Rang Saint-Arthur
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rang Saint-Arthur clear sky chart from 2005-11-06 to 2025-12-18.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	16% 13% 4 10% 6% 5% 5% 4 5% 6% 26%	126869
	Night	14% 13% 5% 11% 7% 6% 5% 4 5% 5% 26%	124241
Transparency	Day	56% 6% 12% 17% 8% 1	130322
	Night	56% 7% 11% 17% 8% 1	128876
Seeing	Day	27% 20% 31% 16% 6% 1	16872
	Night	24% 19% 30% 20% 7% 1	25274

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	10% 18% 9% 16% 8% 6% 6% 5% 4 4 15%	3575
	11th to 20th	Day	6% 14% 7% 21% 12% 8% 6% 3 4 3 16%	3570
	21st to end	Day	5% 8% 6% 21% 8% 7% 7% 4 6% 6% 22%	3960
February	1st to 10th	Day	10% 15% 6% 14% 11% 9% 6% 4 4 4 17%	1653
	11th to 20th	Day	11% 18% 7% 15% 7% 5% 6% 4 4 3 20%	1893
	21st to end	Day	9% 16% 6% 14% 8% 5% 5% 3 4 5% 24%	1650
March	1st to 10th	Day	14% 18% 7% 14% 6% 4 4 3 3 4 21%	4476
	11th to 20th	Day	11% 19% 5% 10% 6% 6% 5% 3 5% 6% 25%	4837
	21st to end	Day	16% 17% 6% 11% 5% 4 3 3 3 5% 27%	5686
April	1st to 10th	Day	20% 18% 5% 10% 5% 4 4 2 3 5% 24%	2400
	11th to 20th	Day	16% 14% 5% 10% 4 4 3 3 4 5% 33%	2507
	21st to end	Day	20% 15% 4 9% 4 3 3 3 4 6% 29%	2600
May	1st to 10th	Day	22% 14% 4 8% 5% 6% 4 4 5% 5% 23%	5886
	11th to 20th	Day	21% 11% 3 9% 6% 5% 5% 4 5% 6% 25%	6310
	21st to end	Day	17% 12% 3 9% 6% 5% 5% 4 5% 6% 27%	7215
June	1st to 10th	Day	21% 10% 3 8% 6% 5% 5% 4 5% 8% 25%	3000
	11th to 20th	Day	20% 8% 3 6% 5% 4 5% 4 7% 7% 29%	3000
	21st to end	Day	23% 11% 4 9% 6% 5% 4 4 5% 6% 24%	3000
July	1st to 10th	Day	17% 9% 4 10% 6% 6% 6% 5% 7% 7% 23%	6643
	11th to 20th	Day	14% 7% 3 7% 6% 6% 6% 5% 7% 8% 31%	6360
	21st to end	Day	12% 7% 2 7% 5% 4 5% 5% 6% 8% 38%	6926
August	1st to 10th	Day	15% 8% 3 7% 6% 6% 6% 5% 7% 9% 28%	4447
	11th to 20th	Day	16% 9% 4 9% 6% 7% 6% 5% 7% 6% 24%	4175
	21st to end	Day	18% 8% 3 8% 5% 4 5% 3 5% 6% 34%	4557
September	1st to 10th	Day	16% 9% 2 8% 4 4 4 4 6% 7% 35%	2385
	11th to 20th	Day	17% 8% 3 5% 3 4 4 3 4 7% 43%	2200
	21st to end	Day	18% 9% 3 7% 4 4 3 3 6% 6% 37%	2200
October	1st to 10th	Day	17% 12% 3 8% 5% 4 4 4 5% 6% 33%	2191
	11th to 20th	Day	19% 16% 4 10% 5% 5% 4 3 5% 4 25%	1852
	21st to end	Day	21% 19% 5% 10% 5% 5% 4 3 3 5% 20%	1980
November	1st to 10th	Day	16% 17% 5% 11% 6% 5% 4 3 4 4 26%	1851
	11th to 20th	Day	16% 17% 6% 12% 7% 6% 3 3 4 4 22%	1853
	21st to end	Day	16% 19% 7% 12% 9% 5% 4 3 4 3 19%	1538
December	1st to 10th	Day	13% 18% 6% 16% 8% 5% 6% 3 4 4 17%	2350
	11th to 20th	Day	14% 16% 6% 15% 7% 6% 4 2 4 4 23%	2667
	21st to end	Day	14% 17% 7% 12% 7% 5% 4 3 5% 4 22%	3476
January	1st to 10th	Night	9% 16% 8% 16% 9% 8% 6% 4 4 4 15%	6367
	11th to 20th	Night	6% 15% 7% 21% 12% 9% 6% 3 4 3 14%	6315
	21st to end	Night	5% 9% 6% 17% 7% 7% 8% 6% 7% 6% 22%	6996
February	1st to 10th	Night	7% 15% 6% 13% 10% 8% 6% 5% 5% 4 22%	2745
	11th to 20th	Night	9% 14% 7% 15% 8% 6% 5% 5% 5% 4 21%	2392
	21st to end	Night	11% 14% 4 13% 8% 6% 4 3 4 5% 28%	1980
March	1st to 10th	Night	16% 17% 5% 11% 6% 4 5% 3 4 6% 23%	5364
	11th to 20th	Night	10% 13% 5% 12% 7% 7% 6% 5% 6% 6% 22%	5122
	21st to end	Night	15% 16% 5% 11% 6% 5% 4 3 4 4 27%	5243
April	1st to 10th	Night	17% 17% 5% 9% 5% 4 5% 4 4 5% 27%	2000
	11th to 20th	Night	15% 14% 3 8% 5% 4 5% 3 5% 7% 30%	1841
	21st to end	Night	19% 14% 4 10% 4 4 4 3 4 5% 30%	1800
May	1st to 10th	Night	20% 12% 5% 7% 5% 4 5% 4 6% 6% 27%	4054
	11th to 20th	Night	21% 11% 3 7% 5% 4 4 3 4 6% 32%	3626
	21st to end	Night	16% 11% 2 7% 4 4 5% 4 5% 7% 36%	3668
June	1st to 10th	Night	20% 11% 3 6% 6% 6% 6% 4 5% 6% 27%	1400
	11th to 20th	Night	15% 8% 3 8% 6% 4 5% 4 6% 7% 36%	1400
	21st to end	Night	19% 10% 3 8% 6% 4 5% 4 5% 7% 29%	1400
July	1st to 10th	Night	13% 10% 4 9% 6% 5% 5% 4 6% 7% 30%	3325
	11th to 20th	Night	18% 9% 3 7% 5% 4 4 4 5% 7% 34%	3600
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	3926
August	1st to 10th	Night	18% 9% 3 7% 4 4 5% 4 6% 6% 33%	2670
	11th to 20th	Night	14% 9% 4 10% 7% 7% 5% 4 6% 6% 28%	2872
	21st to end	Night	17% 8% 3 7% 5% 4 4 3 4 6% 38%	3338
September	1st to 10th	Night	15% 7% 2 7% 5% 5% 4 4 4 6% 41%	2015
	11th to 20th	Night	15% 8% 2 6% 4 3 4 3 5% 6% 43%	2200
	21st to end	Night	16% 9% 3 7% 4 5% 4 3 4 5% 40%	2200
October	1st to 10th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 38%	2207
	11th to 20th	Night	19% 13% 4 8% 5% 5% 5% 3 4 5% 29%	2545
	21st to end	Night	19% 15% 4 10% 5% 5% 4 3 4 4 25%	2860
November	1st to 10th	Night	16% 14% 5% 11% 7% 5% 5% 3 5% 5% 24%	2667
	11th to 20th	Night	14% 12% 5% 12% 7% 6% 4 3 4 6% 26%	2766
	21st to end	Night	13% 17% 7% 13% 8% 6% 5% 4 4 4 19%	3078
December	1st to 10th	Night	11% 13% 7% 17% 11% 10% 5% 3 4 5% 13%	5060
	11th to 20th	Night	15% 15% 7% 14% 9% 6% 4 3 4 4 19%	5697
	21st to end	Night	15% 18% 6% 12% 6% 4 4 3 4 3 25%	7502

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	73% 4 8% 10% 6%	3576
	11th to 20th	Day	71% 4 7% 10% 7% 1	3600
	21st to end	Day	59% 5% 7% 12% 15% 1	3960
February	1st to 10th	Day	68% 6% 6% 11% 8% 2	1653
	11th to 20th	Day	68% 5% 8% 12% 7% 1	1902
	21st to end	Day	61% 4 7% 18% 7% 3	1650
March	1st to 10th	Day	66% 4 7% 10% 10% 4	4479
	11th to 20th	Day	59% 6% 14% 13% 8% 1	4837
	21st to end	Day	60% 4 6% 15% 12% 3	5686
April	1st to 10th	Day	64% 4 8% 13% 10% 2	2400
	11th to 20th	Day	53% 5% 8% 20% 11% 2	2507
	21st to end	Day	58% 3 9% 17% 11% 2	2600
May	1st to 10th	Day	61% 5% 8% 16% 9% 1	5897
	11th to 20th	Day	57% 7% 13% 16% 5% 2	6314
	21st to end	Day	54% 8% 11% 17% 8% 2	7215
June	1st to 10th	Day	56% 6% 11% 20% 7% 1	3000
	11th to 20th	Day	49% 7% 13% 21% 8% 1	3000
	21st to end	Day	60% 6% 10% 16% 7%	3000
July	1st to 10th	Day	54% 8% 15% 17% 6%	6571
	11th to 20th	Day	44% 9% 19% 24% 3	6285
	21st to end	Day	40% 8% 17% 28% 7%	6728
August	1st to 10th	Day	48% 9% 20% 18% 5%	4433
	11th to 20th	Day	54% 9% 18% 15% 4	4214
	21st to end	Day	48% 7% 15% 19% 10% 1	4557
September	1st to 10th	Day	46% 7% 15% 23% 10%	2385
	11th to 20th	Day	41% 5% 13% 29% 11%	2200
	21st to end	Day	47% 5% 13% 24% 11% 1	2200
October	1st to 10th	Day	53% 6% 11% 23% 7%	3568
	11th to 20th	Day	52% 9% 14% 20% 5%	2990
	21st to end	Day	53% 7% 15% 18% 8%	3180
November	1st to 10th	Day	60% 6% 12% 16% 6%	1851
	11th to 20th	Day	64% 6% 8% 15% 7% 1	1853
	21st to end	Day	69% 5% 8% 14% 4	1538
December	1st to 10th	Day	67% 6% 9% 9% 8%	2350
	11th to 20th	Day	62% 6% 7% 15% 9%	2667
	21st to end	Day	65% 3 8% 9% 11% 3	3476
January	1st to 10th	Night	69% 6% 9% 11% 5%	6367
	11th to 20th	Night	71% 8% 7% 10% 4 1	6360
	21st to end	Night	55% 7% 10% 14% 13% 2	6996
February	1st to 10th	Night	61% 7% 8% 14% 9% 1	2745
	11th to 20th	Night	64% 5% 8% 15% 7% 1	2394
	21st to end	Night	58% 5% 7% 17% 12% 1	1980
March	1st to 10th	Night	61% 5% 7% 13% 12% 1	5374
	11th to 20th	Night	58% 8% 16% 11% 7% 1	5123
	21st to end	Night	59% 5% 5% 17% 13% 1	5243
April	1st to 10th	Night	59% 5% 6% 15% 13% 3	2000
	11th to 20th	Night	52% 6% 8% 21% 12% 1	1841
	21st to end	Night	57% 4 8% 18% 11% 2	1800
May	1st to 10th	Night	56% 6% 11% 17% 11% 1	4060
	11th to 20th	Night	53% 8% 12% 21% 7%	3627
	21st to end	Night	46% 7% 16% 18% 12% 1	3669
June	1st to 10th	Night	55% 7% 12% 19% 7%	1400
	11th to 20th	Night	45% 7% 13% 22% 11% 2	1400
	21st to end	Night	53% 6% 16% 17% 8% 1	1400
July	1st to 10th	Night	47% 9% 20% 20% 4	3287
	11th to 20th	Night	45% 9% 21% 22% 3	3552
	21st to end	Night	39% 9% 22% 22% 8%	3809
August	1st to 10th	Night	48% 7% 18% 23% 4	2660
	11th to 20th	Night	49% 11% 18% 17% 5%	2899
	21st to end	Night	45% 7% 15% 20% 11% 2	3338
September	1st to 10th	Night	42% 7% 13% 27% 9% 1	2015
	11th to 20th	Night	41% 6% 17% 27% 8% 1	2200
	21st to end	Night	46% 6% 11% 24% 12% 1	2200
October	1st to 10th	Night	45% 6% 12% 28% 9%	3610
	11th to 20th	Night	49% 10% 10% 24% 6%	4146
	21st to end	Night	49% 7% 18% 18% 8% 1	4610
November	1st to 10th	Night	58% 7% 12% 18% 5%	2667
	11th to 20th	Night	58% 6% 11% 14% 10% 1	2766
	21st to end	Night	65% 7% 10% 13% 4	3078
December	1st to 10th	Night	65% 11% 10% 11% 3	5060
	11th to 20th	Night	65% 7% 6% 15% 7%	5698
	21st to end	Night	63% 4 6% 13% 11% 2	7502

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	26% 30% 36% 8% 1 1	200
	11th to 20th	Day	29% 28% 33% 8% 3 1	200
	21st to end	Day	21% 31% 32% 12% 3 1	220
February	1st to 10th	Day	37% 24% 27% 9% 2	253
	11th to 20th	Day	30% 27% 32% 9% 2	469
	21st to end	Day	27% 29% 29% 13% 2	414
March	1st to 10th	Day	29% 26% 30% 11% 3	500
	11th to 20th	Day	29% 25% 29% 10% 6%	500
	21st to end	Day	31% 24% 25% 13% 5% 1	550
April	1st to 10th	Day	38% 23% 21% 13% 4 1	500
	11th to 20th	Day	32% 24% 26% 14% 3 1	492
	21st to end	Day	35% 18% 31% 10% 4 1	500
May	1st to 10th	Day	30% 17% 34% 15% 2 1	503
	11th to 20th	Day	27% 17% 31% 19% 6% 1	527
	21st to end	Day	23% 20% 34% 17% 5% 1	683
June	1st to 10th	Day	25% 15% 36% 17% 7%	730
	11th to 20th	Day	24% 17% 28% 23% 8% 1	730
	21st to end	Day	30% 14% 29% 20% 6% 1	730
July	1st to 10th	Day	21% 18% 34% 20% 6% 1	650
	11th to 20th	Day	21% 16% 40% 16% 6% 1	524
	21st to end	Day	19% 16% 39% 20% 5% 1	576
August	1st to 10th	Day	21% 14% 34% 23% 7% 2	526
	11th to 20th	Day	19% 13% 33% 24% 10% 1	519
	21st to end	Day	21% 17% 30% 20% 8% 3	583
September	1st to 10th	Day	19% 19% 29% 20% 9% 3	530
	11th to 20th	Day	23% 12% 33% 19% 10% 3	530
	21st to end	Day	20% 20% 30% 16% 9% 4	530
October	1st to 10th	Day	24% 19% 27% 20% 8% 2	524
	11th to 20th	Day	29% 22% 29% 13% 7% 1	420
	21st to end	Day	36% 25% 25% 10% 3	462
November	1st to 10th	Day	36% 20% 28% 12% 3	425
	11th to 20th	Day	27% 27% 24% 15% 6%	430
	21st to end	Day	34% 29% 27% 8% 2	288
December	1st to 10th	Day	31% 21% 31% 13% 4	220
	11th to 20th	Day	37% 27% 26% 8% 1	214
	21st to end	Day	34% 24% 27% 11% 4	220
January	1st to 10th	Night	24% 27% 34% 11% 3	803
	11th to 20th	Night	24% 26% 30% 17% 3	800
	21st to end	Night	16% 25% 37% 18% 4	880
February	1st to 10th	Night	20% 30% 32% 14% 4	800
	11th to 20th	Night	25% 26% 33% 12% 3 1	783
	21st to end	Night	24% 22% 34% 16% 4	660
March	1st to 10th	Night	25% 21% 35% 15% 4	800
	11th to 20th	Night	22% 25% 33% 14% 5% 1	800
	21st to end	Night	24% 21% 27% 21% 6%	880
April	1st to 10th	Night	30% 19% 29% 17% 5%	800
	11th to 20th	Night	26% 17% 30% 21% 5% 1	657
	21st to end	Night	32% 14% 27% 20% 6% 1	600
May	1st to 10th	Night	26% 12% 31% 23% 7%	600
	11th to 20th	Night	23% 11% 29% 26% 10% 1	419
	21st to end	Night	23% 13% 30% 27% 7% 1	433
June	1st to 10th	Night	26% 10% 30% 25% 10% 1	400
	11th to 20th	Night	20% 10% 25% 33% 12% 1	400
	21st to end	Night	24% 11% 25% 31% 9% 1	400
July	1st to 10th	Night	18% 10% 35% 26% 12%	390
	11th to 20th	Night	20% 8% 31% 30% 10% 1	388
	21st to end	Night	17% 8% 27% 37% 11%	438
August	1st to 10th	Night	18% 6% 28% 30% 16% 1	443
	11th to 20th	Night	20% 7% 26% 33% 13% 1	592
	21st to end	Night	17% 11% 29% 30% 12% 2	723
September	1st to 10th	Night	20% 11% 30% 27% 11% 1	800
	11th to 20th	Night	21% 9% 28% 26% 14% 2	800
	21st to end	Night	21% 13% 28% 25% 13% 1	800
October	1st to 10th	Night	21% 16% 28% 25% 8% 1	800
	11th to 20th	Night	27% 19% 28% 20% 7% 1	800
	21st to end	Night	31% 21% 28% 15% 5%	880
November	1st to 10th	Night	30% 22% 29% 15% 4	820
	11th to 20th	Night	26% 22% 26% 18% 6% 1	859
	21st to end	Night	28% 26% 27% 14% 4 1	950
December	1st to 10th	Night	27% 23% 31% 13% 4 1	950
	11th to 20th	Night	27% 26% 26% 16% 5%	936
	21st to end	Night	29% 24% 26% 16% 4	990

Page generated at 2025-12-15 01:13:56. Version ac07

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