What weather conditions are important to consider when planning and conducting a prescribed fire?

The primary weather conditions to consider when planning and conducting a prescribed fire are relative humidity, wind speed and direction, temperature, and mixing height. Most prescribed fires are conducted with relative humidity between 30–50%. Burning with relative humidity below 25% can be tricky as spotfires are likely and suppression becomes challenging. Burning with relative humidity above 50% is often ineffective because of high fine fuel moisture content. Forecast wind speed is for winds 20 feet above surrounding obstructions. The actual wind speed at ground level is usually less. Wind speeds of 2-15 miles per hour are common for most prescriptions. Winds greater than 15 miles per hour can create containment challenges in the event of an escape or spotfire. Mixing height is the distance aboveground where smoke is mixed with the atmosphere. The mixing height and transport wind speeds at that elevation influence smoke dispersal. A mixing height of at least 1,700 feet is recommended for most prescriptions.