LA's fog disappearing due to urban heat

A new study reports that coastal fog in Southern California is on the decline, especially in heavily urbanized areas.

In particular, Los Angeles saw a 63 percent decrease over the last 60 years.

You can blame the heat island effect created by city streets and buildings, said the study's author Park Williams of Columbia University’s Lamont-Doherty Earth Observatory in New York.

Fog may be a nuisance for drivers, but according to Williams, it also plays a crucial role in hydrating many costal ecosystems.

These include mountains with coastal forests and hillsides covered in chaparral, which easily burns when conditions are too dry.

"They all receive water directly from fog and benefit from the shading of these clouds," Williams said.

In fact, he noted that in some parts of Southern California, fog may provide plants with almost as much water as rain does. Williams says this loss of coastal fog could impact the regional environment.

Fog typically forms when the air is cool enough for clouds to condense close to ground level. This often happens at night and in the early morning.

However, Williams said this process is being upset by all the concrete in urban areas, which absorbs heat in the day and slowly releases it over night, raising temperatures.

"When you increase the temperature of the surface of the Earth, then you essentially need to go higher up into the atmosphere before [it] is cool enough to promote condensation," Williams explained.

The end result is that as cities heat up, clouds rise and fog disappears.

Data for the study came from the detailed logs of the 24 coastal airports between Santa Barbara and San Diego.

"Of course airports have been collecting really good data on clouds because the presence of clouds and their height in the atmosphere really affects air travel," he said.

Many of these logs had hourly updates on cloud height, some dating back to the 1940s.

Using this information, Williams and his colleagues determined that the greatest loss of fog occurred in Ontario where there was a nearly 90% decrease over the last 60 years.

Other airports such as LAX, Burbank's Bob Hope, Long Beach Airport and John Wayne Airport in Orange County also saw a considerable decrease in the average amount of fog.

However, less urban areas like Santa Barbara and the undeveloped the Channel Islands remained quite misty.

Williams says this trend is concerning because man-made climate change is expected to heat things up even more in the future.

Coastal fog can help cool an area down but as cities continue to bake, they will gather and emit even more heat, driving away even more fog.

"That can then feedback until the cloud layer is eaten away entirely in the daytime," he said.

Soon, Williams hopes to explore how much water fog provides Southern California in general to see whether the continued loss of these low clouds could dry out the region even more.

His current paper appears in the journal Geophysical Research Letters.