Stability and cloud development Chapter 6
What do we mean by conditionally unstable atmosphere?
(Fig. 6.9 Prevails when environmental lapse rate is between the moist adiabatic rate and the dry adiabatic rate In the environment, most days experience this state (blue skies, stability prevails) Afternoon (warmer) Unsaturated parcel of air at each elevation is colder then warmer after it crosses over Start with environmental rate warmer (unsaturated air rising--> point where now saturated (cloud forms)
What do we mean by unstable atmosphere (absolute instability?
(fig. 6.8) Prevails when environment al lapse rate is GREATER THAN (cools faster) dry adiabatic rate Rising air is always warmer than the air surrounding it Condition that can develop severe weather/ thunderstorms/ tornadoes because it is UNSTABLE Clouds will form (towering type) and thunder (cumulonimbus) Violent weather Rising, unsaturated air parcel at each level is warmer and lighter than the air around it Temperature of environment is cooler temp of dry and moIst/wet adiabatic rate AIR RISING WILL ALWAYS BE WARMER Surface is warmer (parcels not able to rise on their own)
**What is Dew Point Lapse Rate?
1 degree/1000 ft Rate of change of dew point temperature of air parcel with height
What is Environmental Lapse Rate?
3.5 degrees/1000ft The rate of temperature decreases with height in the troposphere
What rate does air cool for Environmental lapse rate?
3.5 degrees/1000ft The rate of temperature decreases with height in the troposphere
What constant rate does unsaturated air rise (cool)/sink (warm)? DAR
5.5 degrees Fahrenheit/ 1000ft
How does adiabatic temperature changes occur?
Adiabatic temperature change comes about when a parcel of air is allowed to expand or be compressed LAPSE RATE TO EVERY 1000M
TB: Moist adiabatic process?
As rising air cools, relative humidity increases as the air temperature approaches dew point temperature. If rising air cools to DPT, RH is 100% Further lifting results in condensation, a cloud forms and latent heat is released inside rising air parcel Because heat added during condensation offsets some of cooling due to expansion, air no cools at DAR but instead at lesser rate or MAR If a saturated parcel containing water droplets were to sink, it would compress and warm the moist adiabatic rate because evaporation of the liquid droplets would offset the rate of compressional warming. Hence, the rate at which rising or sinking saturated air changes temperature- moist adiabatic rate- is less than DAR
TB: Earth's surface?
At the Earth's surface, the parcel has the same temperature and pressure as the air surrounding it The higher the pressure as it is lowers squeezes (compresses) the parcel back into its original (smaller) volume. This squeezing increases the average speed of the air molecules and the parcel temperature rises The lower the pressure it rises pushes the parcel walls outward, expanding the parcel= lower temperature
What conditions bring about stable atmosphere?
Atmosphere is stable when ELR is small (when difference in temperature between surface air and air aloft is small) Atmosphere tends to be more stable (stabilizes) as as air aloft warms or the surface air cools If air aloft is replaced by warmer air (warm advection) and the surface air is not changing appreciably, ELR decreases and atmosphere becomes more stable ELR decreases and atmosphere becomes more stables when lower layer cools The cooling of the surface air may be due to: 1. nighttime radiational cooling of the surface 2. an influx of cold surface air brought in by the wind (cold advection) 3. Air moving over a cold surface On a typical day, atmosphere usually most stable in early morning around sunrise, when lowest air temp is recorded If surface air becomes saturated in stable atmosphere, layer of haze or fog may form Also is when an entire layer of air sinks
What constant rate does saturated air rise (cool) on average? MAR
Average rate is 3.3 degrees/1000 ft
TB: Atmospheric stability?
Condition of equilibrium Stable- After being lifted or lowered, it tends to return to its original position- it resists upward and downward motion Unstable- given a little push, it will move farther away from original position
In our environment, which kind of atmosphere (stability wise) do we experience most days?
Conditionally unstable
How does Dry Adiabatic Rate differ from Moist Adiabatic Rate?
DAR- rate of adiabatic cooling and warming of unsaturated air MAR- rate of cooling of saturated air
** Adiabatic process continued
Ex. Outside temp 60 degrees--> smaller air parcel is 80 degrees and air needs to rise because it has a lot of contents As it moves up the heat energy spreads out (1000m from surface) and cools to 70 degrees--> 2000m= 60 degrees The air molecules and water vapor PLUS the heat energy (little dots inside circles) means that if the air parcel is a higher temperature than the surrounding air it will need to rise WARMER AIR WILL ALWAYS RISE As it rises it expands (but has the same amount of water vapor and heat energy)
TB: Adiabatic process?
If a parcel of air expands and cools, or compresses and warms, with no interchange of heat with its surroundings As long as air in parcel is UNSATURATED (less than 100% RH), the rate remains constant Rate of heating or cooling= 10C for every 1000m or 5.5F per 1000 ft
What does stable air differ from unstable air?
If a rising parcel of air is cooler than the surrounding atmosphere it will tend to sink back to its original position. This is because cool air is more dense or heavier than warmer air. This is referred to as stable air. If a rising parcel of air is warmer than the surrounding atmosphere it will continue to rise. This is because warm air is less dense or lighter than cool air. This is referred to as unstable air. TB: Stable air- if the rising air is colder than the environment, it will be more dense and tend to sink down to its original level (STABLE BECAUSE RESISTS UPWARD MOVEMENT) Unstable air- if rising air is warmer and therefore less dense (lighter) than the surrounding air, it will will continue to rise until it reaches the same temperature as its environment
How does atmospheric stability affect the atmosphere?
It affects cloud development and rainfall patterns
**What happens as the air parcel moves up away from surface?
It expands and gets cooler (same amount of water vapor and heat energy) Expands because atmospheric pressure decreases with height
**What happens as the air parcel sinks down towards surface?
It gets warmer and condenses/ compresses (same amount of water vapor and heat energy) Compression causes sinking air parcel to warm up
How is stability of the atmosphere determined?
It is determined by comparing curves of DALR and MALR with that of ELR Meteorologists say conditions are stable/unstable
** What is true about warmer air?
It will always need to rise
** If/when it sinks, what temperature will it be? What will happen?
It will be 80 degrees; energy will not be lost but instead maintained The contents (water vapor and heat energy) cannot let any energy leave
What is the lapse rate?
Lapse rate to every 1000m
** Does air rise on its own?
No- WIND (will always resist any vertical momentum) Wind is sometimes strong enough/STOPS
As it sinks/warms up, will anything leave or enter air parcel?
No- don't expect outside air to enter it. It doesn't allow things to go through Heat is NOT added to air parcel as it warms It is heating adiabatically (Relative Humidity rises until 100% inside air parcel--> CLOUD)--> condensation (water vapor to tiny droplets of water)
Is heat energy lost as air parcel cools? How is it cooling?
No- it is cooling adiabatically (until it reaches Dew Point Temperature and 100% Relative Humidity)--> CLOUD
TB: MAR?
Not constant but varies greatly with temperature and with moisture content Warm saturated air produces more liquid than cold saturated air MAR is less than DAR when rising air is warm Two rates nearly same when rising air is cold 3.3F/1000ft
What is adiabatic process?
Process in which air parcel "cools"/"warms" without heat energy added to it nor subtracted Rising air parcel expands Expands because atmospheric pressure decreases with height When it expands, it cools Heat energy is not lost as air parcel cools It is cooling adiabatically (until it reaches Dew Point Temperature and 100% Relative Humidity)--> CLOUD When the air parcel sinks to the surface, it compresses. Compression causes sinking air parcel to warm up (but don't expect the outside air to enter it because it doesn't allow anything to go through it) Heat is not added to air parcel as it warms It is warming adiabatically Relative Humidity until 100% inside air parcel--> CLOUD CONDENSATION (water vapor to tiny droplets of water)
TB:What is a parcel of air?
Small volume of air- can expand and contract freely, it does not break apart but remains as a single unit Neither external air not heat can mix with the air inside the parcel
What are the 3 conditions of the atmosphere?
Stable Atmosphere (Absolute Stability) Unstable Atmosphere (Absolute Instability) Conditionally Unstable Atmosphere (Conditional Instability)
What conditions bring about unstable atmosphere?
The atmosphere becomes more unstable as the environmental lapse rate steepens; as air temp drops rapidly with increasing height Air aloft becoming colder or surface air becoming warmer The combination of cold air aloft and warm surface air can produce a steep lapse rate and atmospheric instability
**Why does the air cool as it rises?
The heat energy is spreading/cooling on its own
What is dry adiabatic rate?
The rate of adiabatic cooling and warming of UNSATURATED air RISING unsaturated air COOLS at a constant rate of 5.5 degrees/1000 ft SINKING unsaturated air WARMS at a constant rate of 5.5 degrees/1000 ft
What is moist adiabatic rate?
The rate of cooling of SATURATED air Rate of cooling of saturated air VARIES: average rate is 3.3 degrees/1000ft It is always changing/not fixed
TB: Dry adiabatic process?
This rate applies only to unsaturated air
What kind of atmosphere (stability wise) has the condition that can develop severe and violent weather?
Unstable atmosphere (or absolute instability)
What do we mean by stable atmosphere (absolute stability)?
Up and up clouds spreading out (FIG. 6.3) p 148 Prevails when Environmental lapse rate is LESS THAN (warms faster) moist adiabatic rate Rising air parcel is always colder than the air surrounding it Lifted unsaturated air at each level is colder and heavier than air around; if given chance, parcel would return to original position Temperature of environment warmer than temperature that is rising Warmer than dry adiabatic lapse rate AIR RISING WILL ALWAYS BE COOLER Unsaturated until it reaches MALR then saturated (still cooler)
Is the moist adiabatic rate fixed or always changing?
Varies ALWAYS CHANGING
You should be able to calculate adiabatic cooling and warming of a parcel of air.
When calculating, once the DPLR is equivilant to the DALR, is it at the LCL level and the DPLR stops rising and the RH level is at 100% Base of cloud starts to be visible and then you change to MALR Once the ELR and DALR/MALR is equivalent, it stops rising and it has reached its equilibrium level.
**Does the air parcel cool and warm on its own?
Yes, it is the ADIABATIC PROCESS
TB: Stable environment and horizontal movement?
the rising air is colder and heavier than the air surrounding it Since air in this state strongly resists upward vertical motion, it will, if forced to rise, tend to spread out horizontally If clouds form in this rising air, they, too will spread horizontally in relatively thin layers and usually have flat tops and bases Ex. might expect to see stratiform clouds
