Topic: Dramatic air temperature variations

[DaleReid]

This morning most of Western Wisconsin had clear sky conditions under high pressure. Light or calm winds.

Using MesoWest to look at the temperatures there are dramatic variations, some reporting by AWOS or DNR calibrated stations, and some by private individuals, which as we all know may not have the megabucks to install top notch equipment or have it calibrated.  Nonetheless, most of those stations have in the past been reporting reasonable temps, and thus not entirely suspect.

https://mesowest.utah.edu/cgi-bin/droman/mesomap.cgi?lat=44.86668&lon=-91.48800&radius=25&rawsflag=290&site=KEAU&unit=0&time=LOCAL&product=&year1=&month1=&day1=00&hour1=00&currTimeChecked=

This morning I had -9 on almost all of my thermometers, both electronic and mercury column.

A neighbor 1.5 miles to my east, down in a wee bit of a valley had -15 and -18 on some good 'quality' thermometers, too.

Around Eau Claire temps ranged from -7 to -19 all within a few miles of each other.  To my south about 40 miles, at Black River Falls airport, the temperature was -25 and a mile or two to the east, -28.

I don't understand, other than radiation cooling, how the temperatures can vary that much, especially within a few miles. In the past I would have offered the explanation of bad equipment or observer bias wanting to be a bit dramatic and having the hottest or coldest temperatures.  A couple decades ago when small radio stations were still independent, one station on the east side of Eau Claire near the lake was always, without exception, dramatically colder than any other reporting station, by as much as 10 to 15 degrees.  Engineers who worked there were hams and said that they'd see the same numbers on the readouts as was being reported, and when car thermometers became standard (the epitome of quality, I'm sure) the temps were very close when one would drive by the station.

Is it all just radiation cooling?  If so, why would one open area be dramatically colder than another?

Is it a myth that cold air 'drains' into the lower areas? 

This has been so noticeable the last few days that it has prompted me to wonder about the reason behind it.  As I mentioned observer bias and cheap advertising quality thermometers were my explanation in the past but now I'm wondering if this phenomenon is real.

Thanks for any thoughts or science.
Dale

[CW2274]

Is it a myth that cold air 'drains' into the lower areas? 

Can't really speak to the other stuff, but this is definitely a fact, especially with light/calm wind.

[Waimarie]

I live in the Wairarapa, a very wide "valley" at the bottom of the North Island of New Zealand.  I used to work in Wellington on the other side of the mountain range on the western side of the valley.  While driving to work, in the mornings, I used to notice the temperature variation as I travelled to work in the car.  When it was cold at home (cold here being -1 or -2 celcius at best) the temperature would steadily rise as I went up the "hill" to maybe +5 or+6 at 2-300ft and it would stay there for a while and then started falling again toward the summit of the road (1800ft).

So yes cold air definitely "pools" in low lying areas and cold air being denser than warm, it will sink and the coldest will find the lowest place.

Have you tried to correlate the temperatures to altitudes?  That would prove interesting.

Ian

[DaleReid]

This whole area of Wisconsin here and to the east is at the bottom of a glaciated area, and pretty darned flat.  A few rolling hills but nothing over a hundred feet or so.

If the glacier 11,000 years ago didn't scrub it flat, then old Lake Wisconsin which spread for over a hundred miles to the east mudded in and was quite sedimentary.

Where I grew up in the central part of Wisconsin, just north of Wisconsin Dells where this whole lake drained out in a matter of days creating the beautiful rock formations there, the ground is so flat that the drop per mile towards the Wisconsin River is only a foot or so per mile.  My wife was amazed at how flat it is.

Anyway, height variations would not explain the temperature differences, other than cold air 'draining' or 'pooling' in the valley areas, as small as they are.

I do recall driving home from our family farm one Christmas eve, in a new (for then) car with an outside thermometer in it.  Wow, high tech.  Anyway, with everyone else asleep I was monitoring the temps (already -20 F which was chilly) and noticing that the temperatures were as much as 3 degrees in variation, which on a crystal clear calm cold high pressure night surprised me that it was changing that much.   

[CW7491]

Peter Sinks, UT is maybe one of the most extreme examples on earth of the combination radiational cooling and cold air drainage can have over a very short distance. And fortunately it is actually monitored with professional equipment ...

https://climate.usu.edu/PeterSinks/index.php

[DaleReid]

Wow, I am glad it has a lot of instrumentation to help follow the changes, and to fascinate those of us with odd interests.

We drove by within 50 miles or so of the spot a few years back, and had no idea it was there.  I'd probably have irritated my wife if I had suggested a diversion to take in the setting and see for ourselves, but who knows, next trip maybe?

Dale

[ocala]

Cold air drainage is for real. NWS in Jacksonville often refers to it in my area.
About 25 miles to my west there is a narrow strip that runs north and south where the temps on cold nights can be up to 8 degrees cooler. It doesn't always happen but I have noticed it several times on cold mornings.
We have a front supposed to come through here Thursday and temps are foretasted to be low to mid 30's. I'll take a look and see if it happens. Although the winds are supposed to about 5-10 so it probably won't. On successive the nights the wind is supposed to be calm so we'll see.

[Kryophil]

In low wind conditions with little or no cloud cover, the night-time radiation can efficiently cool the ground. The ground in turn cools the air above it, thus creating a layer of cold air near the ground.

Snow is favourable for the in-situ formation of cold air, fluffy fresh snow is best, it has excellent insulating properties against the ground heat flow.

Since this cold air has a higher density compared to warmer air, a cold air flow is generated when the terrain has a sufficient gradient. This outflow of cold air follows the terrain: it can accumulate at indentations (e.g. the narrowing of a valley), where a lake of cold air is created (imagine by analogy that cold air behaves like water, it flows downwards following gravity).

This works best in sinkholes with no outflow, which occur mainly in karst areas. Here the cold air can come to rest completely and cool down further.

I operate weather stations myself in such cold air pools and in extreme cases temperature differences of up to 30 degrees Celusius (=54 degrees Fahrenheit) can occur over a vertical distance of 75 m (246 ft).

Here is an example, which I captured on video almost 3 years ago: [vimeo]https://vimeo.com/214170397[/vimeo]. The temperature dropped from -8 °C (17.6 °F) to -21 °C (-5.8 °C) on an altitude difference of 45 m (147 ft).