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Why southern Nevada is a desert and Nashville, TN is not a desert?

  1. Dec 25, 2017 #1
    The Mojave Desert is at around 36 degrees North Latitude in southern Nevada. Here is a summary of my understanding of barometric pressure and precipitation on Earth: "Areas with low barometric pressure get a lot of rain. Areas with high barometric pressure get little precipitation. The equator has low barometric pressure and a lot of rain. The North Pole and South Pole are high barometric pressure areas that get little rain. In terms of latitude, the center of the United States of America is at around 40 degrees North Latitude. 40 degrees north Latitude is in between belts of high and low pressure. Therefore, the USA tends to experience alternating periods of rain and sunshine. The portion of the Earth's surface at around 30 degrees latitude has high barometric pressure. Therefore, the portion of the Earth's surface at around 30 degrees Latitude tends to have very little rainfall such as the southwest USA and northern Mexico, the Sahara Desert, the Middle East, etc. The Mojave Desert is at around 36 degrees Latitude. It's my understanding that the Mojave Desert gets so little rainfall because the Mojave Desert is in an area with high barometric pressure."

    Nashville, TN is at approximately 36 degrees North Latitude, just like the Mojave Desert. However, if the Mojave Desert gets so little rainfall because it's in a high barometric pressure zone because the Mojave Desert is at approximately 36 degrees North latitude, why is Nashville, TN not a desert also?
  2. jcsd
  3. Dec 25, 2017 #2


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    The Gulf of Mexico, which is a relatively warm body of water, provides plenty of moisture for precipitation in the middle part of the US and eastward. Off California, the ocean is relatively cold, hence the desert in S. California. The mountain ranges, Sierra Nevada and Rockies, capture a lot of the moisture that would otherwise move eastward.
  4. Dec 25, 2017 #3
    Interesting. That sounds plausible to me. I remember hearing that even in the Summer, surfers in California have to wear "wet suits" in order to surf because the water is so cold.

    Here is my new understanding of this: the water in relatively warm bodies of water tends to evaporate more, which will lead to more precipitation. The water in relatively cold bodies of water tends not to evaporate, so relatively cold bodies of water don't cause much increased precipitation in their vicinity. Therefore, the Gulf of Mexico causes increased precipitation in the middle part of the USA and eastward, which causes Nashville to have a moderate amount of precipitation (as opposed to a low amount of precipitation), despite being in a high barometric pressure zone. Off California, the ocean is relatively cold, hence the desert in S. California"---is my understanding of this correct?

    One more thing. I believe you that the fact that the Pacific Ocean around California is cold is part of the reason that there is a desert in S. California. However, my understanding of this is that the coldness of the Pacific Ocean around Southern California is only part of the reason that S. California is a desert. Isn't it true that the fact that S. California is in a high barometric pressure zone also part of the reason that S. California is a desert?

    I think that Northern California gets a little bit more precipitation than Southern California. And I think that the reason for this is that Northern California is at a higher latitude, and thus, Northern California has lower barometric pressure than Southern California.

    I understand that the prevailing winds are from west to east, so the Rockies would capture a lot of the moisture that would move eastward. I don't really understand the significance of this statement though. Or, at least, I don't understand if this statement helps explain the answer to the topic of this thread. Perhaps the fact that the Rockies capture a lot of the moisture that would otherwise move eastward is both true and also does not answer the topic of this thread. The Rockies capturing a lot of the moisture that would have otherwise moved eastward would seem to me to reduce the precipitation in Nashville, not increase it.
  5. Dec 26, 2017 #4


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    it means that the east side of those mountain ranges is in a rain shadow which produces dry to very dry ( arid) conditions and totally different climatic conditions further east of the ranges keep that eastern region moist

    Astronuc's and my comments completely answer the topic Q

    you didnt read the first sentence of his response did you :wink:

    and therefore the USA southern half of the mid-west and further east get lots of rainfall
    Consider where all the storms occur

  6. Dec 26, 2017 #5

    jim hardy

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    The answer is on your TV weather satellite pictures during the news broadcasts.

    Watch the next few days as the current high pressure moves out

    Here's today's water vapor image which good meteorologists will also show to you on TV's morning and evening news.
    (orange notation mine)
    Watch next few days and you'll see how gulf moisture inundates Eastern US.
    Both maps from weather.com .
    More complete ones at Noaa's GEOS site https://www.star.nesdis.noaa.gov/GOES/index.php

    Rockies is why eastern Colorado and western Nebraska are so dry.

    Dry air has molecular weight 29 while water vapor is just 18 . So dry air is more dense and that's why high pressure is associated with dry weather.

    Use your basics to make sense of things.........

    old jim
    Last edited: Dec 26, 2017
  7. Dec 27, 2017 #6


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    California has a very long coastline. Water temps in the north are considerably cooler than in the southern part of the state. I used to surf back in the 60s in southern California, in south Orange County. In the summer almost no one wore a wet suit there, but they did in the winter months. Although cold water can influence rainfall, this isn't the only factor. The water temperatures off the Washington Coast are much colder than anywhere in California, yet the Olympic Peninsula gets up to 200" of precipitation each year -- hardly a desert.
    As far as I know, there aren't any regions where the barimetric pressure is consistently high.

    I'm reasonably sure latitude and barimetric pressure are unrelated. Where did you get this idea?
  8. Dec 27, 2017 #7

    jim hardy

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  9. Dec 27, 2017 #8
    I told you: At 30 degrees north Latitude, the barometric pressure is usually high. That's why there is a line of deserts through much of the world at 30 degrees North Latitude: Northern Mexico/Southwest USA, the Sahara Desert, the Middle East, Afghanistan, and the deserts of Western China.

    The Barometric pressure is even more consistently high at the North Pole and South Pole than at 30 degrees North Latitude.

    Latitude is related to barometric pressure because Latitude has a relationship with temperature. The ocean is the hottest around the equator. This causes the air molecules above the ocean around the equator to expand upward. Hot air rises. This expansion of the air molecules causes there to be consistently low barometric pressure around the equator. That's why there is a lot of rainfall around the equator.

    Edit: Here a link to a website which supports my contentions: https://www.thoughtco.com/low-and-high-pressure-1434434
    Last edited: Dec 27, 2017
  10. Dec 27, 2017 #9


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    "atmospheric circulation models on an ideal/billiard ball earth; Google "hadley" or "ferrel cells."
  11. Jan 3, 2018 #10
    The Sierras and the Southern California mountain ranges produce a "rain shadow" effect as the weather moves eastward. What happens is, as the air moves, it rises with the terrain of the land. As the altitude increases, both temperature and atmospheric pressure generally decrease. When this happens, the air cannot carry as much water as vapor, so it condenses and falls as rain or snow. Then, on the eastern side of the mountain ranges, as the elevation decreases, the air temperature and atmospheric pressure increase, but the air now carries much less moisture than it had previously, which, as stated, wasn't too much to begin with due to the cold waters of the Pacific Ocean. Basically, the mountain ranges act in a way similar to wringing the water out of a wet sponge. As the air continues eastward, it is influenced by many other factors, the biggest of which is that it picks up a lot of moisture from the warm waters coming off the Gulf of Mexico. This warm, moist air tends to be unstable, which causes rain and storm events in the central and southeast parts of the country.
  12. Jan 4, 2018 #11
    The answers to the original question can be found in almost any textbook on introductory climatology. There are three basic controls of climate: latitude, altitude, and continental position. I think that you will find that the west coast between 30° and 40° is relatively dry compared to the east coast on almost all continents. This is caused by relative cool offshore waters and the prevailing Westerly's. At these latitudes, west coast waters will be some ten degrees Celsius cooler than east coast waters--check any map of sea temperatures.

    In forty years of teaching atmospheric sciences at both the undergraduate and the graduate levels, I cannot recall a single scientific paper relating atmospheric pressure to climate in any significant fashion. Sinking air masses tend to warm adiabatically with a consequent drop in relative humidity, but the decrease in humidity is caused by the warming, not by the increase in pressure. As long as the temperature is constant, humidity (or its inverse, aridity) is not affected by pressure changes.

    The role of altitude as one of the three controls of climate is primarily to reduce temperature with increased elevation. As several posters noted, it also functions to produce orographic precipitation on the windward side and the matching "rain shadows" on the leeward side of mountain barriers to prevailing winds.

    In Post #3, your basic geography is a bit off. It is the Sierra Nevada Mountains that put southern Nevada in a rain shadow, not the Rocky Mountains.
  13. Jan 16, 2018 #12

    jim hardy

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    @bluemoonKY (@ not working again)

    Remember post #5 about the daily weather map ? Here's today's from Weather dot com, with my comments.

    whyTennesseeis wet.jpg

    and the NOAA Geos satellite picture.


    There's a stunningly beautiful lady meteorologist on Weather Channel explaining it right now as i type.

    We're fortunate to have the Gulf of Mexico.

    Thought experiment -
    In your mind's eye imagine a spinning basketball sized globe viewed from the top.
    Pour a viscous liquid like molasses on it right at the top.
    As your liquid flows away from the pole it must acquire tangential velocity to keep up with the globe's surface.
    In our molasses thought experiment the viscous shear probably makes all the force you need to tangentially accelerate the liquid.. But with a continent sized air mass it'll require some pressure differential. Hence that low pressure area along east edge of the high.

    I read about that effect in Dad's "Climate and Man" textbook when i was a kid, before we had satellites. First animated satellite loop i ever saw cemented the concept for me.

    I hope above helps you.

    old jim
    Last edited: Jan 16, 2018
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