(CNN) - Wildfires can be unfathomably huge and destructive. Right now, more than a dozen fires in Northern California have engulfed thousands and thousands of acres and killed almost 20 people.
The damage and scope of such an event can make things seem hopeless: How is it possible to contain such a blaze? How does it even start, and spread, and what does life look like after it has died down? Here are some essential things to know about wildfires.
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They have a lot of causes, but most are man-made
While there are certainly natural phenomena that cause wildfires, like lightning, a NASA study says 84% of wildfires are caused by human carelessness. It doesn't take much, either -- a campfire, discarded cigarette or errant firework can be enough to trigger a huge blaze. Even all terrain vehicles (ATVs) can be a wildfire risk, because the hot exhaust can ignite dry brush.
There have also been recorded cases where wildfires were blamed not on carelessness, but arson.
Certain weather conditions can make them worse
It's no suprise that, say, dry conditions increase fire risk. But windy conditions can also push wildfires forward and spread embers. Hot conditions can make the ground more likely to burn, and on a hot day, rising warm air can create a path for fire to travel up steep landscapes like mountains. In fact, fire will burn faster uphill than downhill.
Very large fires can also create their own weather -- strong winds, and in some cases, fire tornadoes.
They burn more than 1.2 million acres in the US a year
According to statistics from the Natural History Museum of Utah, more than a million acres of US land are burned by wildfires a year. Large wildfires are classified by the United States Geological Survey (USGS) as being 300 acres or bigger. According to the National Park Service, a vast majority of fires never get that big, and 2% to 3% of all fires make up 95% of total areas burned every year.
Wildfires can happen anywhere, especially given that human error is responsible for so many of them. But forested areas, and ones in dry or drought-like conditions, are more susceptible. According to maps from FEMA, wildfires occur most frequently in the western part of the US -- especially southern California and the juncture of Nevada, Idaho and Oregon. Other hot spots appear in south Florida and parts of the Gulf Coast.
You don't just use water to put them out
While water -- and dampness and humidity in general -- are crucial to fire containment, there are an array of both hi-tech and low-tech ways to fight a blaze.
When you hear news about a wildfire, you usually hear about firefighters "containing," rather than putting out, the fire. Here are two major methods they use:
Flame retardant: If you've ever seen photos of an aircraft flying over a wildfire and wondered what smoky substance they're dumping, it's fire retardant, known as "slurry." It's mostly a mixture of fertilizer and water, designed to coat vegetation and slow down the spread of the fire. It's dyed that ominous red so firefighters know exactly where it's being released. In a large wildfire, tens of thousands of gallons of this retardant can be dropped to slow the blaze.
Firelines: Fire spreads as it ignites brush, vegetation, structures and other flammable material. A "fireline" is an area that firefighters clear, so the fire has nowhere to go. Firefighters completely clear an area and scrape the ground down to the soil to ensure there's nothing for the fire to burn.
In addition to these hands-on methods, scientists and firefighters use satellite imaging and modeling to get an idea of how big a wildfire is and where it's headed.
They're not always bad for the environment, in the long run
While fires can obviously be devastating for vegetation and even more so for populated areas, there are some unexpected benefits to a cleansing blaze. According the the National Park Service, wildfires can curb pest populations, purge non-native and invasive plant species, and provide nutrients and new spots of sunlight for the plants that remain afterward.
Some animals can also benefit, because fires change the landscape of an area, and with it, local feeding and hunting habits. There are even two species of pine trees that can only reproduce when fire opens their cones, releasing the seeds within.
They can return from the dead
Just because a fire isn't spreading across the ground doesn't mean it's dead. It could be burning underground, where decaying organic material called peat can keep it alive for months or more.
This obviously presents a serious danger, since a fire that is contained and supposedly beaten in the summer can, theoretically, survive underground during the winter and spread again when conditions become hot and dry again.
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