Hijacking the Winds of Change

Guest post by Jim Steele

published in the Pacifica Tribune February 25, 2020

What’s Natural


Low-tech weathervanes have provided farmers with sage weather advice. If winds were coming from the north, temperatures would be colder than normal; if from the south, temperatures would be warmer. Most fascinating, if winds descended down a mountain slope, they could expect fluctuating extreme weather, with temperatures bouncing between extreme warmth and cold. Across the globe, local downslope winds cause dramatic weather changes and so are given special names such as Chinooks or Foehn winds.

In the western USA, warm dry winter winds descending from the Rocky Mountains are called Chinooks. Because chinooks can melt a foot of snow in one day, Native American Blackfeet people called chinooks “snow-eaters”. Long before it became fashionable to blame warm events on CO2 global warming, standard physics explained extreme warming events. When moist winds are forced over a mountain range, the water vapor condenses at higher elevations releasing precipitation as well as latent heat that warms the air. Then as the winds descend, the air further warms by 5.5°F for every 1000-foot drop in elevation.

For a historical perspective, read the peer-reviewed account of the “Battle of the Chinook Wind at Havre, Montana”. In December 1933 the onset of Chinook winds raised temperatures 27°F in just 5 minutes, and over the next 36 hours temperatures rose by 53°F. When the Chinooks relaxed, typical cold winter air returned, and temperatures fell 40°F in just 2 hours. Montana is a local hot spot for extreme Chinooks. The world record for the greatest warming in 24 hours happened January 1972, just 60 miles away from Havre, as temperatures jumped 103°F (−54 °F to 49 °F).


Such dramatic warming in winter seems unbelievable, but the laws of physics steadfastly state increasing pressure increases temperature without adding heat. As air moves down slope and compresses, it warms. Amazingly, tribes from Borneo to the Philippines beneficially applied the physics of warming long ago. They started fires by rapidly compressing air in a tube. Many modern backpackers carry a similar device called a “fire piston”.

When downslope winds warm and dry the air, they also drive major wildfires. In southern California these periodic wind events are called the Santa Anas, or the Diablo winds in northern California. These rapidly warming wind events dry out grasses and twigs in just a few hours making them easily ignited even in winter. Simultaneously those winds fan the flames, rapidly spreading the fire. A 1960s government report warned those winds made California vulnerable to fire all year long.

In the Swiss Alps, these downslope winds are called foehn winds. And similarly, due to foehn winds fire season in some Swiss valleys peak in the cooler months of March and April. In southeastern Australia where bushfires recently devastated the land, large fires are more likely downslope of the local “Australian Alps” due to foehn-like winds. When human ignitions and poorly managed forests coincide with foehn events, deadly conflagrations ensue.

Antarctica doesn’t experience wildfires, but foehn winds bring extreme temperatures and melt ice. Unlike most of Antarctica, the Esperanza weather station is uniquely situated between cold winds blowing from the continent that battle warm subtropical winds from the north. It is also located in a regional hot spot for foehn winds. On February 6, 2020, a foehn wind raised Esperanza’s normal 32°F summer temperatures to 64.9°F in just 6 hours. A record for the continent but much below nearby Signy Island’s 1982 record of 67.6°F, also driven by foehn winds. Twelve hours after the foehn wind relaxed, the cold winds returned dropping Esperanza’s temperatures back to its normal 32°F. Similar foehn wind events are implicated in the collapse of that region’s Larsen B ice shelf.


Climate change requires 30+ years to detect, but the sharp spike in Antarctic warming came and went over a period of hours. But talking heads mistakenly blamed the warm event on climate change, despite the region’s 2 decade long cooling trend. MSNBC interviewed New York Times reporter Kendra Pierre-Louis, who claimed Esperanza’s extreme temperature verified that climate “models are right” so we must “rein in our greenhouse gas emissions”. But nobody ever mentioned the warming was a natural foehn event. Nor did they mention that the only way to prevent such dramatic warming from foehn events, would require leveling all the mountains of the world. Looking more ill-informed, when Kendra stated this event was “not great for the animals that live in Antarctica”, MSNBC flashed a photo of polar bears.

Instead of informing the public about the science behind these amazing wind effects, they’ve hijacked natural warm weather events to create sensational and misleading climate crisis stories. The public should demand more rigorous scientific reporting!

Jim Steele is Director emeritus of San Francisco State’s Sierra Nevada Field Campus and authored Landscapes and Cycles: An Environmentalist’s Journey to Climate Skepticism

116 thoughts on “Hijacking the Winds of Change

  1. I fear that H0mo Retardus has almost achieved evolutionary status, as evidenced by that MNSBC interview.
    (It’s an inescapable result of the “Anthropocene”)

    • I would have said Homo Cretinus which is evolutionary adaptation caused by the complete absence of science education. Very common among UK politicians and UK mainstream media reporters especially the BBC.

        • And when better informed and wiser people call them on their idiocy they just get angry.
          Dunning Krueger combined with massive self belief and entitlement.

    • Lower and lower “qualified” individuals seem to be occupying visible positions not only in media today (as evidence above) but also in government on account of the same low caliber of individual performing the “check” role (re: checks and balance) on said govt …

  2. Thank you Jim. I was looking for a rational explanation for this event widely misreported here by Australia’s activist publicly funded ABC and SBS networks.

    • ditto from me.

      I thought it must be a load of crap just from who was ‘reporting’ it but had not considered the how and why.

      As it happens, I have a 20 yo daughter who is full on green-woke etc and who gave me a climate alarmist book for Christmas. Out of respect for my otherwise intelligent child I have been grinding my way through the thing and taking some notes as to my upcoming, very long epistle to her explaining precisely what a load of tosh so much of it is. This episode will feature a paragraph or two thanks to Jim.

      • Are you managing to have civil discussions with your youngster KK ?
        I can’t even pose an incontroversial question to mine without getting a factless rant in response 😦

        • The 170(ish) year public education experiment of child concentration day camps has failed. If you care about your children’s future, you should consider removing them from the propaganda source.

      • Just a word of caution from personal experience. Many years ago, just after I had finished my masters in biology, my wife and I were invited to my older brother’s house for Thanksgiving. Having been away for 4 years of military service, followed by 5 years of school, I had no idea that he had gotten deeply involved in a fundamentalist sect enamored with creation “science”. After dinner, he handed me a short booklet that was obviously about creation science, asked me to read it and give him my opinion of it. I told him I’d rather not, as I didn’t want to be disagreeable. He insisted, stating he valued my biologically-educated opinion. So, I read it and found its basic premise faulty in its reliance on a faulty use of thermodynamic principles. After finishing it, he pushed me hard for an opinion and I told him the entire booklet was faulty logic because it was based upon a faulty initial premise. An emotional lecture followed in which I was asked if I “believe” in evolution, to which I retorted that evolution was a basic tenet of modern biology. The figurative explosion that followed included calling me a “tool of Satan” and that I was “going to hell” and was swiftly escorted from his home. We didn’t speak for more than ten years and, to this day, I refuse to engage in any conversation with him concerning anything of any consequence. Take heed! If someone has been brainwashed in climate alarmism, it is likely to be equally as consequential.

  3. In large swaths of the Northwest, a “Chinook wind” is simply a winter wind that causes the snow to sublimate away. It has little to do with descending mountainsides or warming.

    In fact Chinooks are often most noticeable in the coldest parts of winter, when cold wind causes the snow to sublimate yet it’s far below 0ºC.

    • A Chinook was originally a warm, wet wind from the Pacific, such as the Pineapple Express, which both melts snow and adds rain. Recent flooding in my native Umatilla County is a classic example.

      The opposite was a Walla Walla, a cold dry wind from the east, such as those which snap ice-laden winds in Portland in winter, aided by the Venturi tube of the Columbia Gorge.

      Chinooks lived along the lower Columbia, but their trade jargon was the lingua franca of Pacific NW tribes and settlers even into the 20th century.

    • Lonny, I am not sure why my reply did not get posted, but you are incorrect. FOr a hundred years Chinooks have referred to downslope foehn winds. Please read the links in the article

      • Nope. You’ve conflated the Blackfoot term for the mountain falling winds with the origin of the term “Chinook”, which was picked up by settlers on the Great Plains and in the Rockies from previous settlers in the NW. The Blackfoot didn’t call the foehn a Chinook.

        This says first use in print was 1860, but in fact it was in common parlance in the region at least by 1852, when my ancestors arrived, and probably from fur trading days.


        • So you could say that Rocky Mt. state and province settlers “hijacked” an existing BC/OR/WA/ID/western MT term to describe a different phenomenon.

          But great article. I’d also not have restricted “föhn” to Switzerland. It’s used throughout the German-speaking Alps. I first heard it in Austria in 1973. Another minor quibble.

          • Different phenomenon but with same result, ie melting snow. Flooding is worse in the NW however because of accompanying rain. Famous Vanport Flood of 1948 a good example.

            In case any readers are unfamiliar with Western American history, the fur trading era was roughly 1790, when CPT Grey discovered the Columbia, to 1840, the year of the last rendezvous, when beaver hats went out of fashion.

          • In Boulder, Colorado the downslope winds can reach 90 to 100 mph, and are sometimes called mountainados. Back in the 1970s when I worked for NOAA and lived in Boulder, our next door neighbor’s roof was blown off and landed in our back yard. During another downslope wind event, a spindly, easily bendable aluminum filament from the same neighbor’s TV antenna came right through the wall of our house.

          • And to add a little bit of “tripe & piffle” ……

            In case any readers are unfamiliar with Western American history, the fur trading era was roughly 1790, when CPT Grey discovered the Columbia (River),

            But don’t forget, ….. the Russian fur traders arrived in the Pacific Northwest (Alaska and BC) in 1784.

            In 1784, Grigory Ivanovich Shelikhov arrived in Three Saints Bay on Kodiak Island, operating the Shelikhov-Golikov Company.

            “HA”, first fur trader in the Northwest …… and first cosmonaut in space. ☹

          • Samuel, I’ll one up your “tripe & piffle”. I don’t know anyone in the Northwest that considers Alaska as part of the Northwest so from that perspective the Russians were not the first fur traders in the Northwest….

        • John, Assuming you are addressing my last post,

          I never claimed the Blackfeet people who live on the east side of the Rockies called those winds Chinooks. I only said they called those winds “snow eaters”.

          However I do claim that the term Chinook has been used for the past 100 years to describe the warn dry foehn winds. That is exemplified by the report I linked to in the “Battle of the Chinook Wind at Havre, Montana” published in the Monthly Weather Review.

          Although it is of some historical interest to quibble about the origins of the word Chinook winds it is irrelevant for this article. For modern day purposes, Chinooks now refer to foeh winds, as illustrated by the AccuWether illustration in the article.

    • Total nonsense, Lonny. Ranchers in Alberta Canada could ride a horse in and out of temperatures below zero and into warm air well above zero at the boundaries of chinooks.

      Google it for goodness sakes. You have the worlds library at your fingertips! This site isn’t kind to people that choose to remain ignorant. Checkout the woke- marxist wikipedia if you don’t trust non lefty sites. Forget the “talking points” and do a little research on your own

  4. I’m sorry, but you have the laws of physics wrong.
    R and n are constants, but P, V and T are variables.
    You can play around by holding one variable constant.
    But that play is not the law.

    • PV = nRT is true for an ideal gas.

      The Earth’s atmosphere, when considered as a whole, is far from an ideal gas. The mere presence of water vapor will ruin the assumption of an ideal gas all by itself.

      • Actually, not exactly true, Tim. The atmosphere from sea level pressures to below one-tenth sea level pressures obeys the ideal gas law remarkably well, even at saturation pressures for water vapor…

  5. Some years back a friend who lived a few miles from the Ontario (ONT) airport in Southern CA told me of the time he drove from his house in Corona, CA to near ONT and the temp difference was nearly 30 deg in about 15 miles because of the Santa Ana winds. Another time he told me he drove in the same day from near Mammoth Lakes CA where it was about 15f below zero and near ONT it was pushing 100 deg f during an especially strong Santa Ana wind event.

    It’s surprising he survived those wild temp differences. /sarc

  6. “Chinook Winds”. “Indian Summer” (in certain parts of North America. Not a PC term anymore but it referred to the warming after the first frost.). “Nor’Easter” in New England, “Santa Anna Winds”, “Pineapple Express”, etc. etc.
    Just how many seasonal and natural events have happened long enough for the locals to give them a name?
    How many of these events have been twisted or hyped by the “Climate Change” crowd to be something “new” or “different now” than before?

      • I couldn’t do it but perhaps it would be a worthy endeavor for someone to catalog such things to counter the hype promoted by such as “The Storm Channel” (formerly and really about “The Weather Channel”)?
        I mean, before cell phone videos, it did actually rain or snow in places where they imply “It never rained like this before!” when it actually did. Often enough that the locals gave the recurring event a name.
        (Of course I’m not talking about specific events that earned a name such as “The Blizzard of ’78” or the “’37 Flood” in the Mid-West but recurring, seasonal events that have a name.)

  7. The article is great in explaining why descending air warms, but the drying also needs explaining: it dries because the RELATIVE humidity of air decreases as it warms, thus drying out the air without losing any moisture.

    • Andre you are absolutely correct. However because I was writing a 800 word local newspaper article for the general public, my experience is the denser the science, the more the public is overwhelmed and stops reading. I get great feedback from my local community. I once had people tell me they lost me when I used the word “anthropogenic”.

      So instead of diving into how warming changes relative humidity, a term most people are unsure of, I chose to just describe Chinooks and Foehn winds as warm and dry.

      • Jim, you have nailed the reason why the alarmist narrative has been so successful in frightening people. As an engineer I understand the science and can see the great logical chasms in the physiscs, the chemistry, the thermodynamics of the alarmist drivel let alone the paedophile like abuse of statistical methods. Large swathe of our communities have little or no science and maths education and are deaf, mute and blind to such red flags signalling the climate fraud and are as susceptible to the ravings of the alarmists as our ancient ancestors were to the ravings of shamans, druids and other ‘holy men’ from time immemorial.

          • Jim,

            No, they don’t.

            The BBC has for some reason decided that “swathes” sounds better than “swaths”, but they’re just as wrong as you are re. Chinook winds.

            In both American, British and all other forms of English, “swath” does not mean the same as “swathe”. It would have taken you no time at all to look it up, if, as seems the case, you didn’t already know. Which as a land scientist you should have. The BBC is too far removed from the land to understand the difference, but you should know better. The distinction goes deep into Old English.


            I guess you never drove a swather.

            It’s not a nitpick, but a vital distinction. You’re welcome for the education.

          • Sigh, with eye-rolling.

            Please read the definition.

            It has nothing to do with a swath of land, but with how to spell a verb.

            I know you’re smarter than this. Clearly your unscientific inability to admit error has made you grasp at blatantly false straws.

            When you’re in a hole, quit digging.

          • Jim,

            There might be English dialects in which a swathe is the same as a swath with a scythe, but in all normal English, it’s a swath, while to swathe is a verb meaning to wrap.

            Hence, the modern usage “swather”. “Swathe” is not a standard noun in any form of English. But for some reason, BBC presenters have adopted it.

          • My OED says that pronouncing the noun “swath” as “swathe” is an archaic northern counties dialectical variation. But in standard British and American English, “swath” is a noun referring to land, based on a scythe swipe, while “swathe” is a verb meaning to wrap, as per the definition I posted.

            Sorry that I can’t post a link to on line OED, since I’m not a subscriber.

          • John Tillman. I looked up in my micrographic paper of the OED, which I bought in 1974, in which historical usages are given. Their references include spellings as swath and swathe for each of the meanings to which you refer. Fowler commented disapprovingly on how, in the 19th century, grammarians created restrictive rules that were not necessarily a reflection of what good writers of English had previously done. Perhaps this is another such example?

          • From my dictionary, and from memory from growing up, a Swath is the line of grass or corn left by one scythe swing. I used to be quite good at using a scythe.
            To Swathe something is to wrap or bandage it.

          • Not wishing to fan the flames. The Cambridge Dictionary says swathe
            (also swath)
            Def a long strip or large area especially of land:

            Swath Def a strip or belt, or a long area of something:

            I’ve always used swathe, but I was educated in Scotland

      • When I was thinking about the falling air gaining energy, the first thing that came into my mind was Wile E Coyote realizing he was about to be crushed by an anvil.

        I’m not sure it would work in this case, but people understand concepts like bricks falling on their heads from a dizzying height. 🙂 example: “A brick falling from the top of that building would have about the same energy.”

      • Andre Den Tandt – February 26, 2020 at 2:28 pm “ but the drying also needs explaining: it dries because the RELATIVE humidity of air decreases as it warms, thus drying out the air without losing any moisture.

        Jim Steele – February 26, 2020 at 2:45 pm “Andre you are absolutely correct.

        Now boys, being the persnickety cuss that I am, I can‘t let ya get by with that.

        Humid air doesn’t “dry out” when it expands, …… it’s H2O vapor ppm simply decreases due to its expansion. The same amount of H2O vapor is still there, but in a larger volume of air.

        And when humid air warms and rises, ……. cooler dry air may, can, will …. flow in to prevent a vacuum.

        • Samuel, In sense it does “dry” out. Warmer air can hold more water vapor, and thus with a given amount of water vapor the relative humidity decreases as air temperature increases.

          However because relative humidity is a relative term, fire and drought researchers now prefer to use the concept of “vapor pressure deficit”. At a given temperature, the air can hold x amount of water vapor. Any amount less is considered a deficit. The larger the vapor deficit the larger the drying effect on the vegetation and soils.

          • Jim, yup, as the air warms, …… it “feels like” it is drying out, ….. in an “open” systen only.

  8. Regarding 2 decades of cooling trend in the Antarctica Peninsula: The cooling started after the late 1990s according to the linked article, which was published in 2016. That article is Turner et al..2016.

    In a previous WUWT article by Jim Steele, https://wattsupwiththat.com/2020/02/09/medias-horribly-dishonest-antarctica-propaganda/ I got linked to
    https://www.sciencedirect.com/science/article/pii/S0048969716327152 which is Turner et al. 2016. There, I saw the cooling trend mentioned as 1999-2014 and shortly after that being mentioned as initiating in 1998 or 1999. This is less than 2 decades. And the trend there through the the UAH reporting of satellite-measured lower troposphere temperature (starting at the end of 1978) is warming.

    • The first climate change refugees! They really have to change their name to MSLSD seeing as all the onair talent is hallucinating!
      Thank you, Jim, for an interesting and informative post. I am much troubled that thou wert harried by pickers of nits; who knew that languages evolve!

    • Not all them.
      According to “Sherman’s Lagoon”, a polar bear named Thornton’s iceberg melted somewhere in the tropics.
      (He likes it there.) 😎

    • These are the same winds that cross the Southern Alps in New Zealand as well as the dividing range of the North Island. Technically we call them fohn winds, but locals on the eastern side of both islands refer to them as “Nor’ Westers.” Often very strong and gusty as they move in ahead of an approaching cold front. They have been responsible for NZ’s hottest ever recorded temperature some 48 years ago. Feb 7th 1973 Rangiora just out of Christchurch hit 42.2C. The approaching front acted as a conveyor of heat reaching all the way back in to the heart of the Australian continent! Interesting that as NZ is considered a part of “Oceania,” which includes all of the islands of the south Pacific, then that temperature record is also the highest for Oceania! Not what most would expect, where most would think the record would reside in the tropics and not at 43 South!

  9. “anthropogenic”.
    Obviously those readers who got lost never took one of those “gender studies” classes they now have.
    Or maybe they did and wonder what’s now missing?
    Or maybe I should quit while I’m behind? 😎

  10. New York Times reporter Kendra Pierre-Louis…claimed Esperanza’s extreme temperature verified that climate “models are right”

    I guess Kendra Pierre-Louis never looked at this graph of models versus measurements from the IPCC AR5 report in 2013. But when was doing research ever important to propagandists reporters today?

  11. Thank you very much for this wonderful post
    One of the best ever on wuwt that I have read
    i used to live in calgary alberta where what’s his name the movie actor guy had declared a chinook event as a climate change event
    Khobp khun khraap

    • Yet again, you fail to make the simple distinction between noun and verb.

      In standard British English, as I’ve repeatedly shown, the correct spelling of “swath” is “swath”, as in every other form of English.

      • Bear in mind John.

        your persistent irrelevant quibbling is totally annoying, meaningless and tiresome. I really do not care anymore what you have to say about linguistics. It is totally meaningless in the context of a weather dynamics discussion. Your linguistic obsession is way too weird, and I will no longer respond.

        • Sorry to learn that you find reality weird.

          That can’t help with selling CACA skepticism, but more power to you just the same.

          You’ve already lost 15 million Pacific Northwesterners who say that our snow has been Chinooked off by the Pineapple Express. Compared to that, misuse of “swath” pales.

        • I agree, and besides he is giving the name of John a bad reputation.

          Thanks, Jim S.
          I always enjoy your essays.

        • Careful, Jim.
          John might hit with a swatch! (Or should that be a schwach? or a swatch (of cloth) or a switch or a Sasquatch or a …)

          (John, time to give it rest.)

        • I also went to grad school in England in the 70s (two, actually), plus a first degree, and infants, primary and grammar school before that, and have lived there ever since. Normal British usage for the noun is definitely swathe, not swath. (I can’t even guess how to pronounce ‘swath’, having never heard it spoken in my 67 years.)
          I cannot believe you are trying to impose your idiolect (a very apposite word, I think) on other people, given that people have given you several references showing that the noun is at least optionally (and in my experience exclusively) swathe in Britain. Your linguistic experience does not define the language.

        • Your use of ‘evidence’ is astonishing. You reply to Jim’s link (that shows that swathe is a British English noun) with another link, and arrogantly tell him he’s way out of his depth. Presumably you assume that nobody will actually follow your link – because if they do, they will find another reference showing that swathe is a British English noun! Similarly, your triumphal attitude when linking to that agricultural machinery website is presumably designed to give the impression that it supports your case, when in fact it uses the word swather not swath. You might read that as a piece of machinery that creates a swath, but you have zero evidence that the person who wrote it didn’t think of it as meaning a piece of machinery that creates a swathe. (And even if you are right about their intentions, that doesn’t show that their choice was forced by the rules of the language, rather than just happening to agree with your idiolect rather than mine.)
          I can’t decide whether you genuinely don’t understand that a piece of evidence that is consistent with your beliefs is not, ipso facto, evidence against someone you disagree with, or whether you are deliberately trolling to disrupt this highly valuable site. Is the deficit in logic, or honour?

  12. It has become a thing to trash ill informed celebrities but this fits right into this whole story. Leonardo DiCaprio was filming a movie just outside of Calgary Alberta which experiences numerous Chinooks every winter. So when a Chinook rolled in, he had to attribute it to global warming. Everyone in Calgary made fun of his obvious stupidity. https://nationalpost.com/news/canada/leonardo-dicaprio-witnesses-a-terrifying-sign-of-climate-change-in-calgary-a-chinook

  13. I remember sitting in the living room of our home in the Fairbanks Alaska area with the outside temp below -40. In Interior Alaska, when it’s that cold, there is no wind whatsoever. Suddenly we would become aware from the sounds that a significant wind is blowing, the temperature would go up to around 40 above or even warmer in a few minutes. The winds would blow for anywhere from a few hours to a couple of days. Snow would be melting rapidly. The wind would then quit as quickly as it started and within very short time we’d be back to -40. It was always amazing an amazing thing to behold.

  14. 1) PV=mRT (where P=pressure, V=volume, m=mass of air, R=gas constant, T= temperature)
    Pressure = Load / Area
    2) Pressure = Mass x gravity / length x breadth (or M x g / l x b)
    3) Volume = length x breadth x height (or V = l x b x h)

    substituting 2 & 3 in equation 1

    (m x g) / (l x b) x (l x b x h) = m x R x T


    m x g x h = m x R x T
    or g x h = R x T
    T = (g x h / R)

    Therefore Temperature is a function of height (or altitude)

    • Or put another way Gravitational potential energy becomes kinetic energy. But there is another component, it rains on the west side thus releasing latent heat of evaporation.

  15. The 2017 Tubbs fire in Napa ans Sonoma was a classic example of this. I was there. Winds over the range separating Napa and Sonoma Counties hit 86 mph in the middle of the night. With little warning, a big chunk of Santa Rosa disappeared.

  16. [QUOTE FROM ARTICLE]”Such dramatic warming in winter seems unbelievable, but the laws of physics steadfastly state increasing pressure increases temperature without adding heat. As air moves down slope and compresses, it warms.”

    Actually, the first law of thermodynamics states that for any system, dU = Q – W, change in internal energy = heat in minus work done by the system. For a downsloping wind, the wind is doing work on the air in the downwind valley, so that the work done is negative, and even without any heat transfer (Q = 0), the change in internal energy is positive, meaning the temperature increases.

    Of course, there must be some driving force (such as a nearby storm) causing the wind to blow up a mountain range and down the other side. On the upwind side of the mountains, moist air blown up the mountainside toward the low-pressure summits expands and cools, and if the colder air is saturated in moisture, it can precipitate out as rain or snow. On the downwind side, the air near the summit may be nearly saturated, but as it descends and warms, the same absolute humidity may be very low relative humidity, so the sky tends to be clear over the downwind side. If a foehn wind occurs during daylight hours, there is further warming from solar heating.

    I’ve seen foehn winds create sharp temperature differences within 50 km in France. The city of Gap, in southern France, is at the western end of a long valley running east-west, with high mountains to the north. I was there in late January, and there was a snowstorm over the northern slopes of the mountains up to the summit, on a northerly wind. Shortly after driving through the pass toward the south, the snow stopped, and there were breaks in the clouds, and by the time I arrived in Gap, the weather was sunny and about 65 F, under a clear blue sky throughout the bottom of the valley. People living along the French Riviera call this wind “le mistral qui balaye le ciel”, or “the north wind which sweeps the sky [clear]”.

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