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When the air is too dry spot fires (small fires that spring up next to the controlled fire) can happen. 

The ideal relative humidity for an area is between 25 and 50 percent, but these can sometimes go as low as 20 percent or as high as 80 percent. When the relative humidity is lower than around 20 percent, it can be a bit dangerous because fires are more intense due to the dryness.
Image of a tree burning as a result of a spot fire.

When the relative humidity is over 50 percent, fires may not be able to burn the area completely or may not get to a high enough temperature to achieve the desired result because of the moisture in the air.

These are some factors in relative humidity's behavior: 

1.A rise in temperature form sunrise to mid-afternoon can reduce relative humidity by a half.

2.An 11 degree temperature drop from mid-afternoon to mid-evening can cause relative humidity to double.

3.When a cold front moves through an area, there is usually a temperature drop and a drop in the relative humidity. The lower humidity occurs because of the movement of a warm moist air mass to a cooler drier air mass. 

Relative humidity is a factor of prescribed burns that is dependant on all of the other factors. This is why it is not a good idea to try and predict fire behavior of a burn solely on relative humidity. Burns can be done at any level, but the recommended level promises better burns. Temperature also plays a big part in how well a prescribed burn goes.
Image of a fire that has gotten out of hand.

The temperature of an area can come into play when conducting burns. In general, the hotter the fire, the (higher intensity) the greater the risk of losing control of it. 

When there is a need for all of the fuel or ground litter--shrubs, trees, etc--to be burned, temperatures of at least 68 degrees Fahrenheit are recommended. When temperatures are rising during early morning hours, this also creates conditions for unpredictable burning. 

Air temperatures between 70 and 90 degrees Fahrenheit are ideal when you desire complete burns in the area. 

Signs of the fire getting serious are: 

1.Flame lengths reach above 1.2 meters or over 4 feet 

2.Spot or random fires occur ahead of main fire front. 

3.Large areas of smoldering fires. 

4.Increase in wind or change in wind direction 

5.Thick, massive smoke clouds close to the ground for long periods of time.

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