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Why is it colder over even a small highland?

  1. Apr 4, 2018 #1

    DaveC426913

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    I'm not talking about the vertical difference in temps at altitude, such as at ground level versus at 1000 ft; I'm talking about where the land rises. So, that even at local ground level it's colder.

    I was watching a news report that indicated rain all across our province, except where the highlands are (Dundalk area, NE of London ON), and there it was snow.

    I get that air masses move across the land, and that the rise in ground causes the air mass to rise. Why does it cool?

    Does it really come down to adiabatic cooling? I know that, when air expands, it cools as per the Ideal Gas Laws. Is the expansion due to the the air rising a mere few hundred feet (from sea level) and - for no other reason than because it is less squished by the weight of air around and above it - it can expand?

    Is the highlighted part the primary mechanism?
     
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  3. Apr 4, 2018 #2

    russ_watters

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    Yes, assuming the amount of that cooling (5.3F/1000ft) is what you were seeing.
    https://en.wikipedia.org/wiki/Lapse_rate#Dry_adiabatic_lapse_rate
     
  4. Apr 4, 2018 #3

    Baluncore

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    It all comes down to rate of heat loss.
    The sky on a clear night is often very cold, sometimes –50°C, measured with an IR thermometer.
    The ground is much warmer say +10°C and rarely falls below freezing if wet.

    1. Vegetation insulates you to some extent from the sky above. There is less vegetation on the top of a hill.

    2. In a valley you are surrounded by the warm hills and vegetation. On top of a hill you are exposed to the greatest angular area of open sky.

    3. Winds are greater on top of hills. Evaporative cooling is therefore greater.

    The “snow line” is determined by the height at which adiabatic cooling falls below zero degrees.

    Depending on water content the “dew point” and the “cloud base” are both dependent on adiabatic cooling.
     
    Last edited: Apr 4, 2018
  5. Apr 5, 2018 #4

    DaveC426913

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    I addresses that in the OP.
    It does not apply in the case of a rise in ground level.

    Might be.
    In this case, no.

    On a highland, the horizon is imperceptibly lower.We're talking a few hundred feet over miles.

    Yes. And this appears to be the case.
    Just trying to figure out the causes.
     
  6. Apr 5, 2018 #5

    Baluncore

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    You cannot assess which is the principle factor in any particular situation unless you list all the possible factors.

    You did not originally specify only a few hundred feet over miles. Having spent a few nights on top of sharp ridges, 1000 feet above the valley below, I appreciate the difference between a vegetated valley, a rock wall and the open sky. The skyline determines the exposure. It is perceptibly lower when you are on a hill rather than in a valley.

    I agree that on small rises in vegetated areas the horizon can be hard to determine. But the dip of the horizon can be very significant if you are taller, or do not live in flatland. https://aty.sdsu.edu/explain/atmos_refr/dip.html

    There are also counter effects to consider. On still nights, cold air flows downhill and can progressively fill a valley with colder air. That local motion makes a big difference to microclimates and lends a new meaning to the “cold light of dawn”.
     
  7. Apr 5, 2018 #6

    olivermsun

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    Agreed.

    To back up a bit, I think more information about the specific topography and observed cold conditions is needed to provide any useful response.
     
  8. Apr 6, 2018 #7

    DaveC426913

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    I guess I thought I had by talking about highlands, where the ground - and thus the air mass - rises, as opposed to straight vertical altitude.

    But I was looking for the specific answer to the general case of air masses are forced up by geography.

    Anyway, thanks for your answers.
     
  9. Apr 6, 2018 #8

    russ_watters

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    Actually, you can. You have one factor with a clear-cut impact (adiabatic cooling), so if the effect seen matches the prediction, it's almost certainly the principle factor.

    @DaveC426913 do you know the elevation difference and the observed temperature difference?
     
  10. Apr 6, 2018 #9

    olivermsun

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    There's usually more than just adiabatic cooling going on, since the observed lapse rate tends to be significantly less than the dry adiabatic lapse rate.
     
  11. Apr 6, 2018 #10

    russ_watters

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    Sure, there's *always* more going on, but I was a bit perplexed that post #3 didn't mention such a significant factor. And if the observed lapse rate tends to be less, that still supports the idea that the primary factor is the adiabatic lapse rate. In either case, I've never checked it, so I'm interested to. So let's look at a real example:

    According to Google Earth and Wunderground:
    Salt Lake City, Utah is at 4300 ft and 50F.
    20 miles to the east,
    Near Park City, a 9300ft peak is at 31F.

    That's 3.8F/1000ft, about 70% of the predicted lapse rate.

    So from this we can say that the adiabatic lapse rate is likely the principal cause of the cooling, but due to other factors the observed cooling is 30% less than from the adiabatic lapse rate alone.
     
    Last edited: Apr 6, 2018
  12. Apr 6, 2018 #11

    DaveC426913

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    This high point near Dundalk is the highest elevation in S. Ontario at 544m.
    If you zoom in on the map, you can see you have to go several km for it to drop even 15m.
    It is a generally flat land, with barely perceptible rolling hills.

    The temps in greater S. Ontario were around 2C, so it wouldn't have had to drop more than a few degrees to turn the rain into snow.

    http://en-ca.topographic-map.com/places/Ontario-700488/

    dundalk.jpg
    dundalk2.jpg

    dundalk3.jpg

    I walked this road many times back in the 80s.
     
    Last edited: Apr 7, 2018
  13. Apr 7, 2018 #12

    Baluncore

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    It may in some situations support the assertion, but it is paralogical to argue that it proves adiabatic cooling to be the “primary mechanism” in every situation.

    It surprised me that in post #2 you did not acknowledge any of the confounding variables that would in some situations be more important than lapse rate. My; Post #3, considered some of the alternative mechanisms that you had ignored. I acknowledged that adiabatic cooling was important in determining cloud base and snow line, both of which influence microclimate. I did not need to consider adiabatic cooling as your post, #2, had covered only that.

    Answers depend on each individual's interpretation of the question, in the light of received replies. There is a need to agree on a definition of “primary factor” before launching into a criticism of the provision of additional information.

    When predicting the temperature difference between two stations at different altitudes, I first apply the ever-present lapse rate. Then I examine microclimate effects such as differences in vegetation, cloud cover and skyline exposure. When the height difference is only a few hundred feet, over many miles, variation in vegetation and exposure may easily be more important than lapse rate.
     
  14. Apr 7, 2018 #13
    For what it is worth, here is the "planting your veggies" information for southern Ontario.
    http://www.omafra.gov.on.ca/english/crops/facts/climzoneveg.htm
    Based on the last frost free day in spring, and first frost day in autumn.( averages ).

    Dundalk has about 2-3 weeks either side of a growing season ( Zone D & E ) , compared to the longest Zone A at the southern tip ( Windsor ).
    To the north of Dundalk there is the Georgian Bay, to the west Lake Huron, to the south Lake Erie, and to the East Lake Ontario.
    As is said, local climate can vary considerably over short distances.
    For this small area of southern Ontario climate changes due to land topography, elevation, drainage ( ie air - nocturnally cold will flow down valleys and depressed areas ), and the presence of near bodies of water.
    And as is as sure of course latitude.

    Dundalk being located at the high altitude of this small area seems to bear out lower temperatures than the other lower regions, ( which are also closer to the large bodies of water )

    Most likely a lot going on in southern Ontario as seen from the contours wrt microclimates.
     
  15. Apr 9, 2018 #14
    Russ, Let's not confuse the normal lapse rate (3.3°F/1000') with the dry adiabatic lapse rate(5.4°F/1000'). In calculating the effect of mountain elevations on temperature, we use the normal lapse rate. In calculating the heat loss in air forced to rise against the pull of gravity, we use the adiabatic rates. This normal lapse rate, of course, is a world-wide year-long estimated average. Local normal lapse rates will always differ somewhat, as indicated by your example.
     
  16. Apr 9, 2018 #15
    Whether you get rain or snow does not depend upon ground temperatures, but upon the temperatures at which the condensation is occurring in the clouds above and the temperatures of the air through which the precipitation is falling. Quite a bit of rain starts out as snow, and some snow (much, much less) starts out as rain.
     
  17. Apr 9, 2018 #16

    DaveC426913

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    Sure, but the air mass is moving as a unit, approaching from North-West (off L. Huron) and sweeping South-East. Where the land forces it to rise, there will be snow if the initial temp is close enough to zero.
     
  18. Apr 11, 2018 #17
    There are a couple of things going on:
    1. The air mass as a whole is rising as it travels those horizontal miles and climbs those few hundred feet. If ti's raining, it will cool at the lower saturated rate lapse rate. But it's enough to make a difference.

    2. This precipitation is likely starting as snow, and entrains some air as it falls. That falling parcel of air is heating up and melts the snow before it reaches the ground if there is enough distance, or doesn't melt. The act of melting absorbs heat. so the lapse rate won't be the usual one, but will have a greater one in the vertical region of melting snow.

    I've been in the mountains in Alberta where the snowline -- between no snow on the grass and shrubs and nearly totally white -- was as little as 10 vertical feet.
     
  19. Apr 11, 2018 #18

    DaveC426913

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    Right. Lapse rate is a matter of quantifying what it's doing.
    My question was why is it doing it?

    The answer, as seems to be the consensus, is that lapse is primarily due to adiabatic cooling.

    Literally, the air, as it rises, is under decreasing pressure from the column of air above (and around) it. It can expand. By the gas laws, increasing volume causes a drop in temperature.
     
  20. Apr 11, 2018 #19
    Spent more time thinking about this while running. If the observable is snow on the hill, but rain lower down, I am putting more weight on the lapse rate being created by the falling snow.

    I know that when you get caught in a summer shower the temperature drops like a rock. Some of that is that your getting wet with cold water which increases the subjective "it's colder" but standing under an overhang at the start of rain I can experience an abrupt drop in temperature. I've never seen any discussion of the modification of lapse rate during precipitation. I'm probably all wet.
     
  21. Apr 11, 2018 #20

    DaveC426913

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    But is that cause? or effect?

    It is entirely reasonable that the reason the temperature drops like a rock is because the rain is the leading edge of a cold front moving in.

    i.e. cold causes rainfall, as opposed to rainfall causes cold.
     
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