Standard Pressure Versus Altitude Chart

Information compliments of UNISYS Weather
                                                                          
  Pressure   Approximate Height      Approximate Temperature
100 mb 16000 m 53000 ft -56 C. -69 F.
200 mb 12000 m 40000 ft -55 C. -67 F.
300 mb 9000 m 30000 ft -45 C. -49 F.
500 mb 5000 m 18000 ft -20 C. -4 F.
700 mb 3000 m 10000 ft -5 C. 23 F.
850 mb 1500 m 5000 ft 5 C. 41 F.
1000 mb 100 m 300 ft 15 C. 59 F.
Sea Level 0 m 0 ft 15 C. 59 F.

The 500 mb chart is often used to determine the jet stream steering flows

Favorable Conditions for Severe Weather

Lifted Index: <0 Thunderstorms Possible. <-4 Severe thunderstorms possible.  >10 denotes a stable atmosphere

CAPE:  >2000 Thunderstorms possible.  >3000 Severe thunderstorms possible.  <1000 relatively stable atmosphere

Helicity:  Shows where rotation/shear is high enough to allow a thunderstorm to develop into a severe thunderstorm.  A value of 400 to 500 is often needed to produce severe storms.

Total totals:  >45 Thunderstorms possible.  > 52 Severe thunderstorms possible.  < 40 stable generally clear skies.  The higher the number the more unstable the atmosphere is.