Wood fire. Image obtained with thanks from bugeaters on Flickr.
First, you need to understand what starts fires and what environments are ideal for them.
Fires thrive in low humidity environments and are most likely to survive on (burn) dry substances such as dead foliage (after it dries out and turns brown), dead grass, wood and paper.
The cause of fires is equally important. Fires can be caused by ignition or autoignition. Autoignition is when a substance spontaneously “catches fire” (ignites) without a flame or electric spark due to intense heat. In order to understand autoignition, you need to understand what really causes all the fires mentioned above, including the flame and spark ignited ones.
Fires are caused by intense heat. Sparks (electric arcs) start fires because they are at a temperature high enough to cause many substances to ignite. Some substances require higher temperatures than others to ignite.
Each substance has an autoignition temperature and that is the temperature at which it ignites. Don’t confuse autoignition with sparked ignition. Even though the underlying cause of fire is the same, they are used in different contexts. Autoignition is specifically used to refer to ignition without a spark or flame.
A very common example of ignition without a spark is in the diesel engine. It is a compression ignition engine. This means that it compresses air inside it’s cylinders until it exceeds the ignition temperature of diesel fuel, so that when the diesel fuel is injected into the cylinder of hot air, it automatically ignites.
Drought helps to cause fires because it is a lack of rain for a prolonged period of time which causes plants to dry out, and as I said above, dry plants are the most likely to ignite. They can be ignited by lightning strikes, arsons, cigarette lighter accidents, and more.
Wildfires are often contributed to by droughts because droughts are geographically large enough to cause foliage over a very widespread area to become dry, so a fire may be started on even one leaf, spread to the rest of the plant, then spread to the other plants until it becomes a large disaster.