Convective instability

Convective instability
This article incorporates public domain text created by the US government.

In meteorology, convective instability or stability of an airmass refers to its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an unstable atmosphere, vertical air movements (such as in orographic lifting, where an airmass is displaced upwards as it is blown by wind up the rising slope of a mountain range) tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather such as thunderstorms.[1]

Adiabatic cooling and heating are phenomena of rising or descending air. Rising air expands and cools due to the decrease in air pressure as altitude increases. The opposite is true of descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed. Adiabatic heating and adiabatic cooling are terms used to describe this temperature change.

The adiabatic lapse rate is the rate at which a rising or falling airmass lowers or increases per distance of vertical displacement. The ambient lapse rate is the temperature change in the (non-displaced) air per vertical distance. Instability results from difference between the adiabatic lapse rate of an airmass and the ambient lapse rate in the atmosphere.

If the adiabatic lapse rate is lower than the ambient lapse rate, an airmass displaced upward cools less rapidly than the air in which it is moving. Hence, such an airmass becomes warmer relative to the atmosphere. As warmer air is less dense, such an airmass would tend to continue to rise.

Conversely, if the adiabatic lapse rate is higher than the ambient lapse rate, an airmass displaced upward cools more rapidly than the air in which it is moving. Hence, such an airmass becomes cooler relative to the atmosphere. As cooler air is more dense, the rise of such an airmass would tend to be resisted.

Moist air cools when rising at a lower rate (given the same vertical movement) than dry air, and hence has a relatively low adiabatic lapse rate. Thus, moist air is generally less stable than dry air. The dry adiabatic lapse rate (for unsaturated air) is 3 °C (5.4 °F) per 1,000 vertical feet. The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to 5 °F) per 1,000 vertical feet.

The combination of moisture and temperature determine the stability of the air and the resulting weather. Cool, dry air is very stable and resists vertical movement, which leads to good and generally clear weather. The greatest instability occurs when the air is moist and warm, as it is in the tropical regions in the summer. Typically, thunderstorms appear on a daily basis in these regions due to the instability of the surrounding air.

The ambient lapse rate differs in different meteorological conditions, but, on average, is 2 °C (3.5 °F) per 1,000 vertical feet.

References

See also

External links


Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • convective instability — The state of the atmosphere in which there is a high relative humidity at low levels and a low relative humidity at upper levels, such that if a low layer of air is lifted bodily, the ELR (environmental lapse rate) value through the layer will… …   Aviation dictionary

  • Convective available potential energy — A skew T plot showing a morning sounding with a large hydrolapse followed by an afternoon sounding showing the cooling which occurred in the mid levels resulting in an unstable atmosphere as surface parcels have now become negatively buoyant. The …   Wikipedia

  • Instability — [ Hydrodynamics simulation of the Rayleigh–Taylor instability [Shengtai Li, Hui Li Parallel AMR Code for Compressible MHD or HD Equations (Los Alamos National Laboratory) [http://math.lanl.gov/Research/Highlights/amrmhd.shtml] ] ] Instability in… …   Wikipedia

  • Convective inhibition — A Skew T diagram with important features labeled Convective inhibition (CIN or CINH) is a numerical measure in meteorology that indicates the amount of energy that will prevent an air parcel from rising from the surface to the level of free… …   Wikipedia

  • Convective condensation level — The convective condensation level (CCL) represents the height where an air parcel becomes saturated when lifted adiabatically to achieve buoyant ascent. It marks where cloud base begins when air is heated from below to the convective temperature …   Wikipedia

  • Convective temperature — The convective temperature (CT or Tc) is the approximate temperature that air near the surface must reach for cloud formation without mechanical lift. In such case, cloud base begins at the convective condensation level (CCL), whilst with… …   Wikipedia

  • Convective overturn — The convective overturn model of supernovae was proposed by Bethe and Wilson in 1985, and received a dramatic test with SN 1987A, and the detection of neutrinos from the explosion. The model is for type II supernovae, which take place in stars… …   Wikipedia

  • instability line — Meteorol. a nonfrontal line of convective activity in the atmosphere, usually several hundred miles long but of relatively brief duration. * * * …   Universalium

  • instability line — Meteorol. a nonfrontal line of convective activity in the atmosphere, usually several hundred miles long but of relatively brief duration …   Useful english dictionary

  • Horizontal convective rolls — producing cloud streets (lower left portion of the image) over the Bering Sea …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”