Sunday, May 21, 2017

Are Air Conditioners Needed in Seattle Now or in the Future?

The Seattle Times published a story on air conditioning in Seattle, suggesting that "as Seattle summers keep getting hotter and hotter" more apartments are being built with A/C.   In fact, they have a plot suggesting that while only around 4-5 percent of Seattle apartments were built with central air conditioning before 2010, during recent years that number has climbed to 25%!

So one might ask:  do you really need A/C in Seattle and have our summers got "hotter and hotter"?

Let's check the facts.  One of the key measures of the need for air conditioning is cooling degree days, which is based on the difference between daily average temperature and 65F.   If the daily average temperature at a location is say 75F, then that day has 10 cooling degree days.  Do this for all days of the year at a location and add them up, and you have annual average degree days.  The idea is that once the daily average temperature reaches 65F (say a high of 80 and low of 50), you start needing A/C.

The map of average annual cooling degree days below shows that western Washington has the some of the lowest numbers of cooling degree days in the nation outside of high terrain.  Much of western WA is white (low numbers) with only central Puget Sound entering the red colors (101-400).  Seattle clearly has the lowest need for cooling of any major US city in the lower 48 states.

In fact, Sperling's Best Places has rated Seattle America's number one summer "Chill City" based on our temperature and humidity.  Seattle Chill--I think I have heard this before.

The reason for Seattle's low number of cooling degree days is clear:  we have some of the coolest summer temperatures in the nation, because western WA generally is covered with cool air from off the Pacific Ocean.  The map of the normal high temperature for August shows this clearly, with our area the only coastal one with a lot of green.   Seattle does better than Portland because we have cool Puget Sound next to the city and there is a sea level passage to cool Pacific air.   Portland has much more need of A/C.

But temperature is only part of the story---humidity is also an important issue for summer comfort.  Higher humidity reduces our ability to cool by sweating, which is a very effective way our species reduces our body temperature.    A good measure of the water content of air is dew point, the temperature at which air becomes saturated (100% RH) when cooled.

Take a look at the dew point map of the US for August below.  The lowland Northwest has LOW dew points (low 50sF)--low levels of moisture in our air.  Which means our relative humidities end up quite low during the day.   Your body can cool effectively from sweating (which may not even be apparent to you).  Compare that situation to the eastern (and particularly southeastern) US where moist, humid air greatly interferes with our natural cooling mechanisms.

Why are our dew points low?  Because of the cool Pacific Ocean....the amount of moisture air can pick up increases with temperature.  Cool water...not much moisture in the air.

But wait...there is more!   Our dry air allows temperatures to cool rapidly at night (water vapor is a powerful greenhouse gas), so our typical daily low temperatures even during a warm spell almost always drop in the 50s.  Open a window or use a window fan, and comfortable sleep is almost always possible.   Finally, even on the warmest days, cooler temperatures are generally very close....just head to the water, since Puget Sound is generally around 50F.

The bottom line of all this is clear:  Seattle is one city where A/C is a luxury that is not particularly needed for buildings in which windows can be opened, except for the most unusual and exceptional days.  Apartments with poor circulation and facing the sun can get warm (and could use A/C).  Typically, Seattle has two days a year when the maximum temperature reaches 90F. 

But what is the trend of Seattle temperatures and how about the future?  Will A/C become a necessity for most, like in Houston?  

To "warm up" the discussion, here are the average June to September temperatures over the Puget Sound lowlands (NOAA WA Climate Division #3) for the past 65 years.  There is a small upward trend with a lot of variability.   Roughly 60.5 F in the 50s to early 70s, and approximately 62F since then.   The transition in the mid-70s may have been due to a mode of natural variability called the Pacific Decadal Oscillation (PDO), which transitioned from the cold to warm phase at that time.  A small human-caused contribution is also possible (from urbanization of our region and increasing CO2).   But the upward trend is modest and quite small for the last 30 years.

There was one crazy warm year recently (2015), which was associated with an usually persistent ridge of high pressure.   Around a DOZEN days reaching 90F at Seattle.  And you will notice some similar heat spikes early in the record.  These transient features were probably the result of natural variability.

The bottom line of this analysis is that there has not been a large warm up over our region during the past decades and increasing temperatures is probably not the reason for more A/C installations.    A/C may be nice on a rare and extreme Seattle hot spell, but generally is unnecessary.  More A/C is like the trend for expensive granite counter tops-- pleasant to have perhaps, but a luxury.  More A/C is probably a better measure of the increasing wealth of our region than of increasing temperatures.

But what about the future?  If A/C may not be necessary now, what about later in the century?
It might be a good idea.   Let me show you the projections from a regional climate simulation that we completed at the University of Washington (credit to Professor Eric Salathe and research scientist Richard Steed).   Specifically, this is number of days per decade that will climb about 90F at Seattle-Tacoma Airport.  This simulation assumes that we keep on burning fossil fuels in a similar way as in the past (which is what is happening I am afraid).

For the 1990s it shows about 50 per decade (4 per year).  The model is overestimating the number of heat waves, probably because it does not have enough resolution to get Puget Sound correct.  2020s?  Pretty much the same story.  2050s--a modest increase to 80 a decade.  But look at the 2090s.  Wow.  A huge increase to 170 a decade (17 a year).  You will probably want air conditioning in Seattle in the 2090s.

Global warming due to human-inspired emissions of greenhouse gases is probably having a small impact on our local temperatures now, but by the end of the century our climate will warm profound.  But we need better information to get a better idea of the local effects.  That includes running regional climate models at higher resolution and running them many times to get a handle on uncertainties.  That is why we are trying to build a regional climate effort at the University of Washington.

And enjoy the perfect weather A/C needed.

Friday, May 19, 2017

80 degrees on Tuesday?

The first 80F day of the year in Seattle is always a reason to celebrate, and it looks like Tuesday (and possibly Monday) may be it.

Here is the latest forecasts of the National Weather Service, showing 80F for Monday and 85F for Tuesday..

And the even more skillful prediction of is going for 82F on Tuesday.

The origin of this warmth is a strong ridge of high pressure that will building over the region on Sunday and Tuesday (the map for 5AM Tuesday for 500 hPa...around 18,000 ft.. is shown below).
The surface pressure and low level temperature map for Monday at 2 PM shows the typical warm-period set up, with a trough of low pressure caused by warm air extending from California into western Washington.

Saturday morning will have considerable low clouds and a few sprinkles over western WA, but by afternoon the clouds should then and temperatures should rise into the mid-60s.  And it is only up from there.....

Wednesday, May 17, 2017

Was Weather Behind the Record Times During the Windermere Crew Races?

On Saturday, May 6th both the men and women's crew broke long-term course records during the famed Windermere Cup races.  As noted by the Seattle Times:

The men’s winning time broke a 20-year-old course record set by Washington in the 1997 Cal dual regatta of 5:30.0. The women’s performance broke the 30-year-old record set by the Soviet Union national team in the 1987 Windermere Cup of 6:11.73.
Picture Courtesy of the University of Washington
What were the chances that both men's and women's teams would beat long-term records on the same day? Even the woman's coach Yasmin Farooq found it hard to understand.

Now I am the last one to detract from the stunning athleticism of the UW men and women's crew teams, some of whom I have had in my classes. They are fierce and disciplined athletes. But perhaps there was a meteorological/environmental factor that enhanced the times of both the Huskies and their competitors that day.

Let me explain.   

None of this was on my radar until I got a call  from someone from the Seattle police department (didn't note his name).  After assuring me that I wasn't in trouble, he told me that he was involved with the crew races and noted that the water seemed to be moving very fast (westward) during the competition in the Montlake cut (see map) ; he wondered whether that might have explained the records.  I mused that we have had a very wet spring and that there was a pulse of water moving into the rivers that weekend from melting mountain snow.  So maybe there was a connection.

The Windermere Cup Crew Races occur upstream of and in the Montlake Cut, a narrow passage between Lake Washington to the east and Lake Union to the west (red oval)

I told I knew who to call:  Larry Schick, lead meteorologist of the U.S. Army Corps of Engineers (USACE) in Seattle.  USACE runs the Hiram M. Chittenden Locks, which controls the water levels in Lake Union and Washington.  Larry was very helpful, and engaged the help of a hydrologist colleague, Adam Price.

The "plumbing"  of the region is relatively simple.  Most of the water that enters Lake Washington does so through the Cedar River (red oval below); that water then flows through the very narrow Montlake Cut (red oval above) into Lake Union and flows westward through the Locks before entering Puget Sound.  The outflow through the Locks and the inflow in the Cedar control the water levels in Lakes Union and Washington (there are minor rivers/streams flowing into the Lake as well).  Generally, the Corps keeps Lake Washington at around 20 feet during the winter and 22 feet in late spring.

This has been a exceptionally wet spring and thus the Army Corps has had to pass a lot of water through the Locks, resulting in strong westward flow in the Montlake cut (the race venue)

However,  the passage of a wet, warm front in early May upped the ante--it not only provided lots of rain, but also melted the bountiful snow in the Cascades.  As a result, the Cedar River surged, as shown by USGS Daily Discharge Data at Renton.  Blue is the actual this month and the orange triangles are normal,  The flow was WAY above normal on May 6th, reaching around 1800 cubic feet per second.  In fact, this was the greatest flow on record at that location on the that date.

According to USACE hydrologist, Adam Price, on race day the Lake Washington level was fairly stable and the Corps maintained an average release rate of 2800 cubic feet per second, enough to balance the Cedar and minor river inflows.  He told me that this was a very high outflow rate in May.   Using the 2800 cubic feet per second number and simple calculations, he determined the flow speed up the narrow Montlake Cut, finding unusually large outflow.  He did the same for normal May flow (850 cfs).   The result demonstrated that the boats were sped up considerably by the strong current, reducing their travel time by roughly 6 seconds.

So the wet spring and rapid warm up  resulted in increased flows that substantially reduced the travel time down the cut, and may have explained (at least partially), the records broken that day.

Interesting, the date of the previous Windermere Cup men's record (May 3, 1997) was also a period of unusually strong flow from the Cedar River into Lake Washington (see below).

The bottom line of all this is that it is possible that a wet spring and rapid snowmelt led to strong flow along the Windermere Cup race course that might have shortened the travel time appreciably.  

And it is only a rumor that the Husky Crew team has suggested that the US Army Corps of Engineers do a rapid release of water during the first Saturday of May 2018 😊

Monday, May 15, 2017

Heavy Rain Followed by Drought

The next week will be a study of contrasts, with a wet weather system saturating the region through Wednesday, followed by a very extended period of sun, warmth, and little or no precipitation.  Get out your hiking boots, bicycles, outdoor gear and will need them.

Tomorrow morning, an unusually strong (for May) upper low will pass through our area (see 500 hPa upper level map).  As Darth Vader would say:  "impressive, most impressive."

And the forecast precipitation for the 48h ending 5 AM Wednesday is also impressive (see below), with the entire northwest from northern CA to BR getting lots of water, including 1-2 inches in most of the mountainous areas.  Even the Sierra Nevada is getting wet, which is really unusual.

But then the long-awaited miracle occurs and the spigot turns off and stays off.   First, a modest ridge of high pressure builds aloft on Wednesday morning.

This low amplitude ridge is distorted a bit over the subsequent few days, with a several weak systems moving to the north.   But the real miracle is later in the weekend when an Olympian ridge develops over the eastern Pacific, and by Monday morning (shown below) extends to Alaska.  Praise to the gods.

Consist with this ridging, the latest NOAA Climate Prediction Center 6-10 day forecast indicates way below normal precipitation for Sunday to Thursday.
Or how about the vaunted European Model forecast?  The 24-h totals ending 6-hr for the next two weeks shows lots of rain the next few days, but dry (or nearly dry) conditions from roughly May 18- 25.

The US/Canadian NAEFS ensemble (many forecast) system is similar to the EC prediction (check out the second panel). So my confidence is quite high in this improving trend.

A fine, dry period sometime in mid-May is not unusual, representing the interregnum between the weakening storms of the winter season and the unpleasant June-gloom of late May and June.  High pressure building north cuts off the storms, but as it strengthens onshore and sinking flow increases the result is  lots of ocean stratus and onshore flow by the end of May.  Only when the ridge builds sufficiently with descending northerly flow does the fine weather of July begin.

Saturday, May 13, 2017

Seattle Times Climate Change Article is Dead Wrong

The big front page story in the Seattle Times today, both online and in print, is about how climate change has caused the death of a 72-year old pine tree in the University of Washington arboretum.  Unfortunately, the underlying premise of the story is false, representing another unfortunate example of exaggerating the impacts of global warming.

The writer of the story, Lynda Mapes, could not have been more explicit:

The cause of death was climate change: steadily warming and drier summers, that stressed the tree in its position atop a droughty knoll.

So, lets check the data and determine the truth.   My first stop was the nice website of the Office of the Washington State Climatologist (OWSC), where they have a tool for plotting climatological data.  Here is the summer (June-August) precipitation for the Seattle Urban Site, about a mile away from the tree in question.   It indicates an upward trend (increasing precipitation) over the period available (1895-2014), not the decline claimed by the article.

Or lets go to the Western Region Climate Center website and plot the precipitation for the same period, considering the entire Puget Sound lowlands (see below) using the NOAA/NWS climate division data set and for June through September.  Very similar to the Seattle Urban Site.  Not much overall trend, but there is some natural variability, with a minor peak in the 70s and 80s.

It is also important to note that summer precipitation is relatively low in our region--most our precipitation arrives in four months from late fall to midwinter. Looking at annual precipitation (see below), we find the same story:  modest upward trend in precipitation.

So the claim that summers in our region are drying is simple false.  Busted.

So what about temperature?  Let's examine the maximum temperature trend at the same Seattle Urban location for summer (June through August).  There is a slight upward trend since 1895 by .05F per decade. Virtually nothing. 

 What about the period in which the poor lived (it was planted in 1948)?  As shown below, temperatures actually COOLED during that period.

You get the message, the claim that warming summer temperatures produced by "climate change" somehow killed this pine is simply without support by the facts.

So the bottom line of all this is that the climate record disproves the Seattle Times claim that warming and drying killed that pine tree in the UW arboretum.  There is no factual evidence that climate change ended the 72-year life of that tree.  The fact that a non-native species was planted in a dry location and was not watered in the summer is a more probable explanation.

Why is an important media outlet not checking its facts before publishing such a front page story? Lynda Mapes is an excellent writer, who has done great service describing the natural environment of our region.  Why was she compelled to put a climate change spin on a story about the death of a non-native tree?

Now something personal.  Every time I correct misinformation in the media like this, I get savaged by some "environmentalists" and media.  I am accused of being a denier, a skeptic, an instrument of the oil companies, and stuff I could not repeat in this family friendly blog.  Sometimes it is really hurtful.  Charles Mudede of the Stranger is one of worst of the crowd, calling me "dangerous" and out of my mind (see example below).

A postdoc at the UW testified at the Environment Committee of  the Washington State House saying that I was a contrarian voice.  I spoke to her in person a few days later and asked where my science was wrong--she could not name one thing.  But she told me that my truth telling was "aiding" the deniers.  We agreed to disagree. 

My efforts do not go unnoticed at the UW, with my department chairman and leadership in the UW Climate Impacts Group telling me of "concerns" with my complaints about hyped stories on oyster deaths and snowpack.    One UW professor told me that although what I was saying was true, I needed to keep quiet because I was helping "the skeptics."  Probably not good for my UW career.

I believe scientists must provide society with the straight truth, without hype or exaggeration, and that we must correct false or misleading information in the media.   It is not our role to provide inaccurate information so that society will "do the right thing."   History is full of tragic examples of deceiving the public to promote the "right thing"--such as weapons of mass destruction claims and the Iraq War.

Global warming forced by increasing greenhouse gases is an extraordinarily serious challenge to our species that will require both mitigation (reducing emissions) and adaptation (preparing ourselves to deal with the inevitable changes).  Society can only make the proper decisions if they have scientists' best projections of what will happen in the future, including the uncertainties.

Addendum:  Why Do I Spend More Time Dealing with Exaggerators Rather Than Skeptics

Some folks have complained that I spend more time in the blog correcting "Exaggerators" and "Hypers" than "Deniers" and "Skeptics".   Thus, they suggest I am a closet Denier or Skeptic myself.   Let me explain.  I deal with exaggerators more for two simple reasons:

1.  I live in Seattle, WA.   The media here (e.g., the Seattle Times, The Stranger, etc.), in concert with the left-leaning, environmental sentiments of the region,  overwhelming tend towards exaggeration of the effects of global warming.   Same thing with local politicians.     If they went the other way (saying that global warming is nonsense), I would comment on it.

2.  There are LOTS of scientists that are fact-checking skeptics but extremely few that are dealing with the exaggerators.   There are a number of reasons for this, including the political leanings of many scientists.


How will Northwest Weather Change Under Global Warming?  Help Us Determine the Local Impacts of Climate Change

Society needs to know the regional impacts of climate change and several of us at the UW are trying to provide this information with state-of-the-art high resolution climate modeling.  With Federal funding unavailable, we are experimenting with a community funding to build this effort.  If you want more information or are interested in helping, please go here.  The full link is:    All contributions to the UW are tax deductible.

Thursday, May 11, 2017

Flood Warnings in Eastern Washington

The National Weather Service has flood warnings and advisories out for several rivers in northeastern Washington, northern Idaho, and northern Montana (see map)

The Northwest River Forecast Center's river map (below) shows flood and moderate floods on several ridges over Idaho and northeast Washington, with many rivers at bankfull.    Even the Columbia River is high from Portland to the coast.

Why are the rivers so high?   The main reason has been a massive snowmelt produced by a well above-normal snowpack and warmer than normal temperatures during the past week.

First, the snowpack.   Take a look at the latest SNOTEL totals (percentage of normal).    Much of the Northwest is well above normal---and that is after quite a bit of melt the last week.

And then there was temperature.    Here are the temperatures for Spokane and Seattle (yellow lines) for the past two weeks (red and blue lines are normal highs and lows, respectively).   There was a surge of well above normal temperatures about a week ago (into the 80s in Spokane!) that initiated a surge of melting. And temperatures have been above normal the last few days.
Or take a look at the deviation of the temperature from normal for the past week.  Really warm for much of the Northwest and northern Rockies, which is consistent with large snowmelt.

The surge of warmth (and some rain that came at the end of it), pushed some of the rivers rapidly higher.  Consider the Naches River on the eastern slopes of the Cascades (see below).  Huge jump to above flood stage on May 5-6th.  Even today it is at bankfull and above normal conditions.

The surge of melt water was quite obvious in the total storage of the Yakima Basin (see below)--really zoomed up starting with the warm spell.

Large snowmelt and surging rivers in Spring are not unusual in our region, and was a real problem for Portland and other riverfront cities before the hydro dams were put in.

Perhaps the region's greatest spring flood occurred June 5-6, 1894, when an above-normal snowpack was melted quickly by a heat waves, pushing the flood level to 34.4 feet in Portland,  and destroying the town of Cascades, Oregon. Another great flood in 1948 destroyed the town of Vanport, Oregon.  With a line of dams storing much of the spring snowmelt, flooding is thankfully only a minor nuisance along the Columbia these days.


How will Northwest Weather Change Under Global Warming?  Help Us Determine the Local Impacts of Climate Change

Society needs to know the regional impacts of climate change and several of us at the UW are trying to provide this information with state-of-the-art high resolution climate modeling.  With Federal funding unavailable, we are experimenting with a community funding to build this effort.  If you want more information or are interested in helping, please go here.  The full link is:    All contributions to the UW are tax deductible.

Tuesday, May 9, 2017

The Homeless Crisis in Seattle: Time for a New Approach

I bicycle to work each day along the Burke Gilman Trail, and during the past few years there have been noticeable changes in the "sights" along the path.

Today it is not unusual to see someone sleeping next to the trail in a sleeping bag, with a number of "regulars" living in the adjacent wooded areas.  Last week I had an excellent view of one of them relieving themselves in the open.  A few weeks ago, someone even put up a tent next to the trail behind U. Village and another inhabited tent went up in Mathews Beach Park.   And when I get to my building, a homeless person is frequently sleeping on a grating adjacent to the structure, shielded by a few bushes.  Many of these homeless folks are suffering from some kind of mental affliction, talking to themselves or acting strangely.

When I drive on I5 or 520, I view encampments of homeless at many locations (including the intersection of 520 and Montlake), and a trip to the International District reveals huge numbers of homeless folks sleeping in clusters under and near I5 (see pics).

I can give a dozen other examples, but many of you have experienced the same thing:  the past few years has seen an explosion of homeless in parks, green areas, under highways and overpass, and even some in doorways of downtown buildings.  According to several surveys, there are  now 3000-4000 homeless living outside in pubic spaces.

Many of these individuals are clearly mentally ill or on drugs---disheveled, incoherent, dirty, and talking to themselves in unusual ways.  Dozens have died during the past year, passing away in the cold and wet.

It is profoundly unethical and immoral to allow this situation to continue. How can any civilized city accept a status quo in which a homeless individuals, roughly half mentally ill or addicted, must live on the street like animals, many mired in filth and violence.   Some have served our country, living with the demons of PTSD. Others are folks whose luck has run out or have experienced some tragic situation.  It is wrong.

This deteriorating situation is disgraceful for a rich and vibrant city.  It is a sign of failure of our current city leadership and in ourselves for letting the situation fester so long.  When I show out-of-town visitors our city, they shake their heads in disbelieve that Seattle has allowed things to get so bad.

The City of Seattle and the various agencies/groups dealing with the problem are clearly losing the battle.  They are not effective as noted in several reports.  Stop-gap measures like roving "tent cities" not only don't work but they leave the homeless in the cold, wet Seattle winter.  Homeless encampment sweeps, a signature approach of the current mayor, move the homeless from one location to another rather than provide a long-term solution

Recently, Paul Allen generously offered 30 million dollars to help house some homeless, with Seattle offering a match of 5 million. But supposedly this $35 million dollars was only enough to shelter a few dozen people (if we assume 3 dozen, that would be nearly a million dollars per person). Something is very wrong with the way money for homeless is spent.  Every year, Seattle spends around $50 million dollars on homeless services, but the situation continues to degrade.

The homeless crisis is, of course, not limited to Seattle, with nearby Portland and San Francisco having major problems.   And we should not forgot some of the major drivers of homelessness, such as the misguided reduction of live-in mental health facilities around the nation during the Reagan administration, without restoration by subsequent governments.

Another Approach

After reading quite a bit about this topic, I have concluded there is only one viable approach to dealing with Seattle's homeless crisis, one that encompasses two steps:

1.  Build large amounts of very low cost housing so that every individual has a bed in a warm place with complete protection from the elements, bathroom and washing facilities, access to basic food, the availability of medical and mental heath clinics, and on-site workforce counseling.  We are talking about thousands of units.

2.   Make it unlawful for any individual to sleep in public places such as under roadways or bridges, on sidewalks, parks, or other outdoor spaces.   And enforce this ban with sufficient resources to make it effective, once sufficient housing is available.

This is not pie in the sky.   The City of Seattle spent 100K on an evaluation of it expenditures and organization regarding homeless services.   The findings were clear:  the city's approach is disorganized and wasteful, and that a more logical approach (Housing First) could rapidly take homeless off the street.

Having scattered facilities around the city is very ineffective and costly; centralization will cut costs.  As shown in several cities around the country, it is far cheaper to get homeless off the streets and properly cared for than having them end up taking expensive police time or in hospital emergency rooms.

Most importantly, providing basic housing and a stabilizing environment has shown to be effective at the ultimate goal: rehabilitating the homeless back into society.

The kind of housing I am talking about would not be luxurious:  high-density housing or very inexpensive modular units.  Perhaps in industrial south Seattle not far from a light rail line or bus service.  But it would be humane and responsive to the substantial needs of this population.

With the money already available, many hundreds or thousands of units could be acquired or constructed.

A New Mayor, A New Start

Seattle will soon have a new mayor.

Can you imagine the joy of Seattle citizens if the new leader would say enough is enough and end Seattle's homeless crisis, following the approach outlined above?

Of if our new mayor would stop wasting time and money on meaningless PR efforts (like the disastrous Pronto bikes) and work on improving Seattle's potholed streets?   Or fix the rough bike trails and create a safe, protected way to bike into and out of the city?

Or if the new mayor would apply creative ways (and non-creative ones) to lessen the crippling traffic of our city?  Yes, even put park and rides where needed and reject "road diets" where they would create massive traffic?

A mayor that would make education a city priority, with close cooperation with the Seattle School District and other groups to improve math and science eductation?

A new mayor that would accommodate the economic boom while ensuring appropriate housing developments to main affordability.

A more moderate mayor that would not be playing groups against each other, but try to bring the city together to deal with the acute infrastructure and other challenges we face?

Seattle has so much potential.  We just need better leadership.

Sunday, May 7, 2017

Wind Shear: When the Atmospheric Seems to be Tearing Itself Apart

Sometimes the atmosphere seems to be acting strangely, with the clouds at one level moving very differently from neighbors above or below.  As described below, the change of horizontal wind with height, known as wind shear (or vertical wind shear), can produce spectacular contrasts in cloud behavior, is sometimes associated with severe thunderstorms, and can endanger aircraft.

There is no better way to be impressed with wind shear than through videos of cloud fields, and who better to provide the imagery than weather cam maestro, Greg Johnson of Skunk Bay Weather (who was spotlighted by Eric Lacitus of the Seattle Times here).

Check out this video, which shows sequences in which clouds, only a few thousand feet apart in the vertical, are moving in nearly opposite directions!

OK, now you are ready for a bit of wind shear 101.    Vertical wind shear can occur in three ways.  One is pure speed shear, where the wind speed changes with height  (see illustration below).  The other is pure directional shear, where the wind direction changes with height.  Finally, there is a combination of both, which is the most frequent situation in the atmosphere.

Now wind shear is very typical near the surface, since the rough surface (with trees, hills, building, and even ocean waves) slows down the wind.  Thus, wind speed generally increases with height above the surface.  The drag from the rough surface also tends to cause a deviation in wind direction, with a component towards lower pressure near the surface.   This variation of wind with height due to surface drag is visualized as the Ekman spiral (see below).

If you ever want to REALLY be impressed with wind shear, go out on a field on a windy day and lie down on the ground.... MUCH less wind near the surface.  You will be amazed.

Terrain can produce lots of wind shear, particularly in the lower atmosphere.  Here in western Washington, our mountains greatly influences the wind field-- blocking incoming flow, causing large whirls and eddies in the lee of barriers such as the Olympics, producing gap flows in narrow troughs in the mountains, and much more.

For example, on Friday evening at 8 PM, there was west-northwesterly flow on the coast, but northerly flow over northern Puget Sound and southwesterly flow over the south Sound--the result was one of our favorite local weather features:  the Puget Sound Convergence Zone.   But aloft the winds approaching our region from off the Pacific Ocean were westerly near the surface, turning
to southwesterly and then southerly aloft--something we know because of the radiosonde (balloon launched weather observations) observation at 5 PM at Forks, on the WA coast (see below)

Wind shear can have another origin:  from horizontal temperature gradients.  When temperature changes horizontally, it causes a tilt of pressure surfaces with height, resulting a changes in winds with height.   For example, because temperature cools to the north in the midlatitudes, there are increasing westerly winds with height.   There is an equation that describes this effect:  the thermal wind equation.

Let me demonstrate this to you.  Here is the forecast wind speed in the upper troposphere (300 hPa, around 30,000 ft above sea level) for 8 PM on April 17th.  The yellow color  shows strong winds speed heading into northern CA.
 And here is an analysis of lower atmosphere temperatures, with warm being red/orange and cool being greens and blues.)  The strong winds (the jet stream) are associated with large horizontal temperature changes (or gradients).

Finally, wind shear can be produced by the outflows of thunderstorms, as cool, moist flow rushed out from the thunderstorm cell below warm air coming from a different direction

So wind changes with height have a variety of causes, but they can produce extraordinarily interesting cloud images and videos, with perhaps the most dramatic being the effects of wind shear on falling precipitation from high cirrus clouds.  Such fallstreaks are often curved and delicate.

Finally, wind shear is a big issue for aviation.  When wind shear is large aloft, light to strong turbulence can occur.   And wind shear near the surface, usually produced by thunderstorms, can be a severe threat to planes taking off and landing.


How will Northwest Weather Change Under Global Warming?  Help Us Determine the Local Impacts of Climate Change

Society needs to know the regional impacts of climate change and several of us at the UW are trying to provide this information with state-of-the-art high resolution climate modeling.  With Federal funding unavailable, we are experimenting with a community funding to build this effort.  If you want more information or are interested in helping, please go here.  The full link is:    All contributions to the UW are tax deductible.