How a fire develops and spreads is affected by its environmental conditions, namely the temperature, humidity, wetness of the fuel and wind. However, once the burning gets underway, the role that these environmental factors play will much less significant.

Temperature

The higher the ambient temperature, the higher the probability of a fire, and one that is severe, of occurring. The ambient temperature affects the tendency for fire spread in two main ways. Firstly, heat dehydrates, therefore, in a hotter environment, the fuel is dryer compared to being in a cooler environment. As explored in the section of the wetness of fuel later, dry fuel is more combustible. Secondly, the temperature of the environment affects the rate of reaction of combustion which approximately doubles by every increase in 10oC. However, the initial temperature of the environment can be overlook in the event of a fully developed fire because the heat generated is much greater than the ambient temperature. Thus the ambient temperature has the greatest effect only in the initial stages of the fire.

Humidity

Humidity is essentially the amount of water vapor in the air. There are 2 ways that humidity can be measured, which are absolute humidity and relative humidity. Absolute humidity is basically the amount of water vapor in the air while relative humidity is the percentage of humidity in the air with respect to the saturation value, i.e. the maximum humidity, that that temperature can reach. The relation between temperature and humidity is the greater the temperature, the greater the humidity saturation value and a lower tendency for the water vapor to condense.
Humidity affects the tendency for fire to start in 2 ways. First, a low relative humidity causes fuel to dry out, thus it becomes more combustible. This is most significant among living vegetation since plants transpire and loose moisture through their leaves’ stomata. It can be shown clearly when plants tend to dry out much faster in dry periods leading to an increase in the number of wildland fires.
Second, environmental humidity affects the build up of static electricity in objects, more notably non-metallic ones. Static electricity is build up through friction, where one object looses electrons to the other object upsetting the static equilibrium making the object positively or negatively charged. Charge dissipates by leakage through surface moisture during high relative humidity. Thus charges tend to accumulate in environments of low relative humidity. The danger this poses is when the charged object discharges to achieve charge neutrality with another object of different electrical potential, sparks are produced. This has a high potential for being an the ignition source in an environment with the components for combustion in place but lacking only an ignition source. For example, a patrol station where there is a high concentration of petrol fuel.