Download Instability Handout

Instability Handout
12/3/14 3:30 PM
Instability
Whole air columns full of air parcels can be forced to rise due to divergence aloft, for example,
associated with surface cyclones.
But there are situations in which single air parcels will spontaneously rise or sink relative to the
surrounding atmosphere which is not moving vertically, or moving at all. The general topic
heading for the set of processes that govern this spontaneous motion is "instability."
Stability is the state in which an air parcel finds itself colder than the air surrounding it at the same pressure (elevation). The air
parcel will spontaneously sink because air parcels that are colder than the surrounding air at the same elevation will be denser
(heavier).
Instability is the state in which an air parcel finds itself warmer than the air surrounding it at the same pressure (elevation). The air
parcel is buoyant. It will spontaneously rise, because air parcels that are warmer than the surrounding air at the same elevation will
be less dense (lighter).
Indirect Measure of Instability
Meteorologists often looks at maps of dew point temperature to infer, as a first guess, areas that might be prone for thunderstorm
development. Air with high dewpoint temperature often will arrive at, say, the 500 mb level, much warmer than the surrounding air. This is because
air with high dew point also has high water vapor content. If this water vapor condenses a large amount of "heat" is given back to the atmosphere.
This heat is called "latent heat".
In the lower atmosphere, saturated air that rises will be warmed at a rate of around 2 deg F for 1000 feet of ascent due to latent heat release, at the
same time that it will be cooled 5.5 deg F due to expansional cooling. Thus, although saturated air cools as it rises, this cooling rate averages out to
be around 3.5 deg F per 1000 feet instead of 5.5 deg F. Ascending saturated air can thus find itself about 2 deg F warmer than its surroundings once
it is lofted 1000 feet.
However, areas of high dew point temperature will not perfectly correspond to areas of instability since the degree to which a rising
air parcel is warmer than the air surrounding it will also be related to how cold or warm the air is aloft.
Direct Measure of Instability
The direct measure of instability is a parameter known as Convective Available Potential Energy (CAPE). CAPE is directly
related to how warm the air parcel is relative to its surroundings for all elevations of its ascent and maps of CAPE can be used to
estimate areas that should be expected to experience thunderstorms. In general, the greater the CAPE the more likely that
thunderstorms will be strong or severe since CAPE relates directly to the strength of the thunderstorm updraft, which in turn relates
to hail size.
The CAPE/CIN chart is either a depiction at current time OR a forecast map that shows regions in which air parcels are either
currently unstable or expected to be unstable. The green color means high potential for unstable air. Yellows and reds means there is
enormous potential for unstable air. Finding areas of instability can help to show high possibilities for thunderstorms.
Thunderstorms usually come from unstable air. The more deep primary colors, the hotter the temperature is than the air around it,
the more unstable the air. There is a correspondence of the unstable areas shown on this chart to the regions of high dewpoint
because dewpoint relates to the amount of water vapor present, which in turn relates to the amount of potential latent heat that can
be released in ascending air parcels making them unstable.
Here are some rules of thumb...
The atmosphere destabilizes (becomes more
unstable)
http://tornado.sfsu.edu/geosciences/classes/m302/Instability/InstabilityA.html
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Instability Handout
12/3/14 3:30 PM
1. the higher the surface dewpoint temperatures
(this is the largest effect)
2. the colder the temperatures are aloft
3. the warmer the temperatures are at the ground
The BOTTOM LINE (what I expect you to get out of this and to be able to apply):
Instability is the state in which an air parcel finds itself warmer than the air surrounding it at the same pressure (elevation).
The air parcel is buoyant. It will spontaneously rise. (If moisture is condensing, the resulting cloud will be cumulus, cumulus
congestus or cumulonimbus (associated with thunderstorms)).
Meteorologists (because of #1 in table just above) often looks at maps of dew point temperature to infer, as a first guess,
areas that might be prone for thunderstorm development. Usually, areas experiencing suface dew point temperatures of 60 deg
F or greater correspond to area of moderate to strong instability. (This is not always true, as explained above).
The direct measure of instability is a parameter known as Convective Available Potential Energy (CAPE). CAPE is directly
related to how warm the air parcel is relative to its surroundings for all elevations of its ascent and maps of CAPE can be used
to estimate areas that should be expected to experience thunderstorms.
http://tornado.sfsu.edu/geosciences/classes/m302/Instability/InstabilityA.html
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