Another Kind of Perfect Storm
July 03, 2013
Photographer: Thomas McGuire; Thomas McGuire’s Earth Science Books
Summary Author: Thomas McGuire
Historical events are often greatly influenced, if not created, by atmospheric conditions and unusual occurrences. Wildfires are a case in point. In the photo above an aerial fire tanker approaches the Doce fire near Prescott, Arizona. This human-caused fire was remarkably similar to the lightning-caused Yarnell Hill fire that occurred 10 days later and 30 miles to the southeast. The Yarnell Hill fire killed 19 brave Arizona “hotshot” fire fighters on June 30 and was shaped by a tragic convergence of climatic factors.
1. Annual rainfall in the High Sonoran Desert of Arizona is only about 10 in (30 cm) per year. Most of it occurs in midsummer monsoons and winter storm patterns. Late spring is the driest and hottest part of the year. Afternoon temperatures in late June, historically the driest month, commonly exceed 100 F (40 C).
2. Dry spring weather sucks moisture out of vegetation making plant cover tinder dry and rich in natural flammable oils. In a wildfire, ground temperatures can exceed 1500 F (800 C).
3. Human-influenced global climate change is likely influencing Arizona’s weather. Climatic conditions in the last decade have been unusually hot and dry. As a result, the five most extensive wildfires in Arizona history have occurred in the last decade.
4. The monsoon season, which usually starts at about the beginning of July, often begins with dry-lightning thunderstorms. This was the cause of the Yarnell fire.
5. Scrub vegetation does not slow down diurnal winds that generally peak in the afternoon. Ambient winds of about 20 mph (30 km/h), with much faster wind gusts, fanned this fire.
6. Intense fires create their own horizontal and upward convection currents carrying burning embers great distances and surrounding fire fighters. In spite of their extensive training, experience, planning and real-time monitoring, fire fighters can find themselves under siege and encompassed in intense flames.
Photo details: Top - Camera Model: Canon PowerShot S110; Focal Length: 11.4mm; Aperture: f/4.0; Exposure Time: 0.0050 s (1/200); ISO equiv: 160; Software: Adobe Photoshop Elements 9.0 Macintosh. Bottom - Camera Maker: NIKON; Camera Model: E5200; Focal Length: 7.8mm (35mm equivalent: 38mm); Aperture: f/4.8; Exposure Time: 0.0020 s (1/512); ISO equiv: 64.