1. What is L57? L57 is the 700 to 500 mb lapse rate in units of degrees Celsius per kilometer. 2. How is L57 determined? Temperature difference between 700 and 500 mb is noted. Thickness of 700 to 500 mb layer is noted. The temperature difference is then divided by the thickness. The sounding at the bottom of this page shows an L57 parameter of 6.2 C/km. Here is how this value was determined: 700 mb temperature = 9.3 C 500 mb temperature = -7.0 C 700 to 500 mb temperature difference = 9.3 + 7.0 = 16.3 units of C 700 mb height = 3,216 m (shown on left side of Skew-T) 500 mb height = 5,830 m 700 to 500 mb thickness = 5,830 m - 3,216 m = 2,614 m = 2.614 km L57 = 16.3 C / 2.614 km = 6.2 C/km 3. Operational significance of L57: As L57 increases, temperatures cool more rapidly with height. Instability and increased instability becomes more likely as the mid-levels cool and the mid-level lapse rate increases. 7 C/km or higher is a very significant cooling with height. 4. Pitfalls: Index by itself is meaningless when it to comes to forecasting if storms or precipitation will occur.  |