Vertical motion and cloud formation
The Adiabatic Process
- The adiabatic process defined:
- When a parcel of air expands and cools or compresses and warms with no interchange of heat from its surroundings
- When a parcel of air is moved aloft, pressure decreases, this allows the parcel to expand
- The expansion of the parcel means the molecules travel farther within this parcel, energy is used by the molecules and their speed decreases
- We already know that temperature and molecule speed are directly related, so with a relative speed decrease comes a decrease in temp
- It is by this process clouds form when the temp cools to the dew point
- If the air descends, the process is reversed like in the Chinook wind
The Adiabatic Process
- As long as the parcel does not lose any of its moisture, the process is reversible
- Known as the Reversible Adiabatic Process
- However if rain or snow falls from the cloud and leaves the parcel, when the parcel sinks it cannot recover this loss
- Net evaporation is less than net condensation in this circumstance
- This is known as the Irreversible Pseudoadiabtic Process
- As long as the parcel of air is unsaturated the rate of adiabatic cooling or warming is constant
- Unsaturated means less than 100% relative humidity
- This is referred to as the dry adiabatic lapse rate
- 3°C /1000 feet or 5.5°F/1000 feet
- The average lapse rate is
- 2°C/1000 feet
- The moist adiabatic lapse rate
- 1.2°C/1000 for warm parcels, lots of latent heat released
- to 3°C/1000 for colder ones, not so much latent heat
- Superadiabatic lapse rate
- Greater than 3°C/1000 but less than 8°C/1000
- Autoconvective lapse rate
- Greater than 8°C/1000
- Dew point also changes with a change in altitude but at a much lower rate
- .5°C/1000 feet
- The graph shows an actual sounding measuring the temperature and dew point
Lapse Rate and Moisture
- Temp and dew point converge at a rate of about 4.4º F or 2. 2º C per 1000 feet
- Given: Temp 14º C Dew Point 3º C
- T-Dp/Convergence Rate
- 11/2.2= 5000
Unsaturated Air Example
Saturated Air Example
Descending Air Example (Irreversible Pseudoadiabatic)
- The book lists 4 types of lifting:
- Orographic effects
- Frictional effects
- Frontal lift
- Orographic lifting occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain.
- As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and under the right conditions, precipitation.
- Note how the temp is cooler on the windward side than the leeward side
- Any time terrain causes a lifting of the air there is usually a rain shadow created on the leeward side
- The most extreme example of this is the Atacama Desert in Chile
- It is in fact rain shadowed from the Andes to the east as well as the Chilean Coast range to the west
- Some weather stations have never recorded rainfall
- Frictional effects refers to the process of descending air associated with a high pressure and ascending air associated with a low pressure
- This is also known as Frontal Wedging
- This may take place along both a cold front or a warm front
- The worst weather is often associated with a fast moving cold front because of this phenomenon
- Usually air becomes buoyant when cooler air moves over warmer ground
- We covered examples of this when we talked about land and sea breezes, mountain and valley winds and lake effect
- Thermals may drive this process as well and results in fair weather cumulus cloud formation as well as Air Mass Thunderstorms
- Atmospheric stability is the property of the ambient air that either enhances or suppresses vertical motion of air parcels
- This concept is very similar to aircraft stability
Adiabatic Change (Review)
- Heat is neither added or taken from an outside source
- Heating and cooling is caused by compression and expansion
- Adiabatic cooling – rising air cools because of expansion
- Adiabatic heating – descending air warms because of compression
Stable vs Unstable
- A stable atmosphere resists any upward or downward displacement
- An unstable atmosphere allows upward and downward movement
- Environmental lapse rate is the lapse rate of the surrounding air
- The environmental lapse rate compared to the lapse rate of the air that is rising tells us if the atmosphere is stable or unstable
- Whether the air is saturated or unsaturated dictates the lapse rate
- Has a lower adiabatic lapse rate
- Lift a parcel of air
- The parcel cools to the dew point
- Moisture condenses
- Latent heat from vapor to water releases heat and lowers the lapse rate
- This causes the parcel to become buoyant and further drives the process
- Has a higher adiabatic lapse rate
- Parcel of air is lifted mechanically
- The parcel will cool adiabatically because of expansion but at a faster rate because of the absence of latent heat
- This parcel of air will be colder than the surrounding air and settle downward resisting vertical movement
STABILITY AND INSTABILITY
- If the parcel is warmer than the ambient air it will continue to rise – thus unstable
- if the parcel is the same temp as the ambient air it is neutrally stable
- if the parcel is cooler than the ambient air it will sink – thus stable
- An inversion will produce a condition known as absolute stability
- This happens when the environmental lapse rate is less than the moist adiabatic lapse rate
- The book lists 5 types of atmospheric stability but there are only 4:
- Absolute Stability
- Neutral Stability
- Absolute Instability
- Conditional Instability
Stability And Instability
- Orographic lifting – is an air mass being forced up a mountain slope
- Cooling from below increases stability
- Heating from below decreases stability
- Thermals are a sign of instability
- Inversions are an example of stability
Clouds Signposts Of The Sky
- Characteristics of stable and unstable air
- Stable air
- Stratiform clouds
- Poor vis
- Steady precipitation
- Smooth ride
- Remember if air is lifted and there is no Latent heat it cools at the same rate or faster and will be stable
- Unstable air
- Cumuliform clouds
- Good vis
- Showery precipitation
- If air is lifted and there is Latent heat released through condensation, the lapse rate will be less and therefor warmer than surrounding air
Clouds Signposts Of The Sky
- Remember as air is lifted because of Latent heat it cools slower
- this causes further instability
- this continues until:
- The temp matches or gets cooler than the ambient temp or
- It hits a cap of stable air
- Sometimes it will be strong enough to push into the stable air causing embedded T storms
Process That Effect Stability
- Wind effects
- Stability increases when
- Cold air advection into the bottom of a column
- Warm air advection into the top of a column
- Stability decreases when
- Warm air advection into the bottom of a column
- Cold air advection into the top of a column
Process That Effect Stability
- Vertical air motion
- When a column of air descends it shrinks and warms due to adiabatic compression
- The upper part shrinks more than the lower part and becomes warmer
- The lower part can’t shrink more because it’s on the surface
- This can produce an isothermal lapse rate or an inversion and is very stable
- This process is known as Subsidence and can effect large chunks of air
- The process is reversed if the air ascends
Process That Effect Stability
Process That Effect Stability
- Diurnal temperature variation
- Diurnal refers to the difference between day time and night time temps
- Mainly influenced by
- Surface type
- Sky cover
- Water vapor content of the air
- Wind speed
- Temperature variation is maximized
- Over land
- At low latitudes
- With a clear sky
- Dry air
- Light wind
- Conversely, temperature variation is minimized
- Over large bodies of water
- At high latitudes
- With a cloudy sky
- Moist air
- Strong wind
- Lifted Index is a common measure of atmospheric instability.
- Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level.
- Negative values indicate instability – the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms.
- However there are no “magic numbers” or threshold LI values below which severe weather becomes imminent.
- Convective Available Potential Energy is a measure of the amount of energy available for convection.
- CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather.
- Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.
- However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent.
- CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)
Measurements of Stability
- Remember these measurements all determine stability based on the lapse rate of the air being lifted to the lapse rate of the surrounding air
- The Skew-T shows
- Drawn at a 45° angle (hence the name)
- These are horizontal in millibars
- Dry Adiabats
- Red curved lines starting at the bottom
- Represent the rate unsaturated air cools
- This is the dry adiabatic lapse rate
- Moist Adiabats
- Green curved lines starting at the bottom
- Represent the rate saturated air cools
- This is the moist adiabatic lapse rate
- Mixing Ratio
- Mass of water vapor compared to mass of dry air
- Dew point
- Wind speed/direction
Lifting Condensation Level
- The height at which a parcel of air becomes saturated when lifted dry adiabatically.
- To obtain the LCL, start with the surface temperature and follow up the dry adiabat until it cross the saturation mixing ratio of the surface dew point.
Convection Condensation Level
- The height at which a parcel of air, if heated from below, will rise adiabatically until it is saturated (humidity equals 100%).
- This is the level of the flat bases of cumulus clouds.
- To obtain the CCL, start with the surface dew point and follow the saturation mixing ration line up until it crosses the temperature curve.
- The surface temperature at which convective clouds will begin to form from heating of the ground.
- To obtain, begin at the convective condensation level (CCL) and follow the dry adiabat down to the surface level and read the temperature at that point.
Level of Free Convection
- The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely.
- To obtain, begin at the Lifting Condensation Level (LCL) and follow the moist adiabat up to where it intersects the temperature line.
- There are times when no LFC will occur as parcel will always remain cooler than the surrounding atmosphere and therefore will never rise freely.
- When this occurs, the atmosphere is absolutely stable.
Usefulness of the Skew-T
- The atmosphere is very moist as indicated by the small amount of separation between the air temperature (red line) and the dew point (blue line). Even though the air temperature increases a few hundred feet above the ground (a temperature inversion) the air temperature, throughout the entire atmosphere, remains below freezing.
- So, when precipitation begins, it will be in the form of snow and will remain frozen as snowflakes reaching the ground.
- As with the previous sounding, the atmosphere is very moist. So much so, the air temperature and dew point are the same from near 900 millibars (3,000 ft. / 1,000 m) to a little above 700 millibars (10,000 ft. / 3,000 m).
- At the surface, an arctic cold front had moved south of the observation station with an air temperature well below freezing. The air temperature begins to decrease with height (which is normal) dropping from 23°F to 12°F (-5°C to -11°C).
- However, the density of the arctic air is such that it lays close to the ground with its vertical extent fairly small, in this case only about 3,000 feet (1,000 meters) deep. Above 900 millibars (3,000 ft. / 1,000 m) the air becomes considerably warmer. This area is called an inversion, where temperature change with height is ‘inverted’ as it increases with height instead of typically decreasing with height. This inversion is often also referred as a ‘warm nose’.
- Eventually, the temperature of the atmosphere will return to the typical decrease with height (near 800 mb) and will continue to cool until it falls to below freezing again (about 720 mb).
- While there may be some precipitation forming as rain in the warm ‘nose’ region where the air temperature is above freezing, the vast majority of precipitation will form as snow in the colder below freezing air above the inversion.
- As snow falls into the ‘warm nose’, it melts into a liquid drop/rain. Then the liquid drops fall back into the arctic air mass (near the ground) that is cold enough and deep enough for the liquid to freeze into ice pellets before reaching the ground.
- The basic pattern for freezing rain is similar to ice pellets. The main difference is the cold, dense air near the surface is very shallow and/or the ‘warm nose’ is large or very warm or both. The melted snow does not have sufficient time to freeze before is reaches the ground.
- Therefore, precipitation falls as rain but then it freezes upon contact with a surface such as a tree, power line, automobile or bridge. As a general rule, elevated surfaces will ice first because the ground cannot keep them warm, allowing them to cool to the air temperature quickly. This allows the rain to freeze to ice on contact with these surfaces.
- Elevated surfaces may be capable of accumulating ice as soon as the air temperature falls below 32°F (0°C). Road surfaces in contact with the ground will generally begin to ice when the air temperature falls to 28°F (-2°C).
- Inside hurricanes, the velocity of the air helps keep the air mixed. Therefore, other than the normal decrease with height, variations in temperature (and dew point) are fairly minimal.