Month: August 2015

PULLMAN, Wash. – Major firestorms burning in parts of the Pacific Northwest are the result of angry skies pitching lightning bolts to the ground when little or no rain is falling. The fast-moving blazes are destroying homes, closing roads and triggering smoke advisories miles away. Where is the lightning coming from and where is the rain?

In a bad-weather phenomena sometimes referred to as dry lightning or dry thunderstorms, the atmosphere  has been so hot and dry that rain produced in a cloud evaporates before it reaches the ground, according to an atmospheric scientist at Washington State University.

This vanishing rain – called virga – can appear as soft streaks exiting the bottom of clouds. While most of the virga never makes it to the ground, the lightning generated in the cloud still does, said professor Brian Lamb, who teaches a course in meteorology and runs the university’s Laboratory for Atmospheric Research.

When a downward spear of lightning sparks even a small flame on parched landscape, “there’s no accompanying rain to extinguish it,” he explained. “With the drought turning timber, brush and grass bone-dry, there’s ample fuel to feed the flames.”

Hazardous fire weather

In north central Idaho, multiple lightning strikes in a single night last week caused a still-raging complex of fires near Kamiah, wiping out more than 30 homes and prompting highway driving restrictions and smoke advisories in communities 100 miles away.

In Washington state, a 15-mile stretch of fire consuming timber and brush and threatening the town of Chelan was started by five small lightning fires that merged over the weekend, according to fire incident reports. Near Mount Adams, Washington’s National Guard has been mobilized to assist in fighting a wildfire that quickly ballooned to 22,000 acres after it was sparked by lightning one week ago, according to the Northwest Interagency Coordination Center.

Fueling the problem is that dry lightning storms are sometimes accompanied by microbursts, said Lamb. These are drafts of cold air pushed downward from a cloud that hit the ground and spread outward as gusts of wind.

“When lightning hits the ground and sparks a fire, a gust of wind can push the fire’s margins very quickly,” he explained.

Where’s the thunder?

If rain doesn’t reach the ground during a dry thunderstorm, does that mean there’s no thunder as well?

There is thunder, said Lamb, even though people might not hear it: “Thunder is a result of lightning. If you see lightning but don’t hear thunder, it’s because the storm cloud is too far away.”

Often, downbursts or downdrafts coming out of a thunderstorm are invisible, especially if it is a dry downburst. This weather phenomenon in the dissipating stage of a thunderstorm can be deadly if the wind direction suddenly changes and affects the direction of spread of a wildfire.

The video above is of a wet downburst. That and the time lapse feature make it possible to clearly see the air and the rain descending, reaching the ground, and then spreading out. Firefighters need to be aware that this is not a rare occurrence and it can affect the wind direction miles away from the cloud.

Wikipedia has more information about downbursts.