kb/data/en.wikipedia.org/wiki/Conditional_symmetric_instability-1.md

title	chunk	source	category	tags	date_saved	instance
Conditional symmetric instability	2/2	https://en.wikipedia.org/wiki/Conditional_symmetric_instability	reference	science, encyclopedia	2026-05-05T13:39:05.397019+00:00	kb-cron

Slantwise displacement C Only case C is unstable. Horizontal acceleration combines with a vertical upward disturbance and allows oblique displacement. Indeed, the

        θ
        
          e
        
      
    
  

{\displaystyle \scriptstyle \theta _{e}}

of the particle is larger than the

        θ
        
          e
        
      
    
  

{\displaystyle \scriptstyle \theta _{e}}

of the environment. While the momentum of the particle is less than that of the environment. An oblique displacement thus produces a positive buoyancy and an acceleration in the oblique displacement direction which reinforces it. The condition for having conditional symmetric instability in an otherwise stable situation is therefore that:

the slope of

        θ
        
          e
        
      
    
  

{\displaystyle \scriptstyle \theta _{e}}

is greater than that of

        M
        
          g
        
      
    
  

{\displaystyle \scriptstyle M_{g}}

Laterally displaced air is almost saturated.

== Potential effects ==

CSI is usually embedded in large areas of vertical upward motion. The ideal situation is a geostrophic flow from the South with wind speeds that increase with height. The environment is well mixed and close to saturation. Since the flow is unidirectional, the u component of the wind can be set equal to zero, which establishes a symmetrical flow perpendicular to the temperature gradient in the air mass. This type of flow is typically found in baroclinic atmospheres with cold air to the west. The image to the right shows such a situation in winter with CSI associated with negative equivalent potential vorticity (

    η
    ≤
    0
  

{\displaystyle \eta \leq 0}

) near a warm front. Banded snow forms along the front, near the low pressure area and the CSI.

=== Slantwise convection ===

If a particle is climbing in a CSI zone, it will cool down and the water vapor will condense upon saturation, giving cloud and precipitation by oblique convection. For example, in front of a warm front, the air mass is stable because the mild air overcomes a cold mass. The geostrophic equilibrium brings back any particle moving perpendicularly from the center of the depression towards it. However, an upwardly oblique displacement by synoptic scale upward acceleration in a CSI layer produces parallel bands of heavy rainfall. Conditional symmetric instability affects a layer that can be thin or very large in the vertical, similar to hydrostatic convection. The thickness of the layer determines the enhancement of convective precipitation within a region otherwise stratiform clouds. As the motion is in an area near saturation, the particle remains very close to the moist adiabatic lapse rate which gives it a limited Convective available potential energy (CAPE). The rate of climb in a slantwise convection zone ranges from a few tens of centimeters per second to a few meters per second. This is usually below the climbing speed limit in a cumulonimbus, i.e. 5 m/s, which gives lightning and limit the occurrence of it with CSI. It is however possible in:

The trailing precipitation region of mesoscale convective systems. Wintertime convection because the lower and colder tropopause is helping the ionization of upward moving ice crystals. In the eyewall during the deepening phase of mature hurricanes, although rarely as it is a region symmetrically neutral and is generally free of lightning activity. Slantwise convection bands have several characteristics:

They are parallel They are parallel to the thermal wind They move with the general circulation The space between the bands is proportional to the thickness of the CSI layer

=== Subsidence === Conversely, if the particle slide downward, it will warm up and become relatively less saturated, dissipating clouds. The snow produced at higher altitude by the slantwise convection will also sublimate in the descending flow and accelerate. It can give it a speed of descent reaching the 20 m/s. This effect is associated with the descent to the ground of the Sting jet.

== References ==

== External links == David M. Schultz; Philip N. Schumacher (December 14, 1998). "The Use and Misuse of Conditional Symmetric Instability". National Severe Storms Laboratory. Retrieved August 23, 2019. "Slantwise convection". COMET Courses. UCAR. Retrieved August 23, 2019. Interactive course that needs a login