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bobbutts

Local climate change article

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Stampone crunched some numbers from the National Weather Service’s official weather station at Concord airport, dating back to 1940, when full data started being recorded. I sometimes crunch numbers for my stories; having a Ph.D. crunch them is much more reassuring.

Stampone found that the number of really hot days per year – the number of 24-hour periods that had a maximum temperature above 90 degrees – didn’t change during this period. The annual average increased by about one day per year, which is not statistically significant.

On the other hand, the number of not-cool nights per year – those where the minimum temperature never fell to 60 degrees or less – did change. Over this period, they have increased on average by 9 nights per year, which is statistically significant, she said.

In case you’re wondering, Concord airport didn’t get down to 60 degrees for a single night during the heat wave. The minimum overnight temperature was at least 61 from June 27 through last Friday.

Stampone also looked at maximum and minimum temperatures each day over the 77-year period during the three months that meteorologists consider to be summer: June, July and August.

“There is no trend in maximum daily temperature from 1940 to 2017, but minimum temperatures have increased by 2.5 degrees Fahrenheit,” she said. “That is a pretty stark difference.”

What causes this difference? It seems to be at least partly based on behavior of the atmospheric boundary layer, the air that is most changed by reflected heat from the ground.

The boundary layer shrinks at night as temperatures fall, going from kilometers high during the day to a few hundred meters at night.

Greenhouse gases are warming the planet by preventing the escape of heat, which doesn’t change much from daytime to nighttime.

Because the volume of atmospheric gases is much smaller at night, this greenhouse gas effect has more effect per volume than it does during the day. It’s sort of like the way a given amount of stovetop heat will raise the water temperature more when the pot is half-full than when it’s completely full.

This is also part of the reason colder areas, especially polar regions, are being hurt more by the changing climate, although many other factors are probably more important, such as patterns of global movement of the atmosphere and ocean currents.

 

http://www.concordmonitor.com/temperature-climate-change-nh-18663171

So if I understand correctly, they are claiming that greenhouse gasses prevent some amount of heat from radiating at the lowest levels of the atmosphere.  Right?  Is the above good science?

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Yes, the overnight lows have the highest trend in temperature and that is what GHGs do. For an individual site like CON though, you'd also have to look at land use around the obs site as that will affect radiational cooling in a big way. We've seen this affect airports that used to be more rural but now have a lot of development around them like IAD.

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20 minutes ago, ORH_wxman said:

Yes, the overnight lows have the highest trend in temperature and that is what GHGs do. For an individual site like CON though, you'd also have to look at land use around the obs site as that will affect radiational cooling in a big way. We've seen this affect airports that used to be more rural but now have a lot of development around them like IAD.

Thanks.

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Yes. This reduction the diurnal temperature range is an expected and predicted behavior of a GHG forcing agent. I'm not sure I agree with the explanation given though. Based on what I've read it's actually the net effect of a lot of different things including but not limited to cloud cover, soil and vegetation evaporation rates, aerosols, urban heat islands, land use changes, etc. Different processes may dominate in different locales. But, a significant reason that is even more subtle than any of the above is that GHGs species like CO2 are mostly transparent to incoming shortwave radiation so the amount of daytime heating occurring before GHG increases isn't substantially different afterward either (all other things being equal that is). In other words, Tmax at many locales saturates out with or without GHGs at a similar pace. But, in the absence of advective processes and whereas Tmax is modulated more by incoming shortwave radiation Tmin is modulated more by outgoing longwave radiation which CO2 and other GHGs are opaque to so in that regard it's intuitive that Tmin responds more to GHGs than Tmax. Then consider that cloud cover (itself having an anthroprogenic influence) responds to both outgoing longwave radiation and incoming shortwave radiation and it should be obvious that more clouds means a smaller diurnal temperature range (DTR). Many studies pin this later simple explanation on most of the DTR observed in many regions. But, it's certainly not the whole story.

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By the way, an interesting extraterrestrial case study of the DTR is that of Venus. It's greenhouse effect and advective processes are so potent that the DTR there is effectively 0.

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interesting explanations but honestly the largest thing I have seen for tmin has been the increase in moisture in the lower levels, the alternate GHG from the well talked about CO2. You keep dp's up they arent dropping temps/radiating them out definitely one of the biggest contributors I have seen over the past couple years to decade. Stagnation of patterns also doesnt help the cause either we use to swing systems in every couple days and tended to have maybe one or two extended periods of real intense dp and high temps but now it seems the patterns are staying around a bit longer in certain locals. Cant speak for NE but down here around the mason dixon line I have noticed this change many of times the front gets stuck or stalls right near us and NYC drops in dp's while many areas still sit in the low 70's.

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It's interesting that the faster rising minimum temperatures occur during the meteorological winter and summer. But this effect doesn't show up during the spring and fall. I ran the numbers for the whole Northeast climate region since 1980. This is the period the greatest global warming.

Northeast climate region 1980 to present.....temperature change per decade

https://www.ncdc.noaa.gov/cag/

DJF

max...+0.5F/Decade

min...+0.9

MAM

max....+0.4

min....+0.3

JJA

max....+0.3

min....+0.6

SON

max...+0.9

min...+0.9

 

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Back to the article the OP mentioned I'm definitely not understanding the explanation given. I'm playing out typical diurnal pressure and temperature perturbations in my mind and plugging them into PV=nRT and I'm just not seeing how the V or n changes much over the entire depth of the troposphere. Even on first principal reasoning it's weird to think the volume of the atmosphere is substantially different on the night side vs the day side of the planet. I mean, I acknowledge that there probably are small diurnal fluctuations in the cross sectional area of H2O and CO2 molecules (probably more so with water vapor than with carbon dioxide), but the changes aren't going to be enough to substantially alter the probability of an outgoing longwave photon getting absorbed on it's trip up to the stratosphere. Right? What am I missing? 

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While there isn't much of a trend from '40-'17, there certainly is from 1980 onwards and it's decidedly positive. Aerosols have a big part to play in that dip from the 40s to the 70s.

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