Instability
 Air parcels can be forced to rise due to divergence aloft, for example. But there are situations in which air parcels will spontaneously rise or sink. The general topic heading for the set of processes that govern this spontaneous motion is "instability."

Handout on 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.

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).

More Detailed Treatment of Instability -- for those of you who want to learn more about it

Working Principles:

  • Warm air at a given pressure (elevation) is less dense than cold air at the same pressure (elevation);
  • Less dense air parcels at a given elevation will rise (analogy...a cork depressed a few feet in a swimming pool);

  • Since an air parcel that is less dense (warmer) than air surrounding it at the same pressure (elevation) will rise, then the development of horizontal temperature differences between air parcels and their surroundings at the same elevation can cause spontaneous (free) vertical motions (either up or down);
  • Since water vapor represents a source of latent heat, then surface air streams that have high dew point imply that air parcels lofted from such streams of air will become more warm than the air around them at the same elevation (see handout on wet adiabatic lapse rate, and/or the development of hurricanes along the coast of Africa);

Other Issues:

  • Even if an air parcel WILL become warmer than its surroundings if it is lofted, there must be a source of lift for single air parcels....it could be an isolated mountain surrounded by lower elevation terrain, it could be a shallow surge of cold air coming out of an existing thunderstorm...but whatever the actual mechanism, it must be so focussed so that only single air parcels (small volumes of the lower atmosphere) are effected

  • Sometimes air parcels that eventually WILL become warmer than their surroundings when the air parcels are lofted, are initially colder than their surroundings right near the ground...in fact, this is a common situation. Such air parcels will actually "resist" the forcible lofting, but will eventually become buoyant at some elevation above the ground.

Ways of Inferring and Measuring Instability

In general, the surface air parcels are prone to spontaneous upward motion (are unstable) in regions characterized by warm surface temperatures in combination with high dewpoint temperatures. For the conditions found in the upper air typically in North America, the atmosphere tends to instability if surface dewpoints exceed: (a) 60F in winter; (b) 60-70F in spring and fall; and, (c) >70F in summer.

The variation occurs because instability is also dependent upon upper air temperatures, which vary from winter to summer. At the level of Metr 302, there are two ways that students can use to either infer or directly assess the risk of thunderstorms (sometimes referred to as "the risk for convection": (a) examination of maps of surface dewpoint (to indirectly assess the instability); and, (b) examination of maps of CAPE (explained below--to directly assess the instability).

 Meteorologists, therefore, 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. However, this 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.

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.

There are other factors that influence how unstable the air mass is or is to become. For example, relatively low dewpoint temperature can be mitigated if the environmental temperatures are very cold aloft.

Here are some rules of thumb...

The atmosphere destabilizes (becomes more unstable)

  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):