6 replies to this topic

[GeorgeLiv]

Posting this to show how SQM changes due to the properties of the air above you. I forced myself up near 4 am almost every morning earlier this September to take advantage of a stretch of clear skies.

 

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Procedure:

 

In a NW suburb of Montréal, using my basic SQM meter, at the same spot from my roof, with the same method (a minimum of 12 readings per session, minus spurious readings), I took down the mean or mode at zenith in mags/arcsec², the limiting magnitude with my best eye-glasses and noted the sky conditions. The job was made pleasant by glancing at the winter stars with Jupiter & Mars.

 

Adding the actual weather parameters recorded & archived in a field just over 5 km (~3.33 miles) away at Montréal's International Airport, at an elevation of 36 meters (118 feet) above MSL, it's an official Environment Canada recording station operated by NAV Canada.

 

Also posting a short gif (about 1½ hours worth) painstakingly collected for the period of observations to show the influence of air sector. It's the IR showing temperatures. Visible loops are impossible at night. Scale in °C is shown at the gif bottom.

 

End Procedure.

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Observations:

 

For the first time since late June, a flow of Polar air had invaded the North American mid-latitudes on September 1st, (and the day before), 2024. I posted an IR GIF here for the entire day of September 1st '24 showing the low that developed, complete with double cold-fronts and marked air types & flows. For the next 16 days, I was under the influence of either sub-tropical, modified sub-tropical and modified polar air. True tropical air having finally receded far south (good riddance), but unfortunately, the sub-tropical air transferred smoke from western regions of the US.

 

From the marked tiny blue X in the 2010 ISS shot, I got the following sets of SQM readings. Each gif that follows shows the same spot as a white dot near center.

 

 

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September 3, 2024, (pre-dawn) near 4:30 am EDT:

SQM = 17.93 mag/arcsec²
Mag limit was at magnitude 4.5, Nu Andromedae, fairly easy by averted vision. Superb sky. A few comments posted right here.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions
4am LST,  10.8°C,  6.7°C,   76% RH,  300 degrees,  11km/h, 24.1km, 101.94 kPa,   Clear
5am LST,  11.0°C,  7.6°C,   80% RH,  230 degrees,   8km/h, 24.1km, 101.99 kPa,   NA

 

 

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September 4, near 4:30 am EDT, 2024:
SQM = 17.54 mag/arcsec²
Mag limit 4.0 by averted vision easy, Nu Andromedae (mag 4.5) not discerned. Fair zenith, thicker horizons, probably smoke. Warm front in the north already pushed out crisp cooler air.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions
4am LST,  14.3°C,  11.7°C,   84% RH,  250 degrees, 12km/h,  24.1km,  102.32 kPa,   Mainly Clear
5am LST,  14.5°C,  11.6°C,   83% RH,  260 degrees, 14km/h,  24.1km,  102.38 kPa,   NA

 

 

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September 5 (pre-dawn), 2024:
SQM = 17.63  mag/arcsec² near 4:40 am EDT.
Mag limit 4.0 by averted vision (Gamma Trianguli) but with difficulty. Smoke at high levels is certain, horizons obscured and bright stars (Sirius, Rigel & Procyon) not twinkling. Better at zenith.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions
4am LST,  16.8°C,  12.7°C,   77% RH,  180 degrees, 7 km/h, 24.1km,  102.38 kPa,   Clear
5am LST,  15.6°C,  12.6°C,   82% RH,       calm,        0 km/h  48.3km,  102.39 kPa,   NA

 

 

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September 6 (pre-dawn), 2024:
SQM = 17.82 mag/arcsec², 4:30 to 4:45am.
Mag limit 4.5 but with good difficulty. Frontal boundary approaching, but flow from south & south-east ushered in cleaner air than past two mornings. Visibility shown on NAV data as 48.3km for 5am.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions
4am LST,  17.0°C,   12.7°C,    76% RH, 160 degrees,  17 km/h, 24.1km, 101.56 kPa,  Clear
5am LST,  16.8°C,   12.5°C,    76% RH, 170 degrees,  15 km/h, 48.3km, 101.53 kPa,   NA

 

 

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Sept 7, '24: Cloudy by Friday night the 6th through most of the morning hours. Clearings surprisingly dark for marm-sector air in am hours (Sept.7th). No serious check, but by quickly finding Triangulum Mag limit was near 4.0. Warm 18°C near 4am, strong Low-Pressure system developing just south of James Bay.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions
4am LST,  17.9°C,  14.7°C,   81% RH, 150 degrees, 17km/h, 24.1km, 100.56 kPa, Mainly Cloudy
5am LST,  17.4°C,  14.6°C,   84% RH, 140 degrees, 15km/h, 24.1km, 100.50 kPa, NA

 

No loop.

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Sept 8, '24: After a fair amount of rain into the evening of the 7th, some clearings late. Up near same time, strong flow of cP air but lots of sheered low clouds with higher variable cloud. Dark skies seen with clearings. SQM near 17.70 m/arcsec² with clouds. Mag limit at 4.5, clearings were brief, very windy and the temperature was a refreshing 7°C (45°F). Later in day on Sunday Sept 8th, temperature reached only 13.1°C for max. Didn't drop much later at night, 11.0°C by midnight, and all cloudy.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions
4am LST,   7.3°C,   4.8°C,   84% RH, 270 degrees,  25km/h, 24.1km, 100.43 kPa, Mostly Cloudy
5am LST,   7.6°C,   4.6°C,   81% RH, 260 degrees,  20km/h, 24.1km, 100.46 kPa, NA

 

The loop is for the entire night of Sept 8th, 2024..

 

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September 9th, Monday morning, 2024:

SQM was around 17.78 m/arcsec² but clouds in west encroaching.
Mag limit at least 4.2. Mostly cloudy with good clearings for 4 to 4:30am. Pi (π) Aurigae, at mag 4.26 quickly spotted (M3 type star actually appears brighter than listed).
 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,  11.9°C,  6.9°C,   72% RH,  240 degrees, 19km/h, 24.1km, 100.88 kPa, Mostly Cloudy
5am LST,  11.6°C,  7.5°C,   76% RH,  250 degrees, 18km/h, 24.1km, 100.88 kPa, NA

 

 

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Sept 10th, at 4:30 am to 4:50, 2024:
SQM was a mean of 18.02 m/arcsec²

Mag limit was at least 4.5 Nu Andromedae. Clear & darker than normal, but a faint band-line of thin cirrus seen in west at end. Star at foot of Gemini 1 Geminorum easy at mag 4.16. Gamma Triangulum (4.0) & Pi Aurigae (4.26) both easy. Tau Aurigae (4.52) by averted vision.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,   8.4°C,  6.8°C,  89% RH, 230 degrees wind, 5km/h, 24.1km, 101.23 kPa, Clear
5am LST,   8.9°C,  6.5°C,  85% RH,      calm,              0km/h,  24.1km, 101.30 kPa, NA

 

 

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Sept 11th, at 4 am,  2024:

SQM mode (very common reading) 17.66 m/arcsec². Clear but bright all horizons, Sirius dimmed & not twinkling.

Mag limit Pi Aurigae 4.26 with averted vision, but Tau Aurigae and Nu Andromedae both suspected at 4.5. Star 1 Geminorum ok at 4.16. Warm front is pushing in some smoke from the north into my east horizon

 

Later at 5am, very similar except all around horizons appeared even brighter & more obscured. Felt quite humid. SQM 17.72 as a mean. This time 4.5 mag Nu Andromedae not suspected as it was lower.

 

Montréal YUL NAV station:
LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,  12.3°C,  10.2°C, 87% RH, 250 degrees,  9km/h, 24.1km, 101.62 kPa, Clear
5am LST,  11.5°C,    9.9°C, 90% RH, 260 degrees,  7km/h, 24.1km, 101.66 kPa, NA

 

 

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Not Finished Yet. Please refrain from replying.

This is an appreciably difficult post to assemble & double-check.

Edited by GeorgeLiv, 26 September 2024 - 10:02 PM.

[GeorgeLiv]

Continued..

 

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Sept 12th, 2024. Thursday morning, cloudy.

For the period near 4 am the radar showed drizzle but nothing felt or wetness seen.

 

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Sept 13, Friday morning near 4:45 am, 2024:

SQM = a mean of 17.57 mag/arcsec². Nice, clear but humid and bright plus obscured horizons.
Mag limit near 4.1..with effort Pi Aurigae, also at mag 4.0 Gamma Trianguli. Nu Andromedae at 4.5 not visible. Again it's likely that Pi Aurigae may be slightly brighter than 4.26. Star 1 Geminorum at 4.16 spotted but difficult. Auroras seen in Scandinavia, but mostly red glows, also imaged over western US. Auroras not visible although suspected in the late evening Sept 12th.

 

Montréal YUL NAV station:

LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,   14.6°C,  13.6°C,   94% RH,    calm,    0km/h, 24.1km, 101.88 kPa, Clear

5am LST,   14.2°C,  13.3°C,   94% RH,    calm,    0km/h, 24.1km, 101.92 kPa, NA

 

 

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Sep 14th, Sat morning 4:45am, 2024:

SQM lots of 17.58 (mode) mag/arcsec².

Mag limit barely 4.0 Gamma Trianguli. 3.97 mag Nu Aurigae also ok. Stars 1 Geminorum, Pi Aurigae only suspected. Mu Andromedae (3.9) difficult and Nu (4.5) not visible at all. Thick atmosphere, somewhat foggy (low on horizons), misty, warm & high-humidity. Probably smoke too.

 

Montréal YUL NAV station:

LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,  16.1°C,  15.7°C,   97% RH, 10 degrees  4km/h,  4.8km  102.06 kPa,     Fog

5am LST,  16.4°C,  15.9°C,   96% RH, 20 degrees  3km/h,  4.8km  102.09 kPa,     Fog

 

 

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Sep 15th Sun morn 4:45, 2024:

SQM was 17.52 mag/arcsec² mean & mode.

Mag < 4.0 at best. Warmish, obscured horizons, 1 Geminorum not visible, Nu Aurigae (3.97) difficult, Zeta Aurigae at 3.75 ok, also Gamma Trianguli ok but difficult at 4.0. High barometric pressure appears meaningless.

 

Montréal YUL NAV station:

LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,  16.5°C,  14.3°C,   86% RH, 40 degrees  10km/h,  24.1km  102.20 kPa,     Clear

5am LST,  16.4°C,  14.4°C,   88% RH, 40 degrees  12km/h,  24.1km  102.24 kPa,     Na

 

 

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Sep 16th, 2024. Mon morning close to 4:15am, a few minutes after moonset:

SQM was a mean of 17.56 mag/arcsec²

Mag limit 4.2 Gamma Trianguli easy, also 4.26 Pi Aurigae, and with difficulty 1 Geminorum. Nu Andromedae (4.5) not or suspected. Better sky than yesterday (slightly).

 

Montréal YUL NAV station:

LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,  17.8°C,  17.2°C,   96% RH, 190 degrees  4km/h,  24.1km  102.08 kPa,     Clear

5am LST,  17.8°C,  16.4°C,   92% RH, 160 degrees  3km/h,  24.1km  102.14 kPa,     Na

 

 

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Sep 17th, 2024. Tue morning close to 4:55am, with the moon nearing full still 10 degrees up!

SQM = 17.28 mag/arcsec², mean, with just a few mid-level clouds across Moon's location low in south-west

Mag Limit 4.0 at zenith. Pi Aurigae not visible, Nu Aurigae (3.97) very difficult, Zeta Aurigae at 3.75 ok & 4.0 Gamma Trianguli (just passed my zenith) ok but difficult. 1 Geminorum not visible. Had gotten humid after a very nice & quite dry day.

 

Montréal YUL NAV station:

LocalTime/Temp/Dewpoint-Temp/R.H./Wind-direction/Speed/Visibility/BaroPressure/Conditions

4am LST,  18.0°C,  16.0°C,   88% RH, 240 degrees  9km/h,  24.1km  101.86 kPa,     Clear

5am LST,  17.9°C,  15.8°C,   87% RH, 260 degrees  8km/h,  24.1km  101.85 kPa,     Na

 

 

 

End of data collecting as the Moon was nearing Full for the next few nights.

You may now comment. Thanks.

Edited by GeorgeLiv, 26 September 2024 - 11:11 PM.

[Ron359]

Communication of time series data is far more understandable with graphs.  How about you post the most significant data points on a time series graph with diff colors for the different types of data points.  

[PEterW]

I have been trying to get some local data recently, ran out after a band of showers passed a few weeks back and got 19.3, but yesterday was fully clear and only 19.1. The Aerosol optical depth was predicted as 0.03 the first time and 0.08 yesterday- it looked “milkier” and greyer. I’d like a total water column measure to see the effect of that on the results too. It’s good to see how and why things are changing rather than just an app saying things are ok on a random scale.
I should memorise a few more faint stars for magnitude estimation to back up the SQM.

Peter

[Tony Flanders]

Communication of time series data is far more understandable with graphs.  How about you post the most significant data points on a time series graph with diff colors for the different types of data points.  

Agreed. I suspect there's some interesting information in the base post, but I'm not eager to paw through multiple screens to try to find out what it is. I would prefer a presentation saying:

 

1. Here's what I'm trying to demonstrate.

2. Here's an overview of the data that demonstrates it.

3. Here's an explanation why that data demonstrates my point.

4. For those who are curious, here are additional details about the data.

[GeorgeLiv]

Nothing else will become of this. Any casual reader will notice my SQM (one site on Earth) varied by as much as 0.50 m/sec² for similar clear conditions (my gifs) near the same time of night when any nocturnal variation, other than temperature, dewpoint, humidity & barometric pressure, has been minimized.

 

As for displaying these numbers on graph(s)...anyone wanting to plot such "surface" parameters to compare to SQM or limiting magnitude is overthinking things. SQM or limiting magnitudes are the result of the whole column of air to your zenith. I'd imagine if I had access to what PEterW describes just above as the Aerosol Optical Depth as a definite number, then, yes, that would be telling.

 

Get your SQM meters out. The darkest nights for the light-polluted N.E. are coming up now in October, just before the leaves fall. This is when colder and hopefully cleaner air descends from northern Canada with increasing frequency & vigor. The length of nights over Ellesmere, Baffin and all the other arctic islands is over 20 hours, 24 hour darkness at the pole now.

[PEterW]

The aerosol optical depth predictions are available as one of the overlay options in windy.com (website and app). The best views will be for clear and low optical depth nights. Still in the early days of noting things when I check the local SQM, I’ll record any other useful parameters j can.

Peter