If the thermal index is poor, which lapse rate is commonly used?

When the thermal index is poor, the commonly used lapse rate is 5.4 degrees Fahrenheit per 1000 feet. This lapse rate reflects a situation where the atmosphere is relatively stable, which typically occurs during periods of poor thermal activity.

In meteorological terms, the lapse rate refers to the rate at which temperature decreases with an increase in altitude. A lapse rate of 5.4 degrees Fahrenheit per 1000 feet suggests a relatively stable atmosphere that does not readily promote the development of strong thermals. When thermals are weak or not prevalent, pilots rely on this lapse rate as a guideline for understanding how lift may be affected in those conditions.

Other lapse rates, such as 6.0 degrees or 4.0 degrees Fahrenheit per 1000 feet, indicate different atmospheric stability conditions that could either be too unstable or suggest stronger thermal activity. Therefore, recognizing the relationship between the thermal index and the corresponding lapse rate aids pilots in assessing the potential for lift based on the stability of the atmosphere.