturation adiabat parallels the dry adiabat. This is the pressure level where all the moisture has been condensed out of the sample
- From this pressure, follow a dry adiabat back to the original pressure. The isotherm value at this point is equal to the equivalent temperature (Te)
What is the Equivalent Potential Temperature (theta-e)?
The temperature a sample of air would have if all its moisture were condensed out by a pseudo-adiabatic process and the sample then brought dry-adiabatically back to 1000 hPa
The Equivalent Potential Temperature is identical to what?
The equivalent temperature
How is the Equivalent Potential Temperature different from the Equivalent Temperature?
The sample is brought dry-adiabatically from the equivalent temperature at the initial level to the equivalent potential temperature at the 1000 hPa level
Equivalent Potential Temperature is commonly expressed in what unit?
Kelvin
How can someone figure out the Equivalent Potential Temperature while using a Skew-T diagram?
- From the dewpoint at the given pressure, draw a line upward along a saturation mixing-ratio line. Also, from the T curve at the given pressure, draw a line upward along a dry adiabat until it intersects the line drawn from the dewpoint at the LCL
- From this intersection, follow a saturation adiabat upward to a pressure where the saturation adiabat parallels the dry adiabat. This is the pressure level where all the moisture has been condensed out of the sample
- From this pressure, follow the dry adiabat down to the 1000 hPa isobar. The temperature where the dry adiabat crosses the 1000 hPa isobar is the equivalent potential temperature (theta-e)
What is the Wet Bulb Temperature?
The temperature to which a parcel of air at a constant pressure cools through the evaporation of water into it
When is the Wet Bulb Temperature reached?
When the air parcel achieves saturation
The Wet Bulb Temperature will always fall between what two things?
The dewpoint and the temperature unless the air is saturated
At saturation, what three things are equal?
The temperature, dewpoint, and the wet bulb temperature
In the real atmosphere, the wet bulb temperature often provides what?
A good estimate of what the surface temperature will become after the onset of precipitation and once conditions become saturated
How can someone figure out the Wet Bulb Temperature while using the Skew-T diagram?
- From the temperature, proceed up along a dry adiabat
- From the dewpoint, proceed up along a mixing ratio line
- From where the two lines intersect, proceed down the saturation adiabat to the original level
What is the Wet Bulb Potential Temperature?
The Wet Bulb Temperature a sample of air would have if it were brought along a saturation adiabat to a pressure of 1000 hPa
What is the Wet Bulb Potential Temperature equivalent to?
The Wet Bulb Temperature
The Wet Bulb Potential Temperature is typically expressed in what unit?
Kelvin
How can someone find the Wet Bulb Potential Temperature while using the Skew-T diagram?
- Find the wet-bulb temperature
- From the wet-bulb temperature, follow the saturation adiabat to the 1000 hPa isobar
- The isotherm value at this intersection equals the wet-bulb potential temperature at the given pressure
What is the Convective Condensation Level?
The height to which a parcel of air, if heated sufficiently from below, will rise adiabatically until it is just saturated
How can someone figure out the Convective Condensation Level while using a Skew-T diagram?
To determine the CCL on a sounding, start at the surface dewpoint, process upward along the saturation mixing ratio line until this line intersects the temperature profile on the sounding
When the moisture content in the near-surface layers varies significantly, an average moisture value of the lower layer may be used in place of the surface-parcel moisture value in computing what?
The CCL
What is the Convective Temperature?
The surface temperature that must be reached to start the formation of convective clouds caused by solar heating of the near-surface layer
How can someone figure out the Convective Temperature while using a Skew-T diagram?
From the Convective Condensation Level on the temperature profile, proceed downward along a dry adiabat to the pressure level at the surface
What is the Wet-Bulb Zero Level?
The lowest level in a sounding at which the wet-bulb temperature is zero degrees Celsius
During the onset of a cool season precipitation event, the higher the initial wet-bulb zero level, the what?
The less chance of the precipitation changing to freezing/frozen precipitation at the surface
During the convective season, lower wet-bulb zero levels can indicate a higher probability of what?
Hail occurrence
What is the Level of Free Convection?
The height at which a parcel of air, when lifted, becomes warmer than its surroundings and thus convectively buoyant
How can someone figure out the Level of Free Convection while using a Skew-T diagram?
From the Lifting Condensation Level, proceed upward along a saturation adiabat until you intersect the sounding temperature curve
What is the Tropopause?
The boundary between the troposphere and the stratosphere
The Tropopause is usually marked by what?
A significant change in lapse rate from less stable below in the troposphere to very stable above in the stratosphere
Since the temperature gradient reverses from cooling with increased height in the troposphere to warming with increased height in the stratosphere, the maximum wind speed is typically observed at or near what level?
The tropopause level
What is the Equilibrium Level?
The height where the temperature of a buoyantly rising parcel again equals the temperature of the environment
The Equilibrium Level may be determined for what?
Surface parcels that are lifted or heated
How can someone figure out where the Equilibrium Level for a Lifted Parcel is when using a Skew-T diagram?
From the LFC, proceed upward along a saturation adiabat until it intersects the temperature profile
How can someone figure out where the Equilibrium Level for a Heated Surface Parcel is when using a Skew-T diagram?
From the CCL, proceed upward along a saturation adiabat until intersecting the temperature profile
How is CAPE represented on a Skew-T diagram?
The area on a Skew-T enclosed by the environmental temperature profile and the saturation adiabat running from the LFC to the EL
CAPE is measured in what?
Joules per kilogram
The larger the positive area on a Skew-T diagram, the higher the what?
CAPE value and instability and the greater potential for strong and perhaps severe convection
What do CAPE values of 0 represent when talking about the stability level of the atmosphere?
That the atmosphere is stable
What do CAPE values of 0 to 1000 represent when talking about the stability level of the atmosphere?
That the atmosphere is Marginally Unstable
What do CAPE values of 1000 to 2500 represent when talking about the stability level of the atmosphere?
That the atmosphere is Moderately Unstable
What do CAPE values of 2500 to 3500 represent when talking about the stability level of the atmosphere?
That the atmosphere is Very Unstable
What do CAPE values of 3500 or greater represent when talking about the stability level of the atmosphere?
That the atmosphere is Extremely Unstable
CAPE is a robust indicator of the potential for what?
Deep convection and convective intensity
What does CAPE provide a measure of?
Stability integrated over the depth of the sounding
Since the computation of CAPE is based on parcel theory, it does not take into account processes such as what three things?
Mixing, water loading, and freezing
Surface layer based CAPE computations may underestimate what?
The convective potential in situations with elevated convection
Since CAPE by itself does not account for wind shear, it may underestimate what?
The potential for severe convection where strong wind shear is present
How is Convective Inhibition represented on a Skew-T diagram?
By the area on the Skew-T enclosed by the environmental temperature profile and the temperature of a parcel lifted from some originating level to the LFC. This area indicates the amount of energy required to lift the parcel to the LFC
Convective Inhibition is measured in units of what?
Joules per kilogram
The larger the negative area on a Skew-T diagram, the blank the CIN value?
The higher the CIN value and the lower the likelihood of convective storms
If the CIN is large but storms manage to form, usually due to increased moisture and or heating overcoming the CIN, then what?
Then the storms are more likely to be severe
What is CIN usually the result of?
A capping stable layer or inversion
How is the Lifted Index calculated?
The difference between the observed temperature at 500 hPa and the temperature of an air parcel lifted to 500 hPa from near the surface
The more unstable the environment, the more negative the what?
The Lifted Index is
If there is an Lifted Index value of -2, what does this mean regarding severe weather potential?
Weak severe weather potential
If there is an Lifted Index value between -3 to -5, what does this mean regarding severe weather potential?
Moderate severe weather potential
If there is an Lifted Index value of -6 or less, what does this mean regarding severe weather potential?
Strong severe weather potential
What is an important takeaway regarding the Lifted Index?
Someone should never rely solely on the Lifted Index to evaluate the convective potential
Why is the Lifted Index limited?
It relies on only 3 sounding inputs: temperature, dewpoint of the boundary layer, and the temperature at 500 hPa level
What does the Lifted Index not take into account?
The account of vertical wind shear, which is often an important element in the severe convective environment
How can someone figure out the Lifted Index while using a Skew-T diagram?
- Find the mean temperature and the dewpoint in the lowest 100 hPa
- From those mean temperature and dewpoint temperature values, located at the midpoint of the layer, find the LCL
- From the LCL, lift the parcel moist-adiabatically to 500 hPa and find the parcel temperature
- Given the 500 hPa sounding temperature, the Lifted Index is calculated by taking the temperature at the 500 hPa level minus the temperature
The Showalter stability index is a popular what?
Severe weather index
What is the difference between the Showalter Stability Index and the Lifted Index?
While the Lifted Index starts with the mean of the lowest 100-hPa AGL layer, the SSI uses a parcel lifted from 850 hPa to 500 hPa. At 500 hPa, the parcel temperature is subtracted from the sounding temperature
More negative Showalter Stability Index values indicate what?
Greater instability
If the SSI values are between +3 to +1, what is the weather event most likely occurring?
Rain showers and some thundershowers
If the SSI values are between +1 to -2, what is the weather event most likely occurring?
Thundershowers
If the SSI values are between -3 to -6, what is the weather event most likely occurring?
Severe thunderstorms
If the SSI values are less than -6, what is the weather event most likely occurring?
Severe thunderstorms and possible tornadoes
What is a limitation of the Showalter Stability Index?
It is intended for use at locations with a station elevation up to about 1000 feet
What does the Showalter Stability Index not take into account?
The vertical wind shear
How can someone figure out the Showalter Stability Index while using a Skew-T diagram?
- Find the temperature and dewpoint temperature at the 850 hPa level
- From that temperature and dewpoint temperature, find the LCL
- From the LCL, lift the parcel moist-adiabatically to 500 hPa and find the parcel temperature
- Given the 500 hPa sounding temperature, the SSI is computed by taking the 500 hPa temperature minus the temperature
What is the K-Index particularly useful for?
Identifying convective and heavy-rain producing environments
What does the K-Index computation take into account?
The vertical distribution of both moisture and temperature
The higher the moisture and the greater the 800-500 temperature difference, the higher the what?
K-Index and the potential for convection
Thunderstorm probability ranges from very low when the K-Index values are from what to what?
Values less than 20 west of the Rocky Mountains to a likelihood of widespread activity when K-Index values are greater than 35 west of the Rocky Mountains
What can the K-Index not be used to infer?
The severity of convection
Where can the K-Index not be applied?
The Rocky Mountain region. This is because it used 850 hPa data where the surface pressure is typically less than 850 hPa
How is the Total Totals Index computed?
Using the temperature and the dewpoint at 850 hPa and the temperature at 500 hPa
The higher the 850 hPa dewpoint and temperature and the lower the 500 hPa temperature, the greater the what?
Instability
If the Total Totals Index value is 44, what weather event is most likely to be occurring?
Thunderstorms
If the Total Totals Index value is 50, what weather event is most likely to be occurring?
Severe thunderstorms are possible
If the Total Totals Index value is 55 or greater, what weather event is most likely to be occurring?
Severe thunderstorms are likely with possible tornadoes
What is a limitation of the Total Totals Index?
It uses data from only two mandatory levels (850 and 500 hPa) and thus does not account for intervening inversions or moist or dry layers that may occur below or between these levels
What does the Total Totals Index not take into account?
The account wind shear
How can someone calculate the Total Totals Index?
TT = (T850 + Td850) - (2 * T500)
How can someone calculate the K-Index?
K index = (T850 - T500) + Td850 - (T700 - Td700)
The Severe Weather Threat (SWEAT) index differs from many of the other severe weather indices in that it takes into account the wind profile in assessing severe weather potential. Inputs include what?
- Total Totals Index (TT)
- 850 hPa dewpoint
- 850 hPa wind speed and direction
- 500 hPa wind speed and direction
In general, what conditions lead to a higher SWEAT Index and the greater probability of severe weather?
- Higher temperature and moisture at low levels
- Cooler temperatures aloft
- Large vertical wind shear
- Wind direction veering with height
If the SWEAT Index values are between 150 and 300, what is the severe weather potential?
Slight chance for severe weather
If the SWEAT Index values are between 300 and 400, what is the severe weather potential?
Severe weather is possible
If the SWEAT Index values are 400 or greater, what is the severe weather potential?
Tornadic storms are possible
What is a limitation of the SWEAT Index?
Inputs are only from 850 and the 500 hPa levels
How can someone calculate the SWEAT Index?
SWEAT= 12(850Td) + 20(TT - 49) + 2(V850) + (V500) + 125(sin(dd500 - dd850) + 0.2)
What things must someone find before being able to calculate the SWEAT Index?
- Total Totals index (TT)
- Dewpoint (°C) at 850 hPa (Td850)
- Wind speed (kt) at 850 hPa (V850)
- Wind direction (°) at 850 hPa (dd850)
- Wind speed (kt) at 500 hPa (V500)
- Wind direction (°) at 500 hPa (dd500)
What is the Bulk Richardson Number?
The ratio of buoyancy to the vertical wind shear of the environment
Updraft strength is directly related to what?
CAPE
The storm structure and its movement are directly related to what?
The vertical shear
What does it mean generally if the BRN is less than 10?
There is much more shear than buoyancy and the storms tend to be torn apart by the shear
What does it mean if the BRN values are between 10 and 35?
The balance between shear and buoyancy tends to favor supercells
What does it mean if the BRN values are greater than 50?
Buoyancy dominates over shear and single- or mult-cell storms are more likely to be observed
How can someone calculate the BRN?
BRN = CAPE / (0.5 * (u6km - u500m)2)
What does Horizontal Vorticity often result from?
Vertical wind shear
A wind profile that maintains a single direction and increases its speed with height generates what?
A shear vector parallel to the wind direction
A wind profile whose speed remains constant but whose direction changes with height generates what?
A sheer vector perpendicular to the mean wind
What is important for someone to note regarding vorticity?
In the real world, vorticity is rarely perfectly crosswise or streamwise
What is the importance of atmospheric stability?
Atmospheric stability provides a crucial control on the ability of air to rise or sink and on the resulting weather
Nearly all techniques routinely used to analyze the stability of the atmosphere employ what?
The parcel method
What is happening when talking about the parcel method?
Stability is determined by lifting or lowering a hypothetical air parcel and comparing its resulting characteristics to those of the surrounding environment. Here, the surrounding environment and associated environmental lapse rate are defined by measurements from a sounding device, which are then plotted on a thermodynamic diagram
What two things does someone assume when using the parcel method?
We assume ascending or descending parcels of air experience temperature and moisture changes associated with two primary processes:
- Pressure change
- The release or uptake of latent heat due to condensation or evaporation
When talking about the parcel method, the temperature of a parcel changes adiabatically as it is displaced a small distance vertically from what?
Its original position
When talking about the parcel method, what is happening regarding saturated conditions?
An ascending parcel experiences latent heat of condensation and will cool at the saturated adiabatic rate, while a descending parcel warms and immediately becomes unsaturated, warming at the dry adiabatic lapse rate
The parcel method defines what three basic states of atmospheric stability?
- Stable
- Neutral
- Unstable
When a lifted parcel has a lower virtual temperature than the surrounding environment, what is occurring?
The parcel will be denser than the environment
A sinking parcel will have a higher what?
Will have a higher virtual temperature than its surroundings, which will cause it to rise toward its original position
What is the normal condition for the atmosphere?
That it is stable, except where certain processes such as strong surface heating, upward motion, render an unstable condition
When a lifted parcel has the same virtual temperature as the surrounding environment, what is occurring?
Its density will be the same as the surrounding environment
If a lifted parcel has a higher virtual temperature than the surrounding environment, what is occurring?
Its density will be less than the environment
A sinking parcel will have what?
A lower virtual temperature than its surroundings and it will accelerate downward away from its original position
What is a key assumption of the parcel method?
That displacements are small, confined to levels adjacent to the parcel level
How can someone be able to accurately determine the stability of the real troposphere?
The entire profile from ground to the tropopause needs to be examined
What happens when a parcel is lifted to a point above an inversion?
It will become unstable and will accelerate upward
If a dry air parcel is displaced upward in a stable atmosphere, what happens to the parcel when it is released?
A) It returns to its original position
B) It remains where it is
C) It accelerates upward
A) It returns to its original position
If a dry air parcel is displaced upward in an unstable atmosphere, what is the virtual temperature of the parcel relative to the ambient virtual temperature?
A) Tv (parcel) > Tv (ambient)
B) Tv (parcel) = Tv (ambient)
C) Tv (parcel) < Tv (ambient)
A) Tv (parcel) > Tv (ambient)
If a dry air parcel is displaced upward in a dry, neutral atmosphere, what is the potential temperature (theta) of the parcel relative to the ambient potential temperature?
A) Theta (parcel) > Theta (ambient)
B) Theta (parcel) = Theta (ambient)
C) Theta (parcel) < Theta (ambient)
B) Theta (parcel) = Theta (ambient)
What is important for someone to note regarding absolute instability?
Absolute instability is a relatively rare condition and is usually confined to a shallow layer near the surface under conditions of strong surface heating
What does the term “Conditional” mean?
The lifted parcel is stable if unsaturated and unstable (above the LFC) if saturated
Not all conditionally unstable atmospheres lead to what?
Unsettled weather
Since moisture is not accounted for in assessing conditional instability, some measure of the moisture profile is what?
Needed to refine the classification of stability
When a conditionally unstable atmosphere is unsaturated, what needs to be evaluated to determine the degree of instability?
CAPE
If the conditionally unstable layer is surmounted by a stable layer such that there is no LCL and the CAPE is zero, no vertical displacement of parcels however large, will produce what?
Any positive buoyancy
If the temperature curve of a sounding is parallel to a saturation adiabat, then what is occurring?
Then a saturated parcel displaced upward would be neither aided nor hindered by the surrounding atmosphere and would tend to remain at the level to which it is displaced
If the Temperature curve of the sounding parallels a dry adiabat, then what is occurring?
An unsaturated parcel displaced upward would tend to remain at the level to which it is displaced
All downward-displaced parcels will be in neutral equilibrium if what?
If the sounding exhibits a dry adiabatic lapse rate
In addition to tracing the path of air parcels, how can the stability also be determined by?
Comparing the environmental lapse rate to the dry and saturated adiabatic lapse rates
When does absolute stability occur?
When the environmental lapse rate is less than the moist adiabatic lapse rate
When does absolute instability occur?
When the environmental lapse rate is greater than the dry adiabatic lapse rate
When does neutral equilibrium occur?
When the environmental lapse rate equals the dry adiabatic lapse rate in an unsaturated environment or when the environmental lapse rate equals the saturated adiabatic lapse rate in a saturated environment
If the environmental lapse rate falls between the dry and moist adiabatic lapse rates, then the environment is said to be what?
Conditionally unstable
If CAPE equals zero, then the parcel is said to be what?
Absolutely stable
If CAPE is greater than zero, then the parcel becomes unstable when lifted beyond what point?
The level of free convection (LFC)
When does an inversion occur?
When the temperature increases with increasing height
Because any air parcel that is displaced upward will cool according to the appropriate adiabatic lapse rate, inversions are what?
A special case of a stable lapse rate
Inversions form in response to several processes including but not limited to what?
- Radiative surface cooling
- Subsidence aloft
- Frontal passage
Several processes in the atmosphere act to do what?
Modify lapse rates
What are the five basic kinds of physical processes that can change the stability at a point or in a given local vertical layer?
- Diabatic heating and cooling
- Advection
- Vertical motion of a layer
- Penetrative convection
- Moistening
Diabatic heating and cooling effects are generally important only near what?
The ground surface and within some clouds
Radiative heating and cooling lead to the formation of what?
Daytime low-level instability and nocturnal low-level stability
Radiative processes in the free air and at the cloud tops are what?
Slow and their effect on the lapse rate are generally minimal
The release of the latent heat of condensation has important local effects, potentially leading to what?
Deep convection
Evaporation and melting effects have a significant impact on lapse rates locally in instances of what?
Heavy precipitation
What causes the surface temperature to rise?
As the ground absorbs solar radiation, this process causes the surface temperature to rise
If the initial lapse rate is stable, then the rise of the surface-heated parcels is what?
Delayed
When will heated parcel “bubbles” rise?
Either when some of them become sufficiently warm to spontaneously rise or when some are impelled upward by mechanical turbulence
Through penetrative convection, heated parcel bubbles warm what?
The lowest atmosphere
With time, warm air rises to higher and higher altitudes in the stable layers above, thereby slowly extending a dry-adiabatic lapse rate from what to what?
The surface to greater altitudes
Surface heating creates instability indirectly through what?
The immediate mechanism of convective mixing. This process is limited by the amount of heat absorbed by the ground and conducted to the air
The inversion and the convective inhibition are wiped out or reduced by surface heating as what happens?
As the surface temperature rises
Pure nocturnal radiative cooling in calm air results in what?
A shallow surface-based inversion. The depth of the inversion increases with greater duration of the cooling while the strength of the inversion increases with the degree of cooling
What does wind complicate?
The effects of cooling the ground on the low-level lapse rate
The turbulent mixing of the air cooled at the ground with warmer air above tends to establish what?
A surface layer with an adiabatic lapse rate capped by a turbulence inversion
Intermediate conditions and combinations between the ground inversion and the turbulence inversion can occur, depending on what?
The relative degrees of wind and cooling
Evaporative cooling of precipitation can produce significant what?
Cooling in an unsaturated boundary layer, resulting in a more stable environment
What happens if sufficient surface cooling takes place?
Saturation results, leading to fog or stratus formation
The latent heat released in fog or stratus is usually too small to do what?
Greatly increase the mixing depth
The formation of fog or stratus greatly reduces or stops what?
The further radiative cooling of the ground
Radiative cooling at the top of the fog or stratus tends to maintain what?
The inversion near the top of fog or cloud
Does the following process make conditions more stable or less stable? Nocturnal radiative cooling at the earth’s surface:
A) More stable
B) Less stable
More stable. Radiative cooling at the earth’s surface decreases the lapse rate and frequently results in an inversion
Does the following process make conditions more stable or less stable? Cold air flowing over a warm lake surface:
A) More stable
B) Less stable
Less stable. Flow over a lake tends to warm and moisten the air at low levels. Both processes act to destabilize the atmosphere
Does the following process make conditions more stable or less stable? Warm air flowing over a snow surface:
A) More stable
B) Less stable
More stable. Since the temperature of snow cannot rise above 0°C, air, the warm air cools in contact with snow. Such cooling at low levels stabilizes the atmosphere
Does the following process make conditions more stable or less stable? Solar heating of the earth’s surface:
A) More stable
B) Less stable
Less stable. Solar heating of the surface conductively warms the air immediately above it. This process tends to steepen lapse rates and can result in superadiabatic lapse rates
Advection, both at the surface and aloft, has a strong influence on what?
The lapse rate through a given region of the atmosphere
The advection effects may be visualized as what two processes?
- Uniform advection of an air mass with a different lapse rate
- Differential advection of temperature due to vertical wind shear
By examining the temperature advection at multiple levels, we can assess the effects of differential advection on what?
The local lapse rate
Fronts mark a boundary between what?
Two air masses, which sometimes may be seen in soundings, most often as a frontal inversion
Observational studies of cyclones and frontal zones have determined that there are two primary types of what?
Cold frontal zones
The vertical motions that affect the lapse rate may be what?
Of any scale or type, from small-scale turbulence to the large-scale mean vertical-motion field associated with synoptic features
In the absence of divergence or convergence, the stability of a dry, ascending layer does what?
Decreases
In the absence of divergence or convergence, the stability of a descending layer does what?
Increases
If a saturated stable layer ascends, what will happen?
It will become less stable with the lapse rate tending toward moist adiabatic (saturation)
If an ascending layer is saturated at the bottom and unsaturated at the top, the bottom and top cool at what?
The moist adiabatic and dry-adiabatic lapse rate. The layer will be destabilized
If an ascending layer is unsaturated at the bottom and saturated at the top, the layer becomes more what?
Stable
Convergence will act to do what?
Destabilize a dry, surface-based layer
Because of the constraining boundary at the earth’s surface, divergence must always accompany what?
Descent
Because of the constraining boundary at the earth’s surface, convergence must always accompany what?
Ascent
What is a subsidence inversion produced by?
The adiabatic warming of air as it sinks
If the initial pre-subsidence lapse rate is substantially less then dry adiabatic, what will happen?
An inversion quickly forms at the base of the subsident layer
Subsidence inversions are usually associated with what?
Anticyclones and or stable air masses
Unlike the vertical motion of a layer in the atmosphere, penetrative convection consists of what?
Local vertical currents having cross-sections of the order of a few meters to a few kilometers across
At some height, what will a stable layer do?
Cap the conditionally unstable layer
The speed with which the penetrative convection changes the lapse rate varies what?
Greatly
What are some of the factors that affect penetrative convection?
- The duration of the convection
- The resistance (stability) of the layers affected
- The size spectrum and pattern of convective cells
Widespread continuous penetrative convection can render whole layers what?
Completely unstable. This frequently happens in layers near strongly heated ground or in layers lifted by a front
Increasing the surface dewpoint can increase what?
The instability
