Question

The table below list heights (in m) and temperatures (in °C) at 18 UTC on February 16, 2009 for several pressure levels at San Diego, CA. The data were derived from a computer simulation of the atmosphere. Although thunderstorms are relatively infrequent along the southern California Coast, there is a tendency for more thunderstorms during winter. This image is the 18 UTC radar from San Diego on February 16, 2009 (the white square shows the radar’s location).

Pressure Level	Height	Temperature (°C)
1000 mb	8	13.7
925 mb	730	7.7
850 mb	1420	2.6
700 mb	2970	-6.9
600 mb	4160	-13.1

1. Compute the environmental lapse rate for the 1000-925 mb layer, the 925-850 mb layer, the 850-700 mb layer, and the 700-600 mb layer. Express your answers in °C/km.
2. Which layers are stable? Which layers are conditionally unstable? How did you make your decisions?

Answer 1

Adiabatic Lapse rate is defined as the ratio of change in temperature to the change in altitude of air parcel ( without gaining of losing any heat to the surrounding )

Therefore, Environmental lapse rate =

for the layer 1000-925 mb;

Lapse rate =

for the layer 925-850mb,

Lapse rate =

for the layer 850-700mb,

Lapse rate =

for the layer 700-600mb,

Lapse rate =

(ALL THESE CALCULATED VALUES ARE IN oC/km.)

To understand the concept of stability, We need to have an understanding of the dry adiabatic lapse rate and the wet adiabatic lapse rate.

Dry adiabatic lapse rate is when the air contains little amount of water in it. It is equal to about 10 oC/km.

Wet adiabatic lapse rate is when the air is moist , it is the rate at which the moist air warms or cools when it moves vertically downwards or upwards. It is equal to about 6oC/km.

CONDITION FOR ABSOLUTE STABILITY IS:

dry adiabatic lapse rate(10)> Moist adiabatic lapse rate(6) > environmental lapse rate

Here,  

for the layer 1000-925mb, 10> 8.31 >6 ....................(a)

for the layer925-850 mb, 10> 7.39> 6 ......................(b)

for the layer 850-700mb, 10> 6.129> 6 .......................(c)

for the layer 700-600mb, 10> 6> 5.21

Here the only absolutely stable layer is the layer 700-600mb , BECAUSE ONLY IT FOLLOWED THE CONDITION FOR STABILITY.

The condition for CONDITIONAL UNSTABILITY IS:

moist adiabatic lapse rate(6)< -environmental lapse rate <dry adiabatic lapse rate(10)

Clearly the equations (a), (b) and (c) follow the above condition.

hence the layers 1000-925mb, 925-850mb, and 850-700mb are conditionally unstable.