Stability indices

Showalter index: Lift an air parcel from 850 mb level to the 500 mb level.

Lifted index:

1. Forecast the afternoon maximum temperature.

2. Mixed the lowest 50 mb layer.

3. Lift an air parcel from this mixed layer to the 500 mb level.

K index

Total total index

Precipitable water

The amount of rainfall if all the water in an air column falls as rain.

M = mass of an air column with area A

M' = mass of water vapor in an air column with area A

pw = precipitable water

Example

q = 2 g/kg = .002 kg/kg

dp = 100000 Pa

g = 9.8 m/s2

pw = 20.4 kg/m2 or 20.4 mm

Convective Condensation Level (CCL) and Mixed Condensation Level (MCL)

How to find CCL and the convective temperature on a skew T chart

1. Starting from the observed surface mixing ratio and draw a line of constant mixing ratio upward. Find the intersection of this line and the observed temperature profile. The level at this intersection is the CCL.

2. Starting from this intersection, draw a constant line downward to the surface, the temperature at the surface is the convective temperature.

Physical reasoning

As the surface temperature increases in the day time due to solar radiation, mixing will start to form a constant layer. The thickness of this constant layer will increase with additional solar radiation.

Observed sounding in the morning:

An adiabatic layer is formed in the low level after some solar radiation.

Addition heating will increase the depth of the constant layer. All the air in the constant layer will be free to move up and down with resistance. As the top of the constant layer reaches the CCL, the surface air will be free to move all the way up to the CCL without resistance. Once it reaches the CCL, it is saturated and a layer of cloud will form. The surface temperature reached at this time is the convective temperature. The concept of CCL will help us to predict the formation of clouds in the afternoon due to solar heating. Some of the clouds may develop into air mass thunderstorms if the sounding is sufficiently unstable.

NOTE:

In this is argument, the potential temperature in a layer is totally mixed due to solar heating. However, we assume that the dew point temperature profile stays the same. If we also mixed the dew point temperature profile the CCL may be higher.

How to find MCL on a skew T chart

1. Determine a layer which is unstable due to strong wind shear.

2. Draw a constant line to represent the temperature sounding after the mixing.

3. Draw a constant w line to represent the dew point temperature (or the mixing ratio) after the mixing.

4. The intersection of these two lines is the MCL.

In the following example, nowhere of the observed sounding (thick line) is saturated. After mixing, the layer about MCL is saturated and a layer of cloud will form there.