Study: UHI in Los Angeles is driving off coastal clouds that dampen California wildfires

From the THE EARTH INSTITUTE AT COLUMBIA UNIVERSITY and “an actual case of man-made climate change” department.

Urbanization and climate change combine to heighten danger

Sunny California may be getting too sunny. Increasing summer temperatures brought on by a combination of intensifying urbanization and warming climate are driving off once common low-lying morning clouds in many southern coastal areas of the state, leading to increased risk of wildfires, says a new study.

Low-level clouds over Los Angeles (seen here in early afternoon) and other urban areas of coastal southern California are becoming rarer, leading to increased fire risk. (Park Williams)

“Cloud cover is plummeting in southern coastal California,” said Park Williams, a bioclimatologist at Columbia University’s Lamont-Doherty Earth Observatory and lead author of the research. “And as clouds decrease, that increases the chance of bigger and more intense fires.” Williams said the decrease is driven mainly by urban sprawl, which increases near-surface temperatures, but that overall warming climate is contributing, too. Increasing heat drives away clouds, which admits more sunlight, which heats the ground further, leading to dryer vegetation, and higher fire risk, said Williams. The study appears this week in the journal Geophysical Research Letters.

The research follows a 2015 study in which Williams first documented a decrease in cloud cover around the sprawling Los Angeles and San Diego areas. Urban pavement and infrastructure absorb more solar energy than does the countryside, and that heat gets radiated back out into the air–a major part of the so-called heat-island effect, which makes cities generally hotter than the rural areas. At the same time, overall temperatures have been rising in California due to global warming, and this has boosted the effect. In the new study, Williams and his colleagues have found a 25 to 50 percent decrease in low-lying summer clouds since the 1970s in the greater Los Angeles area.

Normally, stratus clouds form over coastal southern California during early morning within a thin layer of cool, moist ocean air sandwiched between the land and higher air masses that are too dry for cloud formation. The stratus zone’s altitude varies with weather, but sits at roughly 1,000 to 3,000 feet. But heat causes clouds to dissipate, and decades of intense urban growth plus global warming have been gnawing away at the stratus layer’s base, causing the layer to thin and clouds to burn off earlier in the day or disappear altogether. Cloud bases have risen 150 to 300 feet since the 1970s, says the study. “Clouds that used to burn off by noon or 1 o’clock are now gone by 10 or 11, if they form at all,” said Williams.

Williams and others have demonstrated a strong link between warming climate and increased wildfire in the western United States. But in southern California the link is more subtle, and clouds are a rarely studied part of the system.

While few scientists have looked in detail at clouds, many California airports large and small have been collecting hourly cloud observations since the 1970s, not for research, but rather for navigational safety. Williams and his colleagues decided to tap this trove to develop a fine-grained picture of changing cloud cover over the region.  They then compared it to a separate large database kept by the U.S. Wildland Fire Assessment System, whose researchers have regularly measured vegetation moisture in the hills outside Los Angeles for decades. By comparing the two sets of data, the team found that periods of less cloud cover during the summer correlated neatly with lower vegetation moisture, and thus more danger of fire.

However, the study did not find that total area burned in summer has increased as a result of decreases in cloud shading. There are too many other factors at play, said Williams. These include yearly variations in rainfall, winds, locations where fires start, and perhaps most of all, decreases in burnable area as urban areas have expanded, and the increased effectiveness of fire-fighting.

“Even though the danger has increased, people in these areas are very good at putting out fires, so the area burned hasn’t gone up,” he said. “But the dice are now loaded, and in areas where clouds have decreased, the fires should be getting more intense and harder to contain. At some point, we’ll see if people can continue to keep up.”

The catastrophic California-wide fires that consumed over 550,000 acres in fall of 2017 were probably not strongly affected by the reductions in summer cloud cover, said Williams. Although he did find that vegetation is drier in fall seasons that follow summers with few clouds, the fall 2017 fires were driven mainly by extreme winds and a late onset of the fall rainy season. And ironically, part of this record wildfire wave resulted not from a recent record four-year drought driven in part by climate change, but rather from record rains that followed the drought, which produced a surfeit of flammable vegetation. Things will vary year to year, but Williams said he expects to see overall fire danger increase in California, as long as there is adequate vegetation to burn.

The other authors of the study are Pierre Gentine of Columbia’s Department of Earth and Environmental Engineering; Max Moritz and Dar Roberts of the University of California, Santa Barbara; and John Abatzoglou of the University of Idaho, Moscow.


The study:

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Thomas Homer
May 31, 2018 10:16 am

“… the team found that periods of less cloud cover during the summer correlated neatly with lower vegetation moisture, and thus more danger of fire.

However, the study did not find that total area burned in summer has increased as a result of decreases in cloud shading”

A theoretical increase in ‘danger of fire’, but no increase in actual fire. Therefore …

John Harmsworth
Reply to  Thomas Homer
May 31, 2018 12:51 pm

Shouting “FIRE” in a crowded wilderness! Wrap your head around that!

May 31, 2018 10:22 am

So it has gotten warmer since the 1970’s? Anyone using an uncooked database for US temperatures would note the 1970’s were the end of a 30 year cooling trend, and whether it is currently warmer or cooler than the late 1930’s is questionable.

Reply to  Tom Halla
May 31, 2018 11:20 am

Bingo – and that 30 yr cooling trend used to be much more of a cooling trend, up and until man-made adjustments were applied.

Reply to  Tom Halla
May 31, 2018 1:20 pm

More importantly than the behavior of the national average, L.A. and all of southern CA experienced and enormous spurt of urbanization starting in the late sixties.

Reply to  Tom Halla
May 31, 2018 5:20 pm

LA has also grown a lot in the last 50 years.

May 31, 2018 11:19 am

Well, look at that it’s 11:00 here in Los Angeles and this morning’s “layer of cool, moist ocean air sandwiched between the land and higher air masses,” is just beginning to break up – a bit earlier than it has been for, I’m going to guess 25 out of the last 30 days, or so. We might even get up to 70 degrees today.

High pressure ridge moving in next week and will see some real warmth finally. I’ll bet that the morning marine cloud layer doesn’t make much of a showing – regardless of that study.

Meanwhile – didn’t we just see a study out that noted that, globally, cloud cover has been increasing over recent decades? Why is CA always different?

John Harmsworth
Reply to  garyh845
May 31, 2018 12:54 pm

This report actually explains a lot. Get Jerry Brown inside. He’s been out in the hot sun too much and gone mental.

Alan Tomalty
May 31, 2018 11:36 am

Well every time the researchers come out with a new cause and effect they always have to add another cause of global warming or they wont get any funding next time. Then they are left with the IMPOSSIBLE task of separating out the nonexistent global warming cause from the real cause. They are left trying to chase their own TALE. This makes their study always tantamount to JUNK SCIENCE because they go through statistical hoops trying to minimize the real cause so that they can pin most of the cause on non existent global warming. Junk science. But what else can you end up with for research that believes in a religion?

May 31, 2018 11:51 am

Oops, I live on the beach in LA and the MayGray andJuneGloom haven’t changed. So, fail.

Mike MacKenzie
Reply to  Wharfplank
May 31, 2018 4:47 pm

Same is true in Northern San Diego county!

Reply to  Wharfplank
June 1, 2018 9:56 am

In “the Valley” it seems MayGray 2018 has been confined to the past few days in May only, most of this month was quite sunny and warm, not like when I first came to LA two decades ago. YMMV

Tom in Florida
May 31, 2018 11:52 am

“the team found that periods of less cloud cover during the summer correlated neatly with lower vegetation moisture”

Well, duh!! Apparently the team never hung their clothes out to dry.

May 31, 2018 12:00 pm

OMG! This study is heresy!

Gavin Schmidt staunchly asserts that there is no meaningful “Urban Heat Island” effect, just as he claims that the counterintuitive upward-correction of measured temperatures to “adjust” for UHI is warranted and robust.

It seems the clouds didn’t get the GISS memo

Reply to  GeologyJim
May 31, 2018 3:32 pm

Here’s what the EPA has to say about it:

Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F (27–50°C) hotter than the air1, while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.

. . atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings.2 On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).

When we hear on the news that large metropolitan city set a new high temperature record yesterday, “Yesterday’s high of 97 degrees shattered the old record of 95, set back in 1888 (happens a lot – like that) one can rest assured that it didn’t, as 2-5 degrees of it is most probably UHI effect . . err . . that is man-made, come to think of it. ha. There should be an effort to adjust for this in the record keeping.

Reply to  garyh845
June 1, 2018 5:34 am

Good, informative post, gary. Thanks.

Stephen Skinner
May 31, 2018 1:38 pm

“Cloud cover is plummeting”. How does it do that?

M Courtney
Reply to  Stephen Skinner
May 31, 2018 1:45 pm

That would be rain.

DC Cowboy
Reply to  M Courtney
May 31, 2018 1:59 pm

+2 (or fog)

DC Cowboy
May 31, 2018 2:03 pm

Sounds a lot like the land use change that affected Kilimanjaro. It wasn’t rising temps, it was deforestation that changed wind patterns.

May 31, 2018 2:10 pm

I don’t think we should be too critical of the study. It emphasizes the urban heat effect and how that can impact cloud cover. While the CO2 stuff is nonsense, I think the urban heat effect is relevant and should be studied more.

Reply to  freeholddave
May 31, 2018 7:31 pm

Urban heat island effect does happen. It makes sense that more heat would affect cloud patterns over the heated area. I’m not surprised that there may be less clouds over the LA area.
If the patterns had changed, does that put more moisture (evaporated from the ocean) somewhere else? Is there less moisture in the air in total or just a lower relative humidity because of the higher temperature? Lots of questions on the overall effect, but not necessarily a reason to have an issue with the study itself.

May 31, 2018 2:40 pm

If true, it sure as hell isn’t working this May. I don’t remember a May Gray this depressingly awful since at least 2003.

Mike MacKenzie
May 31, 2018 4:44 pm

Well, I’m getting a kick out of this since I drove up from San Diego to LA this morning treated to plenty of low coastal cloud cover. Also getting a kick out of the photo with this article since I was just out on the edge of the breakwater at the entrance to Marina del Rey (pictured here) photographing the sunset last week.

Also could it be that the reason the reduction in cloud cover and increase in dryness isn’t increasing actual acres of wild land burned, because the changes are localized over urban areas which don’t have much wild land available to burn anyway?

John in Oz
May 31, 2018 5:21 pm

Urban pavement and infrastructure absorb more solar energy than does the countryside, and that heat gets radiated back out into the air–a major part of the so-called heat-island effect, which makes cities generally hotter than the rural areas. At the same time, overall temperatures have been rising in California due to global warming, and this has boosted the effect.

If the temperature readings they are using are affected by UHI (which is a lot of them according to Anthony’s research) then UHI is a major cause of temperatures rising.

These researchers appear to separate UHI from ‘climate change’ (bolded text).

It is also not clear that they do/do not accept UHI:

Definition of so-called
1 : commonly named the so-called pocket veto
2 : falsely or improperly so named deceived by a so-called friend

Perhaps they use both depending on who they are talking to (eg – no.2 for Gavin)

May 31, 2018 5:59 pm

Looking at it from another angle, the UHI in the LA area is going up even faster than just increases in population and asphalt would otherwise indicate.
Time to increase those adjustments.

Thomas Ryan
May 31, 2018 6:39 pm

My first experience in Southern California was in the fall of 1968 doing carrier qualification for the Navy. We did not need radar or TACAN to find San Diego. We just followed the plume of smoke from the ubiquitous annual fires to NAS San Diego.

May 31, 2018 10:00 pm

Perth in West Aus has a very similar climate to S California. It has also experienced a step in rainfall begining in the late 70’s. There has been much conjecture over this, but two papers have pointed at the shifting jetstreams as a result of the ozone hole see below
Its possible similar conditions prevail in California.

Reply to  ChasTas
June 1, 2018 8:04 am

I lived in Perth for 8 years and the weather there is probably the best I have ever experienced! It gets very hot (40 C is not at all unusual) quite early in the day as the sun heats the ground, but then this rising hot air pulls in cooler air from the ocean – referred to as the Fremantle Doctor – which keeps the nights cool – rarely over 20. In the morning the sand could be quite cool on the feet, but we would leave the beach by 11am most days as it became uncomfortably hot and the wind could whip the dry sand up into a bit of a flurry which really stung on bare skin. Although there is a decent annual rainfall, the big changes mean that humidity is normally very low when the temperature is high such that we never had AC in our apartment – ceiling fans were enough.

This weather pattern is controlled more by the surrounding area than any urban heat island I suspect. The ‘Doctor came right through the city and circulated the air nicely. Having said that, the city has grown a lot in the 18 years since we left. I am due back for a visit next week and checking on the map shows suburbs have grown up all round the city, but especially north and south along the coast. I am really looking forward to seeing how much has changed!

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