University of Illinois

> online guides
  educational cd-rom
  current weather
  about ww2010

Online Guides
> meteorology
  remote sensing
  reading maps
  projects, activities

  air masses, fronts
  clouds, precipitation
  el nino
  forces, winds
  hydrologic cycle
  light, optics
  midlatitude cyclones
> severe storms
  weather forecasting

Severe Storms
  dangers of t-storms
  types of t-storms
> tstorm components

Tstorm Components
> updrafts/downdrafts
  wind shear
  outflow phenomena
  wall clouds

> introduction
  lemon techique (lt)
  multicell lt-analysis
  supercell lt-analysis
  supercell matures

User Interface
> text

NOTE: We've guessed that you're not using a client that supports colored tables and have tried to compensate. Low graphics mode looks much better on clients that do... we recommend switching to Netscape 3.0 or Microsoft Internet Explorer.
rising and sinking air

All thunderstorms require instability (potential) and lift. The lift is the mechanism that releases the instability. Lift is produced by such things as fronts and low pressure troughs, or by air rising upslope.

We say that the atmosphere is unstable when air rising in a cloud is warmer than its environment, like a hot-air balloon. It is the heat released by condensation within a cloud that permits the rising air to stay warmer than its surroundings, and thus to be buoyant through great depths.

In the same way, air that is cooler than its environment tends to sink as long as it can stay cooler than its surroundings. The upward moving air in a thunderstorm is known as the updraft, while downward moving air is the downdraft. The atmosphere can be unstable for updrafts but stable for downdrafts, stable for updrafts but unstable for downdrafts, stable for both, or unstable for both. The degree of atmospheric instability is one of the two major factors in determining the strengths of thunderstorm updrafts and downdrafts. Furthermore, vertical draft strengths basically determine the degree of storm severity. If we consider a "generic" storm, there are four possible combinations of weak and strong draft strengths.

[Image: draft strength combinations (46K)]

When the low-level air is unstable but relatively dry and adequate mid-level moisture is present, a storm may develop with a weak updraft but a strong downdraft with the latter the result of strong negative buoyancy and cooling through evaporation of precipitation into the dry air. This high-based storm resembles high terrain, western U.S. storms which occasionally produce dry microbursts. Significant hail and rain are unlikely.

A storm which contains a strong updraft and weak downdraft; will not produce wind damage, but can foster heavy rains and/or damaging hail. Single and multicell storms comprise this category. They include storms that dump heavy rain, but little or no hail because of warm conditions aloft, and multicell storms that are capable of producing hail because of lower environmental freezing levels. Strong updraft, weak downdraft storms often form in very moist atmospheres where there is little, if any, dry air and evaporational cooling to drive downdrafts.

[Image: more updraft/downdraft combinations (48K)]

Relatively weak updrafts and downdrafts are found with non-severe showers and thunderstorms. The last possible combination is a storm with strong updrafts and downdrafts. These storms frequently produce destructive downbursts, hail, heavy rain, and tornadoes. As one would expect, the most severe storms, including supercells, have strong vertical drafts and occur in the most unstable atmospheres.

Tstorm Components
Terms for using data resources. CD-ROM available.
Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.

lemon techique (LT)