From "Managing Michigan's Wildlife: A landowner's guide"
Weather has an overriding effect on a prescribed burn. A burn plan will outline the weather conditions, which must be met before the burn is conducted. It is very important to have the latest and most updated weather conditions available before starting the burn. Relative humidity is an important factor to consider when planning a controlled burn. If the relative humidity is below 50%, the dryness of the grass is prone to causing very hot fires. If the relative humidity is above 70% the fire will have a hard time catching at all. Therefore a relative humidity between 50% and 70% works best.
Temperature is also important when laying out a burn plan because of its relation to relative humidity. Below 32 degrees Fahrenheit grass mats will rarely burn, and above 80 degrees Fahrenheit burning is hazardous. Between 40 degree Fahrenheit and 60 degrees Fahrenheit is ideal.
Wind direction and speed should both be taken into account as well . The wind speed should be between three and seven mph, and the wind direction should remain steady. If either varies greatly, the fire can shift with gusts of wind, and may burn too quickly with an increase in wind speed. Both of these variables can severely hinder safety precautions if not watched closely. In general, wind is calmer in the morning and the evening. Smoke management is crucial. Always warn your neighbors of your burn, and prevent smoke from hindering any roadways by planning your burn when the wind direction is going away from the road.
Of all the weather parameters the wind speed and direction are most critical. Unless professionals are included in the burn crew a burn at over seven mph is not recommended. Faster burns are less effective. They may not remove all the litter and unwanted species. In addition, safety comes first. Often the safest time to burn is in the evening between 7 pm and 10 pm. This is when humidity is on the rise, temperature decreases, and wind speed dies down, creating the setting for a slower (and safer) fire. However, burning after dark can be dangerous. Areas still smoldering can be missed.
The National Weather Service should have a telephone number listed in your area. They will be able to keep you up to date with changing weather conditions.