|GRAPHICAL AREA DISPLAY:|
850 to 500 mb LAPSE RATE
METEOROLOGIST JEFF HABY
A lapse rate is a change in temperature with height. This value can be used to infer instability. The higher the value then the higher the
instability. Values less than 6 are generally stable while values of 7.5 or above are very unstable. Values between 6 and 7.5 generally
mean there is instability and it can be released if there is adequate dynamic lifting to lift a parcel into an unstable elevation in the
troposphere. The same holds for values of 7.5 and above, but the convection is more likely to be explosive if instability release occurs at
these high values.
The 850 and 500 mb lapse rate has units of C/km. The C value is found by taking the temperature difference between 850 and 500 mb. The
km value is the vertical distance between the 850 and 500 mb pressure surfaces. For example, if the temperature is 25 C at 850 mb, the
temperature is -10 C at 500 mb, and the distance from 850 to 500 mb is 4.2 km, then the lapse rate is (25 (-10) / 4.2 = 35/4.2 = 8.3 C/km. For
higher elevation regions, such as locations where the 850 mb level is near or below the ground surface, then the 700 to 500 mb lapse rate
is used in order to represent the lapse rate.
The image below shows an example of a lapse rate chart. The values are on the high side, thus thunderstorms that develop should have strong
updrafts. This increases the potential for severe weather.