Wildfire Smoke Blows Into US

One unfortunate factor behind California wildfires is the lack of relief from the weather in the summer and early fall.

Rain is more common in California during the winter, but summer is a dry season, offering almost no relief to any fires when they're ignited.

(MORE: California's Carr Fire One of Largest in State History)

Traditional California Fire Season

The peak time of year for California wildfires generally runs from June through October, especially across the southern half of the state. All but one of the state's 20 largest wildfires on record dating back to 1932 have all occurred during those months.

The climate over a significant portion of California is classified as Mediterranean, meaning that most rain falls from November to early April, and the summer months are very dry. This includes metro areas like Los Angeles and San Francisco and the surrounding higher elevations.

Occasionally, monsoonal moisture works its way into parts of California during the summer months. During those periods, there can be some thunderstorms, but most produce little or no rain, unlike the deluges of heavy rain that more often occur in parts of the Rockies or Desert Southwest.

These thunderstorms instead produce lightning igniting new fires, and strong, shifting winds which can make a wildfire behave erratically.

As summer transitions to fall, conditions typically remain dry from late September through at least the first half of October.

Recipe for Wildfires

During the winter months, the jet stream often dips southward into California, allowing storms to roll in from the Pacific Ocean.

By May and June, a high-pressure ridge – a northward bulge in the jet stream – builds in the Southwest U.S.

Therefore, during the summer months, any Pacific frontal system comes in well to the north of California, and dry conditions prevail.

The ridge also produces extreme heat, especially away from the immediate coast.

(MORE: Most of California's 10 Most Destructive Fires Have Happened Since 2017)

A pressure gradient –  or a difference in air pressure – occurs between the cooler coast and hot inland areas, driving gusty winds, especially in higher elevations and in and near the mouth of canyons, where fires may start or are active.

By September and October, Santa Ana winds become more common. These winds most often come from an easterly or northeasterly direction, and can rapidly spread any wildfires.

All of this occurs during a season when there is no rain to mitigate the effects of wildfires.

Growing Concerns

This latest spate of California wildfires is being worsened by a chain of events.

The state was soaked by its second-wettest winter December 2016-February 2017, then seared through its record hottest summer June-August, 2017. The vegetation that grew in response to the wet winter quickly dried out months later and provided fuel for wind-driven wildfires to burn in the fall.

Instead of typical, soaking rain and mountain snow, this past winter was the state's second driest, and 2018's first seven months are among the top 10 hottest on record in many California locations, according to the Southeast Regional Climate Center.

Vegetation moisture is at record or near-record lows for late July in parts of the state, according to Dr. Daniel Swain, a UCLA climate scientist.