What indicates a period of unstable atmosphere during critical fire weather conditions?

The indication of a period of unstable atmosphere during critical fire weather conditions is associated with very low relative humidity. Low humidity levels mean that there is less moisture in the air, which can cause the air to heat up more quickly and rise more freely. This rising air leads to a greater potential for fire behavior to become erratic and unpredictable. When humidity is low, the potential for fire spread increases, as fuels can ignite more readily and support combustion.

In contrast, high atmospheric pressure is generally associated with stable conditions that inhibit vertical motion in the atmosphere, which would not promote instability. Stable temperature gradients refer to conditions where the temperature decreases gradually with altitude, which tends to keep the air from rising. Calm wind conditions can contribute to a stable atmosphere as well, suppressing fire behavior by reducing the movement of flames and air currents that might otherwise enhance fire spread. Hence, low relative humidity stands out as a key factor indicating an unstable atmosphere conducive to critical fire weather.