A sprawling wildfire in northern Colorado nearly doubled in size again Monday, spewing plumes of smoke and forcing the evacuation of thousands.

The fire grew to 36,930 acres, authorities said Monday. It had been estimated at 20,000 acres Sunday night.

The Red Cross, Humane Society and other aid groups mobilized to help evacuees while at least 400 firefighters, aided by air tankers and helicopters from as far away as Canada battled the fire about 15 miles west of Fort Collins, Colorado.

As wildfire season flares up, here's a look at how some of these dangerous events start and how much damage they've caused in the past:

About Wildfires:

• Wildfires are sometimes called "wildland fires."

• Wildfires can originate from a dropped match, cigarette embers, campfires, exhaust sparks from a train, or arson.

• Many wildland fires are ignited by lightning.

• There are no official rules, but the first responders usually name a fire after a meadow, creek, city, or type of plant they see.

• Wind, temperature, and humidity all influence wildfires. Strong winds push flames toward new fuel sources. Wind can pick up and transfer burning embers and sparks, starting "spot fires."

• During the day, sunlight heats the ground and warm air rises, allowing hot air currents to travel up sloped landscapes. At night, the ground cools and air currents travel down the slopes.

• Humidity dampens fuel, slowing the spread of flames. Humidity is greater at night, so fires usually burn less intensely then.

• Large fires can create their own winds and weather, increasing their flow of oxygen.

• A really large fire can generate hurricane-force winds, up to 120 mph. The high temperatures preheat fuels in the fire's path, preparing them to burn more readily.

A look at the number of past fires, damage caused

Year                                       Number of fires                  Acres burned          

2000                                       92,250                                        7,393,493

2001                                       84,079                                        3,570,911

2002                                       73,457                                        7,184,712

2003                                       63,629                                        3,960,842

2004                                       65,461                                        8,097,880*

2005                                       66,753                                        8,689,389

2006                                       96,385                                        9,873,745

2007                                      85,705                                        9,328,045

2008                                      78,979                                        5,292,468

2009                                      78,792                                        5,921,786

2010                                      71,971                                        3,422,724

* 2004 fires and acres do not include state lands for North Carolina

Source: The National Interagency Fire Center